Serial Peripheral Interface (SPI)¶

SPI is the “Serial Peripheral Interface”, widely used with embedded systems because it is a simple and efficient interface: basically a multiplexed shift register. Its three signal wires hold a clock (SCK, often in the range of 1-20 MHz), a “Master Out, Slave In” (MOSI) data line, and a “Master In, Slave Out” (MISO) data line. SPI is a full duplex protocol; for each bit shifted out the MOSI line (one per clock) another is shifted in on the MISO line. Those bits are assembled into words of various sizes on the way to and from system memory. An additional chipselect line is usually active-low (nCS); four signals are normally used for each peripheral, plus sometimes an interrupt.

The SPI bus facilities listed here provide a generalized interface to declare SPI busses and devices, manage them according to the standard Linux driver model, and perform input/output operations. At this time, only “master” side interfaces are supported, where Linux talks to SPI peripherals and does not implement such a peripheral itself. (Interfaces to support implementing SPI slaves would necessarily look different.)

The programming interface is structured around two kinds of driver, and two kinds of device. A “Controller Driver” abstracts the controller hardware, which may be as simple as a set of GPIO pins or as complex as a pair of FIFOs connected to dual DMA engines on the other side of the SPI shift register (maximizing throughput). Such drivers bridge between whatever bus they sit on (often the platform bus) and SPI, and expose the SPI side of their device as a struct spi_master. SPI devices are children of that master, represented as a struct spi_device and manufactured from struct spi_board_info descriptors which are usually provided by board-specific initialization code. A struct spi_driver is called a “Protocol Driver”, and is bound to a spi_device using normal driver model calls.

The I/O model is a set of queued messages. Protocol drivers submit one or more struct spi_message objects, which are processed and completed asynchronously. (There are synchronous wrappers, however.) Messages are built from one or more struct spi_transfer objects, each of which wraps a full duplex SPI transfer. A variety of protocol tweaking options are needed, because different chips adopt very different policies for how they use the bits transferred with SPI.

struct spi_statistics¶: statistics for spi transfers

Definition

struct spi_statistics {
  spinlock_t lock;
  unsigned long           messages;
  unsigned long           transfers;
  unsigned long           errors;
  unsigned long           timedout;
  unsigned long           spi_sync;
  unsigned long           spi_sync_immediate;
  unsigned long           spi_async;
  unsigned long long      bytes;
  unsigned long long      bytes_rx;
  unsigned long long      bytes_tx;
#define SPI_STATISTICS_HISTO_SIZE 17;
  unsigned long transfer_bytes_histo[SPI_STATISTICS_HISTO_SIZE];
  unsigned long transfers_split_maxsize;
};

Members

lock: lock protecting this structure
messages: number of spi-messages handled
transfers: number of spi_transfers handled
errors: number of errors during spi_transfer
timedout: number of timeouts during spi_transfer
spi_sync: number of times spi_sync is used
spi_sync_immediate: number of times spi_sync is executed immediately in calling context without queuing and scheduling
spi_async: number of times spi_async is used
bytes: number of bytes transferred to/from device
bytes_rx: number of bytes received from device
bytes_tx: number of bytes sent to device
transfer_bytes_histo: transfer bytes histogramm
transfers_split_maxsize: number of transfers that have been split because of maxsize limit

struct spi_delay¶: SPI delay information

Definition

struct spi_delay {
#define SPI_DELAY_UNIT_USECS    0;
#define SPI_DELAY_UNIT_NSECS    1;
#define SPI_DELAY_UNIT_SCK      2;
  u16 value;
  u8 unit;
};

Members

value: Value for the delay
unit: Unit for the delay

struct spi_device¶: Controller side proxy for an SPI slave device

Definition

struct spi_device {
  struct device           dev;
  struct spi_controller   *controller;
  struct spi_controller   *master;
  u32 max_speed_hz;
  u8 chip_select;
  u8 bits_per_word;
  bool rt;
  u32 mode;
#define SPI_CPHA        0x01                    ;
#define SPI_CPOL        0x02                    ;
#define SPI_MODE_0      (0|0)                   ;
#define SPI_MODE_1      (0|SPI_CPHA);
#define SPI_MODE_2      (SPI_CPOL|0);
#define SPI_MODE_3      (SPI_CPOL|SPI_CPHA);
#define SPI_MODE_X_MASK (SPI_CPOL|SPI_CPHA);
#define SPI_CS_HIGH     0x04                    ;
#define SPI_LSB_FIRST   0x08                    ;
#define SPI_3WIRE       0x10                    ;
#define SPI_LOOP        0x20                    ;
#define SPI_NO_CS       0x40                    ;
#define SPI_READY       0x80                    ;
#define SPI_TX_DUAL     0x100                   ;
#define SPI_TX_QUAD     0x200                   ;
#define SPI_RX_DUAL     0x400                   ;
#define SPI_RX_QUAD     0x800                   ;
#define SPI_CS_WORD     0x1000                  ;
#define SPI_TX_OCTAL    0x2000                  ;
#define SPI_RX_OCTAL    0x4000                  ;
#define SPI_3WIRE_HIZ   0x8000                  ;
  int irq;
  void *controller_state;
  void *controller_data;
  char modalias[SPI_NAME_SIZE];
  const char              *driver_override;
  int cs_gpio;
  struct gpio_desc        *cs_gpiod;
  struct spi_delay        word_delay;
  struct spi_statistics   statistics;
};

Members

dev: Driver model representation of the device.
controller: SPI controller used with the device.
master: Copy of controller, for backwards compatibility.
max_speed_hz: Maximum clock rate to be used with this chip (on this board); may be changed by the device’s driver. The spi_transfer.speed_hz can override this for each transfer.
chip_select: Chipselect, distinguishing chips handled by controller.
bits_per_word: Data transfers involve one or more words; word sizes like eight or 12 bits are common. In-memory wordsizes are powers of two bytes (e.g. 20 bit samples use 32 bits). This may be changed by the device’s driver, or left at the default (0) indicating protocol words are eight bit bytes. The spi_transfer.bits_per_word can override this for each transfer.
rt: Make the pump thread real time priority.
mode: The spi mode defines how data is clocked out and in. This may be changed by the device’s driver. The “active low” default for chipselect mode can be overridden (by specifying SPI_CS_HIGH) as can the “MSB first” default for each word in a transfer (by specifying SPI_LSB_FIRST).
irq: Negative, or the number passed to request_irq() to receive interrupts from this device.
controller_state: Controller’s runtime state
controller_data: Board-specific definitions for controller, such as FIFO initialization parameters; from board_info.controller_data
modalias: Name of the driver to use with this device, or an alias for that name. This appears in the sysfs “modalias” attribute for driver coldplugging, and in uevents used for hotplugging
driver_override: If the name of a driver is written to this attribute, then the device will bind to the named driver and only the named driver.
cs_gpio: LEGACY: gpio number of the chipselect line (optional, -ENOENT when not using a GPIO line) use cs_gpiod in new drivers by opting in on the spi_master.
cs_gpiod: gpio descriptor of the chipselect line (optional, NULL when not using a GPIO line)
word_delay: delay to be inserted between consecutive words of a transfer
statistics: statistics for the spi_device

Description

A spi_device is used to interchange data between an SPI slave (usually a discrete chip) and CPU memory.

In dev, the platform_data is used to hold information about this device that’s meaningful to the device’s protocol driver, but not to its controller. One example might be an identifier for a chip variant with slightly different functionality; another might be information about how this particular board wires the chip’s pins.

struct spi_driver¶: Host side “protocol” driver

Definition

struct spi_driver {
  const struct spi_device_id *id_table;
  int (*probe)(struct spi_device *spi);
  int (*remove)(struct spi_device *spi);
  void (*shutdown)(struct spi_device *spi);
  struct device_driver    driver;
};

Members

id_table: List of SPI devices supported by this driver
probe: Binds this driver to the spi device. Drivers can verify that the device is actually present, and may need to configure characteristics (such as bits_per_word) which weren’t needed for the initial configuration done during system setup.
remove: Unbinds this driver from the spi device
shutdown: Standard shutdown callback used during system state transitions such as powerdown/halt and kexec
driver: SPI device drivers should initialize the name and owner field of this structure.

Description

This represents the kind of device driver that uses SPI messages to interact with the hardware at the other end of a SPI link. It’s called a “protocol” driver because it works through messages rather than talking directly to SPI hardware (which is what the underlying SPI controller driver does to pass those messages). These protocols are defined in the specification for the device(s) supported by the driver.

As a rule, those device protocols represent the lowest level interface supported by a driver, and it will support upper level interfaces too. Examples of such upper levels include frameworks like MTD, networking, MMC, RTC, filesystem character device nodes, and hardware monitoring.

void spi_unregister_driver(struct spi_driver *sdrv)¶: reverse effect of spi_register_driver

Parameters

struct spi_driver *sdrv: the driver to unregister

Context

can sleep

module_spi_driver(__spi_driver)¶: Helper macro for registering a SPI driver

Parameters

__spi_driver: spi_driver struct

Description

Helper macro for SPI drivers which do not do anything special in module init/exit. This eliminates a lot of boilerplate. Each module may only use this macro once, and calling it replaces module_init() and module_exit()

struct spi_controller¶: interface to SPI master or slave controller

Definition

struct spi_controller {
  struct device   dev;
  struct list_head list;
  s16 bus_num;
  u16 num_chipselect;
  u16 dma_alignment;
  u32 mode_bits;
  u32 buswidth_override_bits;
  u32 bits_per_word_mask;
#define SPI_BPW_MASK(bits) BIT((bits) - 1);
#define SPI_BPW_RANGE_MASK(min, max) GENMASK((max) - 1, (min) - 1);
  u32 min_speed_hz;
  u32 max_speed_hz;
  u16 flags;
#define SPI_CONTROLLER_HALF_DUPLEX      BIT(0)  ;
#define SPI_CONTROLLER_NO_RX            BIT(1)  ;
#define SPI_CONTROLLER_NO_TX            BIT(2)  ;
#define SPI_CONTROLLER_MUST_RX          BIT(3)  ;
#define SPI_CONTROLLER_MUST_TX          BIT(4)  ;
#define SPI_MASTER_GPIO_SS              BIT(5)  ;
  bool slave;
  size_t (*max_transfer_size)(struct spi_device *spi);
  size_t (*max_message_size)(struct spi_device *spi);
  struct mutex            io_mutex;
  spinlock_t bus_lock_spinlock;
  struct mutex            bus_lock_mutex;
  bool bus_lock_flag;
  int (*setup)(struct spi_device *spi);
  int (*set_cs_timing)(struct spi_device *spi, struct spi_delay *setup, struct spi_delay *hold, struct spi_delay *inactive);
  int (*transfer)(struct spi_device *spi, struct spi_message *mesg);
  void (*cleanup)(struct spi_device *spi);
  bool (*can_dma)(struct spi_controller *ctlr,struct spi_device *spi, struct spi_transfer *xfer);
  bool queued;
  struct kthread_worker           *kworker;
  struct kthread_work             pump_messages;
  spinlock_t queue_lock;
  struct list_head                queue;
  struct spi_message              *cur_msg;
  bool idling;
  bool busy;
  bool running;
  bool rt;
  bool auto_runtime_pm;
  bool cur_msg_prepared;
  bool cur_msg_mapped;
  bool last_cs_enable;
  bool last_cs_mode_high;
  bool fallback;
  struct completion               xfer_completion;
  size_t max_dma_len;
  int (*prepare_transfer_hardware)(struct spi_controller *ctlr);
  int (*transfer_one_message)(struct spi_controller *ctlr, struct spi_message *mesg);
  int (*unprepare_transfer_hardware)(struct spi_controller *ctlr);
  int (*prepare_message)(struct spi_controller *ctlr, struct spi_message *message);
  int (*unprepare_message)(struct spi_controller *ctlr, struct spi_message *message);
  int (*slave_abort)(struct spi_controller *ctlr);
  void (*set_cs)(struct spi_device *spi, bool enable);
  int (*transfer_one)(struct spi_controller *ctlr, struct spi_device *spi, struct spi_transfer *transfer);
  void (*handle_err)(struct spi_controller *ctlr, struct spi_message *message);
  const struct spi_controller_mem_ops *mem_ops;
  struct spi_delay        cs_setup;
  struct spi_delay        cs_hold;
  struct spi_delay        cs_inactive;
  int *cs_gpios;
  struct gpio_desc        **cs_gpiods;
  bool use_gpio_descriptors;
  u8 unused_native_cs;
  u8 max_native_cs;
  struct spi_statistics   statistics;
  struct dma_chan         *dma_tx;
  struct dma_chan         *dma_rx;
  void *dummy_rx;
  void *dummy_tx;
  int (*fw_translate_cs)(struct spi_controller *ctlr, unsigned cs);
  bool ptp_sts_supported;
  unsigned long           irq_flags;
};

Members

dev

device interface to this driver

list

link with the global spi_controller list

bus_num

board-specific (and often SOC-specific) identifier for a given SPI controller.

num_chipselect

chipselects are used to distinguish individual SPI slaves, and are numbered from zero to num_chipselects. each slave has a chipselect signal, but it’s common that not every chipselect is connected to a slave.

dma_alignment

SPI controller constraint on DMA buffers alignment.

mode_bits

flags understood by this controller driver

buswidth_override_bits

flags to override for this controller driver

bits_per_word_mask

A mask indicating which values of bits_per_word are supported by the driver. Bit n indicates that a bits_per_word n+1 is supported. If set, the SPI core will reject any transfer with an unsupported bits_per_word. If not set, this value is simply ignored, and it’s up to the individual driver to perform any validation.

min_speed_hz

Lowest supported transfer speed

max_speed_hz

Highest supported transfer speed

flags

other constraints relevant to this driver

slave

indicates that this is an SPI slave controller

max_transfer_size

function that returns the max transfer size for a spi_device; may be NULL, so the default SIZE_MAX will be used.

max_message_size

function that returns the max message size for a spi_device; may be NULL, so the default SIZE_MAX will be used.

io_mutex

mutex for physical bus access

bus_lock_spinlock

spinlock for SPI bus locking

bus_lock_mutex

mutex for exclusion of multiple callers

bus_lock_flag

indicates that the SPI bus is locked for exclusive use

setup

updates the device mode and clocking records used by a device’s SPI controller; protocol code may call this. This must fail if an unrecognized or unsupported mode is requested. It’s always safe to call this unless transfers are pending on the device whose settings are being modified.

set_cs_timing

optional hook for SPI devices to request SPI master controller for configuring specific CS setup time, hold time and inactive delay interms of clock counts

transfer

adds a message to the controller’s transfer queue.

cleanup

frees controller-specific state

can_dma

determine whether this controller supports DMA

queued

whether this controller is providing an internal message queue

kworker

pointer to thread struct for message pump

pump_messages

work struct for scheduling work to the message pump

queue_lock

spinlock to syncronise access to message queue

queue

message queue

cur_msg

the currently in-flight message

idling

the device is entering idle state

busy

message pump is busy

running

message pump is running

rt

whether this queue is set to run as a realtime task

auto_runtime_pm

the core should ensure a runtime PM reference is held while the hardware is prepared, using the parent device for the spidev

cur_msg_prepared

spi_prepare_message was called for the currently in-flight message

cur_msg_mapped

message has been mapped for DMA

last_cs_enable

was enable true on the last call to set_cs.

last_cs_mode_high

was (mode & SPI_CS_HIGH) true on the last call to set_cs.

fallback

fallback to pio if dma transfer return failure with SPI_TRANS_FAIL_NO_START.

xfer_completion

used by core transfer_one_message()

max_dma_len

Maximum length of a DMA transfer for the device.

prepare_transfer_hardware

a message will soon arrive from the queue so the subsystem requests the driver to prepare the transfer hardware by issuing this call

transfer_one_message

the subsystem calls the driver to transfer a single message while queuing transfers that arrive in the meantime. When the driver is finished with this message, it must call spi_finalize_current_message() so the subsystem can issue the next message

unprepare_transfer_hardware

there are currently no more messages on the queue so the subsystem notifies the driver that it may relax the hardware by issuing this call

prepare_message

set up the controller to transfer a single message, for example doing DMA mapping. Called from threaded context.

unprepare_message

undo any work done by prepare_message().

slave_abort

abort the ongoing transfer request on an SPI slave controller

set_cs

set the logic level of the chip select line. May be called from interrupt context.

transfer_one

transfer a single spi_transfer.

return 0 if the transfer is finished,
return 1 if the transfer is still in progress. When the driver is finished with this transfer it must call spi_finalize_current_transfer() so the subsystem can issue the next transfer. Note: transfer_one and transfer_one_message are mutually exclusive; when both are set, the generic subsystem does not call your transfer_one callback.

handle_err

the subsystem calls the driver to handle an error that occurs in the generic implementation of transfer_one_message().

mem_ops

optimized/dedicated operations for interactions with SPI memory. This field is optional and should only be implemented if the controller has native support for memory like operations.

cs_setup

delay to be introduced by the controller after CS is asserted

cs_hold

delay to be introduced by the controller before CS is deasserted

cs_inactive

delay to be introduced by the controller after CS is deasserted. If cs_change_delay is used from spi_transfer, then the two delays will be added up.

cs_gpios

LEGACY: array of GPIO descs to use as chip select lines; one per CS number. Any individual value may be -ENOENT for CS lines that are not GPIOs (driven by the SPI controller itself). Use the cs_gpiods in new drivers.

cs_gpiods

Array of GPIO descs to use as chip select lines; one per CS number. Any individual value may be NULL for CS lines that are not GPIOs (driven by the SPI controller itself).

use_gpio_descriptors

Turns on the code in the SPI core to parse and grab GPIO descriptors rather than using global GPIO numbers grabbed by the driver. This will fill in cs_gpiods and cs_gpios should not be used, and SPI devices will have the cs_gpiod assigned rather than cs_gpio.

unused_native_cs

When cs_gpiods is used, spi_register_controller() will fill in this field with the first unused native CS, to be used by SPI controller drivers that need to drive a native CS when using GPIO CS.

max_native_cs

When cs_gpiods is used, and this field is filled in, spi_register_controller() will validate all native CS (including the unused native CS) against this value.

statistics

statistics for the spi_controller

dma_tx

DMA transmit channel

dma_rx

DMA receive channel

dummy_rx

dummy receive buffer for full-duplex devices

dummy_tx

dummy transmit buffer for full-duplex devices

fw_translate_cs

If the boot firmware uses different numbering scheme what Linux expects, this optional hook can be used to translate between the two.

ptp_sts_supported

If the driver sets this to true, it must provide a time snapshot in spi_transfer->ptp_sts as close as possible to the moment in time when spi_transfer->ptp_sts_word_pre and spi_transfer->ptp_sts_word_post were transmitted. If the driver does not set this, the SPI core takes the snapshot as close to the driver hand-over as possible.

irq_flags

Interrupt enable state during PTP system timestamping

Description

Each SPI controller can communicate with one or more spi_device children. These make a small bus, sharing MOSI, MISO and SCK signals but not chip select signals. Each device may be configured to use a different clock rate, since those shared signals are ignored unless the chip is selected.

The driver for an SPI controller manages access to those devices through a queue of spi_message transactions, copying data between CPU memory and an SPI slave device. For each such message it queues, it calls the message’s completion function when the transaction completes.

struct spi_res¶: spi resource management structure

Definition

struct spi_res {
  struct list_head        entry;
  spi_res_release_t release;
  unsigned long long      data[];
};

Members

entry: list entry
release: release code called prior to freeing this resource
data: extra data allocated for the specific use-case

Description

this is based on ideas from devres, but focused on life-cycle management during spi_message processing

struct spi_transfer¶: a read/write buffer pair

Definition

struct spi_transfer {
  const void      *tx_buf;
  void *rx_buf;
  unsigned len;
  dma_addr_t tx_dma;
  dma_addr_t rx_dma;
  struct sg_table tx_sg;
  struct sg_table rx_sg;
  unsigned cs_change:1;
  unsigned tx_nbits:3;
  unsigned rx_nbits:3;
#define SPI_NBITS_SINGLE        0x01 ;
#define SPI_NBITS_DUAL          0x02 ;
#define SPI_NBITS_QUAD          0x04 ;
  u8 bits_per_word;
  u16 delay_usecs;
  struct spi_delay        delay;
  struct spi_delay        cs_change_delay;
  struct spi_delay        word_delay;
  u32 speed_hz;
  u32 effective_speed_hz;
  unsigned int    ptp_sts_word_pre;
  unsigned int    ptp_sts_word_post;
  struct ptp_system_timestamp *ptp_sts;
  bool timestamped;
  struct list_head transfer_list;
#define SPI_TRANS_FAIL_NO_START BIT(0);
  u16 error;
};

Members

tx_buf: data to be written (dma-safe memory), or NULL
rx_buf: data to be read (dma-safe memory), or NULL
len: size of rx and tx buffers (in bytes)
tx_dma: DMA address of tx_buf, if spi_message.is_dma_mapped
rx_dma: DMA address of rx_buf, if spi_message.is_dma_mapped
tx_sg: Scatterlist for transmit, currently not for client use
rx_sg: Scatterlist for receive, currently not for client use
cs_change: affects chipselect after this transfer completes
tx_nbits: number of bits used for writing. If 0 the default (SPI_NBITS_SINGLE) is used.
rx_nbits: number of bits used for reading. If 0 the default (SPI_NBITS_SINGLE) is used.
bits_per_word: select a bits_per_word other than the device default for this transfer. If 0 the default (from spi_device) is used.
delay_usecs: microseconds to delay after this transfer before (optionally) changing the chipselect status, then starting the next transfer or completing this spi_message.
delay: delay to be introduced after this transfer before (optionally) changing the chipselect status, then starting the next transfer or completing this spi_message.
cs_change_delay: delay between cs deassert and assert when cs_change is set and spi_transfer is not the last in spi_message
word_delay: inter word delay to be introduced after each word size (set by bits_per_word) transmission.
speed_hz: Select a speed other than the device default for this transfer. If 0 the default (from spi_device) is used.
effective_speed_hz: the effective SCK-speed that was used to transfer this transfer. Set to 0 if the spi bus driver does not support it.
ptp_sts_word_pre: The word (subject to bits_per_word semantics) offset within tx_buf for which the SPI device is requesting that the time snapshot for this transfer begins. Upon completing the SPI transfer, this value may have changed compared to what was requested, depending on the available snapshotting resolution (DMA transfer, ptp_sts_supported is false, etc).
ptp_sts_word_post: See ptp_sts_word_post. The two can be equal (meaning that a single byte should be snapshotted). If the core takes care of the timestamp (if ptp_sts_supported is false for this controller), it will set ptp_sts_word_pre to 0, and ptp_sts_word_post to the length of the transfer. This is done purposefully (instead of setting to spi_transfer->len - 1) to denote that a transfer-level snapshot taken from within the driver may still be of higher quality.
ptp_sts: Pointer to a memory location held by the SPI slave device where a PTP system timestamp structure may lie. If drivers use PIO or their hardware has some sort of assist for retrieving exact transfer timing, they can (and should) assert ptp_sts_supported and populate this structure using the ptp_read_system_*ts helper functions. The timestamp must represent the time at which the SPI slave device has processed the word, i.e. the “pre” timestamp should be taken before transmitting the “pre” word, and the “post” timestamp after receiving transmit confirmation from the controller for the “post” word.
timestamped: true if the transfer has been timestamped
transfer_list: transfers are sequenced through spi_message.transfers
error: Error status logged by spi controller driver.

Description

SPI transfers always write the same number of bytes as they read. Protocol drivers should always provide rx_buf and/or tx_buf. In some cases, they may also want to provide DMA addresses for the data being transferred; that may reduce overhead, when the underlying driver uses dma.

If the transmit buffer is null, zeroes will be shifted out while filling rx_buf. If the receive buffer is null, the data shifted in will be discarded. Only “len” bytes shift out (or in). It’s an error to try to shift out a partial word. (For example, by shifting out three bytes with word size of sixteen or twenty bits; the former uses two bytes per word, the latter uses four bytes.)

In-memory data values are always in native CPU byte order, translated from the wire byte order (big-endian except with SPI_LSB_FIRST). So for example when bits_per_word is sixteen, buffers are 2N bytes long (len = 2N) and hold N sixteen bit words in CPU byte order.

When the word size of the SPI transfer is not a power-of-two multiple of eight bits, those in-memory words include extra bits. In-memory words are always seen by protocol drivers as right-justified, so the undefined (rx) or unused (tx) bits are always the most significant bits.

All SPI transfers start with the relevant chipselect active. Normally it stays selected until after the last transfer in a message. Drivers can affect the chipselect signal using cs_change.

(i) If the transfer isn’t the last one in the message, this flag is used to make the chipselect briefly go inactive in the middle of the message. Toggling chipselect in this way may be needed to terminate a chip command, letting a single spi_message perform all of group of chip transactions together.

(ii) When the transfer is the last one in the message, the chip may stay selected until the next transfer. On multi-device SPI busses with nothing blocking messages going to other devices, this is just a performance hint; starting a message to another device deselects this one. But in other cases, this can be used to ensure correctness. Some devices need protocol transactions to be built from a series of spi_message submissions, where the content of one message is determined by the results of previous messages and where the whole transaction ends when the chipselect goes intactive.

When SPI can transfer in 1x,2x or 4x. It can get this transfer information from device through tx_nbits and rx_nbits. In Bi-direction, these two should both be set. User can set transfer mode with SPI_NBITS_SINGLE(1x) SPI_NBITS_DUAL(2x) and SPI_NBITS_QUAD(4x) to support these three transfer.

The code that submits an spi_message (and its spi_transfers) to the lower layers is responsible for managing its memory. Zero-initialize every field you don’t set up explicitly, to insulate against future API updates. After you submit a message and its transfers, ignore them until its completion callback.

struct spi_message¶: one multi-segment SPI transaction

Definition

struct spi_message {
  struct list_head        transfers;
  struct spi_device       *spi;
  unsigned is_dma_mapped:1;
  void (*complete)(void *context);
  void *context;
  unsigned frame_length;
  unsigned actual_length;
  int status;
  struct list_head        queue;
  void *state;
  struct list_head        resources;
};

Members

transfers: list of transfer segments in this transaction
spi: SPI device to which the transaction is queued
is_dma_mapped: if true, the caller provided both dma and cpu virtual addresses for each transfer buffer
complete: called to report transaction completions
context: the argument to complete() when it’s called
frame_length: the total number of bytes in the message
actual_length: the total number of bytes that were transferred in all successful segments
status: zero for success, else negative errno
queue: for use by whichever driver currently owns the message
state: for use by whichever driver currently owns the message
resources: for resource management when the spi message is processed

Description

A spi_message is used to execute an atomic sequence of data transfers, each represented by a struct spi_transfer. The sequence is “atomic” in the sense that no other spi_message may use that SPI bus until that sequence completes. On some systems, many such sequences can execute as a single programmed DMA transfer. On all systems, these messages are queued, and might complete after transactions to other devices. Messages sent to a given spi_device are always executed in FIFO order.

The code that submits an spi_message (and its spi_transfers) to the lower layers is responsible for managing its memory. Zero-initialize every field you don’t set up explicitly, to insulate against future API updates. After you submit a message and its transfers, ignore them until its completion callback.

void spi_message_init_with_transfers(struct spi_message *m, struct spi_transfer *xfers, unsigned int num_xfers)¶: Initialize spi_message and append transfers

Parameters

struct spi_message *m: spi_message to be initialized
struct spi_transfer *xfers: An array of spi transfers
unsigned int num_xfers: Number of items in the xfer array

Description

This function initializes the given spi_message and adds each spi_transfer in the given array to the message.

bool spi_is_bpw_supported(struct spi_device *spi, u32 bpw)¶: Check if bits per word is supported

Parameters

struct spi_device *spi: SPI device
u32 bpw: Bits per word

Description

This function checks to see if the SPI controller supports bpw.

Return

True if bpw is supported, false otherwise.

struct spi_replaced_transfers¶: structure describing the spi_transfer replacements that have occurred so that they can get reverted

Definition

struct spi_replaced_transfers {
  spi_replaced_release_t release;
  void *extradata;
  struct list_head replaced_transfers;
  struct list_head *replaced_after;
  size_t inserted;
  struct spi_transfer inserted_transfers[];
};

Members

release: some extra release code to get executed prior to relasing this structure
extradata: pointer to some extra data if requested or NULL
replaced_transfers: transfers that have been replaced and which need to get restored
replaced_after: the transfer after which the replaced_transfers are to get re-inserted
inserted: number of transfers inserted
inserted_transfers: array of spi_transfers of array-size inserted, that have been replacing replaced_transfers

note

that extradata will point to inserted_transfers**[**inserted] if some extra allocation is requested, so alignment will be the same as for spi_transfers

int spi_sync_transfer(struct spi_device *spi, struct spi_transfer *xfers, unsigned int num_xfers)¶: synchronous SPI data transfer

Parameters

struct spi_device *spi: device with which data will be exchanged
struct spi_transfer *xfers: An array of spi_transfers
unsigned int num_xfers: Number of items in the xfer array

Context

can sleep

Description

Does a synchronous SPI data transfer of the given spi_transfer array.

For more specific semantics see spi_sync().

Return

zero on success, else a negative error code.

int spi_write(struct spi_device *spi, const void *buf, size_t len)¶: SPI synchronous write

Parameters

struct spi_device *spi: device to which data will be written
const void *buf: data buffer
size_t len: data buffer size

Context

can sleep

Description

This function writes the buffer buf. Callable only from contexts that can sleep.

Return

zero on success, else a negative error code.

int spi_read(struct spi_device *spi, void *buf, size_t len)¶: SPI synchronous read

Parameters

struct spi_device *spi: device from which data will be read
void *buf: data buffer
size_t len: data buffer size

Context

can sleep

Description

This function reads the buffer buf. Callable only from contexts that can sleep.

Return

zero on success, else a negative error code.

ssize_t spi_w8r8(struct spi_device *spi, u8 cmd)¶: SPI synchronous 8 bit write followed by 8 bit read

Parameters

struct spi_device *spi: device with which data will be exchanged
u8 cmd: command to be written before data is read back

Context

can sleep

Description

Callable only from contexts that can sleep.

Return

the (unsigned) eight bit number returned by the device, or else a negative error code.

ssize_t spi_w8r16(struct spi_device *spi, u8 cmd)¶: SPI synchronous 8 bit write followed by 16 bit read

Parameters

struct spi_device *spi: device with which data will be exchanged
u8 cmd: command to be written before data is read back

Context

can sleep

Description

The number is returned in wire-order, which is at least sometimes big-endian.

Callable only from contexts that can sleep.

Return

the (unsigned) sixteen bit number returned by the device, or else a negative error code.

ssize_t spi_w8r16be(struct spi_device *spi, u8 cmd)¶: SPI synchronous 8 bit write followed by 16 bit big-endian read

Parameters

struct spi_device *spi: device with which data will be exchanged
u8 cmd: command to be written before data is read back

Context

can sleep

Description

This function is similar to spi_w8r16, with the exception that it will convert the read 16 bit data word from big-endian to native endianness.

Callable only from contexts that can sleep.

Return

the (unsigned) sixteen bit number returned by the device in cpu endianness, or else a negative error code.

struct spi_board_info¶: board-specific template for a SPI device

Definition

struct spi_board_info {
  char modalias[SPI_NAME_SIZE];
  const void      *platform_data;
  const struct property_entry *properties;
  void *controller_data;
  int irq;
  u32 max_speed_hz;
  u16 bus_num;
  u16 chip_select;
  u32 mode;
};

Members

modalias: Initializes spi_device.modalias; identifies the driver.
platform_data: Initializes spi_device.platform_data; the particular data stored there is driver-specific.
properties: Additional device properties for the device.
controller_data: Initializes spi_device.controller_data; some controllers need hints about hardware setup, e.g. for DMA.
irq: Initializes spi_device.irq; depends on how the board is wired.
max_speed_hz: Initializes spi_device.max_speed_hz; based on limits from the chip datasheet and board-specific signal quality issues.
bus_num: Identifies which spi_controller parents the spi_device; unused by spi_new_device(), and otherwise depends on board wiring.
chip_select: Initializes spi_device.chip_select; depends on how the board is wired.
mode: Initializes spi_device.mode; based on the chip datasheet, board wiring (some devices support both 3WIRE and standard modes), and possibly presence of an inverter in the chipselect path.

Description

When adding new SPI devices to the device tree, these structures serve as a partial device template. They hold information which can’t always be determined by drivers. Information that probe() can establish (such as the default transfer wordsize) is not included here.

These structures are used in two places. Their primary role is to be stored in tables of board-specific device descriptors, which are declared early in board initialization and then used (much later) to populate a controller’s device tree after the that controller’s driver initializes. A secondary (and atypical) role is as a parameter to spi_new_device() call, which happens after those controller drivers are active in some dynamic board configuration models.

int spi_register_board_info(struct spi_board_info const *info, unsigned n)¶: register SPI devices for a given board

Parameters

struct spi_board_info const *info: array of chip descriptors
unsigned n: how many descriptors are provided

Context

can sleep

Description

Board-specific early init code calls this (probably during arch_initcall) with segments of the SPI device table. Any device nodes are created later, after the relevant parent SPI controller (bus_num) is defined. We keep this table of devices forever, so that reloading a controller driver will not make Linux forget about these hard-wired devices.

Other code can also call this, e.g. a particular add-on board might provide SPI devices through its expansion connector, so code initializing that board would naturally declare its SPI devices.

The board info passed can safely be __initdata … but be careful of any embedded pointers (platform_data, etc), they’re copied as-is. Device properties are deep-copied though.

Return

zero on success, else a negative error code.

int __spi_register_driver(struct module *owner, struct spi_driver *sdrv)¶: register a SPI driver

Parameters

struct module *owner: owner module of the driver to register
struct spi_driver *sdrv: the driver to register

Context

can sleep

Return

zero on success, else a negative error code.

struct spi_device * spi_alloc_device(struct spi_controller *ctlr)¶: Allocate a new SPI device

Parameters

struct spi_controller *ctlr: Controller to which device is connected

Context

can sleep

Description

Allows a driver to allocate and initialize a spi_device without registering it immediately. This allows a driver to directly fill the spi_device with device parameters before calling spi_add_device() on it.

Caller is responsible to call spi_add_device() on the returned spi_device structure to add it to the SPI controller. If the caller needs to discard the spi_device without adding it, then it should call spi_dev_put() on it.

Return

a pointer to the new device, or NULL.

int spi_add_device(struct spi_device *spi)¶: Add spi_device allocated with spi_alloc_device

Parameters

struct spi_device *spi: spi_device to register

Description

Companion function to spi_alloc_device. Devices allocated with spi_alloc_device can be added onto the spi bus with this function.

Return

0 on success; negative errno on failure

struct spi_device * spi_new_device(struct spi_controller *ctlr, struct spi_board_info *chip)¶: instantiate one new SPI device

Parameters

struct spi_controller *ctlr: Controller to which device is connected
struct spi_board_info *chip: Describes the SPI device

Context

can sleep

Description

On typical mainboards, this is purely internal; and it’s not needed after board init creates the hard-wired devices. Some development platforms may not be able to use spi_register_board_info though, and this is exported so that for example a USB or parport based adapter driver could add devices (which it would learn about out-of-band).

Return

the new device, or NULL.

void spi_unregister_device(struct spi_device *spi)¶: unregister a single SPI device

Parameters

struct spi_device *spi: spi_device to unregister

Description

Start making the passed SPI device vanish. Normally this would be handled by spi_unregister_controller().

void spi_finalize_current_transfer(struct spi_controller *ctlr)¶: report completion of a transfer

Parameters

struct spi_controller *ctlr: the controller reporting completion

Description

Called by SPI drivers using the core transfer_one_message() implementation to notify it that the current interrupt driven transfer has finished and the next one may be scheduled.

void spi_take_timestamp_pre(struct spi_controller *ctlr, struct spi_transfer *xfer, size_t progress, bool irqs_off)¶: helper for drivers to collect the beginning of the TX timestamp for the requested byte from the SPI transfer. The frequency with which this function must be called (once per word, once for the whole transfer, once per batch of words etc) is arbitrary as long as the tx buffer offset is greater than or equal to the requested byte at the time of the call. The timestamp is only taken once, at the first such call. It is assumed that the driver advances its tx buffer pointer monotonically.

Parameters

struct spi_controller *ctlr: Pointer to the spi_controller structure of the driver
struct spi_transfer *xfer: Pointer to the transfer being timestamped
size_t progress: How many words (not bytes) have been transferred so far
bool irqs_off: If true, will disable IRQs and preemption for the duration of the transfer, for less jitter in time measurement. Only compatible with PIO drivers. If true, must follow up with spi_take_timestamp_post or otherwise system will crash. WARNING: for fully predictable results, the CPU frequency must also be under control (governor).

void spi_take_timestamp_post(struct spi_controller *ctlr, struct spi_transfer *xfer, size_t progress, bool irqs_off)¶: helper for drivers to collect the end of the TX timestamp for the requested byte from the SPI transfer. Can be called with an arbitrary frequency: only the first call where tx exceeds or is equal to the requested word will be timestamped.

Parameters

struct spi_controller *ctlr: Pointer to the spi_controller structure of the driver
struct spi_transfer *xfer: Pointer to the transfer being timestamped
size_t progress: How many words (not bytes) have been transferred so far
bool irqs_off: If true, will re-enable IRQs and preemption for the local CPU.

struct spi_message * spi_get_next_queued_message(struct spi_controller *ctlr)¶: called by driver to check for queued messages

Parameters

struct spi_controller *ctlr: the controller to check for queued messages

Description

If there are more messages in the queue, the next message is returned from this call.

Return

the next message in the queue, else NULL if the queue is empty.

void spi_finalize_current_message(struct spi_controller *ctlr)¶: the current message is complete

Parameters

struct spi_controller *ctlr: the controller to return the message to

Description

Called by the driver to notify the core that the message in the front of the queue is complete and can be removed from the queue.

int spi_slave_abort(struct spi_device *spi)¶: abort the ongoing transfer request on an SPI slave controller

Parameters

struct spi_device *spi: device used for the current transfer

struct spi_controller * __spi_alloc_controller(struct device *dev, unsigned int size, bool slave)¶: allocate an SPI master or slave controller

Parameters

struct device *dev: the controller, possibly using the platform_bus
unsigned int size: how much zeroed driver-private data to allocate; the pointer to this memory is in the driver_data field of the returned device, accessible with spi_controller_get_devdata(); the memory is cacheline aligned; drivers granting DMA access to portions of their private data need to round up size using ALIGN(size, dma_get_cache_alignment()).
bool slave: flag indicating whether to allocate an SPI master (false) or SPI slave (true) controller

Context

can sleep

Description

This call is used only by SPI controller drivers, which are the only ones directly touching chip registers. It’s how they allocate an spi_controller structure, prior to calling spi_register_controller().

This must be called from context that can sleep.

The caller is responsible for assigning the bus number and initializing the controller’s methods before calling spi_register_controller(); and (after errors adding the device) calling spi_controller_put() to prevent a memory leak.

Return

the SPI controller structure on success, else NULL.

struct spi_controller * __devm_spi_alloc_controller(struct device *dev, unsigned int size, bool slave)¶: resource-managed __spi_alloc_controller()

Parameters

struct device *dev: physical device of SPI controller
unsigned int size: how much zeroed driver-private data to allocate
bool slave: whether to allocate an SPI master (false) or SPI slave (true)

Context

can sleep

Description

Allocate an SPI controller and automatically release a reference on it when dev is unbound from its driver. Drivers are thus relieved from having to call spi_controller_put().

The arguments to this function are identical to __spi_alloc_controller().

Return

the SPI controller structure on success, else NULL.

int spi_register_controller(struct spi_controller *ctlr)¶: register SPI master or slave controller

Parameters

struct spi_controller *ctlr: initialized master, originally from spi_alloc_master() or spi_alloc_slave()

Context

can sleep

Description

SPI controllers connect to their drivers using some non-SPI bus, such as the platform bus. The final stage of probe() in that code includes calling spi_register_controller() to hook up to this SPI bus glue.

SPI controllers use board specific (often SOC specific) bus numbers, and board-specific addressing for SPI devices combines those numbers with chip select numbers. Since SPI does not directly support dynamic device identification, boards need configuration tables telling which chip is at which address.

This must be called from context that can sleep. It returns zero on success, else a negative error code (dropping the controller’s refcount). After a successful return, the caller is responsible for calling spi_unregister_controller().

Return

zero on success, else a negative error code.

int devm_spi_register_controller(struct device *dev, struct spi_controller *ctlr)¶: register managed SPI master or slave controller

Parameters

struct device *dev: device managing SPI controller
struct spi_controller *ctlr: initialized controller, originally from spi_alloc_master() or spi_alloc_slave()

Context

can sleep

Description

Register a SPI device as with spi_register_controller() which will automatically be unregistered and freed.

Return

zero on success, else a negative error code.

void spi_unregister_controller(struct spi_controller *ctlr)¶: unregister SPI master or slave controller

Parameters

struct spi_controller *ctlr: the controller being unregistered

Context

can sleep

Description

This call is used only by SPI controller drivers, which are the only ones directly touching chip registers.

This must be called from context that can sleep.

Note that this function also drops a reference to the controller.

struct spi_controller * spi_busnum_to_master(u16 bus_num)¶: look up master associated with bus_num

Parameters

u16 bus_num: the master’s bus number

Context

can sleep

Description

This call may be used with devices that are registered after arch init time. It returns a refcounted pointer to the relevant spi_controller (which the caller must release), or NULL if there is no such master registered.

Return

the SPI master structure on success, else NULL.

void * spi_res_alloc(struct spi_device *spi, spi_res_release_t release, size_t size, gfp_t gfp)¶: allocate a spi resource that is life-cycle managed during the processing of a spi_message while using spi_transfer_one

Parameters

struct spi_device *spi: the spi device for which we allocate memory
spi_res_release_t release: the release code to execute for this resource
size_t size: size to alloc and return
gfp_t gfp: GFP allocation flags

Return

the pointer to the allocated data

Description

This may get enhanced in the future to allocate from a memory pool of the spi_device or spi_controller to avoid repeated allocations.

void spi_res_free(void *res)¶: free an spi resource

Parameters

void *res: pointer to the custom data of a resource

void spi_res_add(struct spi_message *message, void *res)¶: add a spi_res to the spi_message

Parameters

struct spi_message *message: the spi message
void *res: the spi_resource

void spi_res_release(struct spi_controller *ctlr, struct spi_message *message)¶: release all spi resources for this message

Parameters

struct spi_controller *ctlr: the spi_controller
struct spi_message *message: the spi_message

struct spi_replaced_transfers * spi_replace_transfers(struct spi_message *msg, struct spi_transfer *xfer_first, size_t remove, size_t insert, spi_replaced_release_t release, size_t extradatasize, gfp_t gfp)¶: replace transfers with several transfers and register change with spi_message.resources

Parameters

struct spi_message *msg: the spi_message we work upon
struct spi_transfer *xfer_first: the first spi_transfer we want to replace
size_t remove: number of transfers to remove
size_t insert: the number of transfers we want to insert instead
spi_replaced_release_t release: extra release code necessary in some circumstances
size_t extradatasize: extra data to allocate (with alignment guarantees of struct spi_transfer)
gfp_t gfp: gfp flags

Return

pointer to spi_replaced_transfers,: PTR_ERR(…) in case of errors.

int spi_split_transfers_maxsize(struct spi_controller *ctlr, struct spi_message *msg, size_t maxsize, gfp_t gfp)¶: split spi transfers into multiple transfers when an individual transfer exceeds a certain size

Parameters

struct spi_controller *ctlr: the spi_controller for this transfer
struct spi_message *msg: the spi_message to transform
size_t maxsize: the maximum when to apply this
gfp_t gfp: GFP allocation flags

Return

status of transformation

int spi_setup(struct spi_device *spi)¶: setup SPI mode and clock rate

Parameters

struct spi_device *spi: the device whose settings are being modified

Context

can sleep, and no requests are queued to the device

Description

SPI protocol drivers may need to update the transfer mode if the device doesn’t work with its default. They may likewise need to update clock rates or word sizes from initial values. This function changes those settings, and must be called from a context that can sleep. Except for SPI_CS_HIGH, which takes effect immediately, the changes take effect the next time the device is selected and data is transferred to or from it. When this function returns, the spi device is deselected.

Note that this call will fail if the protocol driver specifies an option that the underlying controller or its driver does not support. For example, not all hardware supports wire transfers using nine bit words, LSB-first wire encoding, or active-high chipselects.

Return

zero on success, else a negative error code.

int spi_set_cs_timing(struct spi_device *spi, struct spi_delay *setup, struct spi_delay *hold, struct spi_delay *inactive)¶: configure CS setup, hold, and inactive delays

Parameters

struct spi_device *spi: the device that requires specific CS timing configuration
struct spi_delay *setup: CS setup time specified via spi_delay
struct spi_delay *hold: CS hold time specified via spi_delay
struct spi_delay *inactive: CS inactive delay between transfers specified via spi_delay

Return

zero on success, else a negative error code.

int spi_async(struct spi_device *spi, struct spi_message *message)¶: asynchronous SPI transfer

Parameters

struct spi_device *spi: device with which data will be exchanged
struct spi_message *message: describes the data transfers, including completion callback

Context

any (irqs may be blocked, etc)

Description

This call may be used in_irq and other contexts which can’t sleep, as well as from task contexts which can sleep.

The completion callback is invoked in a context which can’t sleep. Before that invocation, the value of message->status is undefined. When the callback is issued, message->status holds either zero (to indicate complete success) or a negative error code. After that callback returns, the driver which issued the transfer request may deallocate the associated memory; it’s no longer in use by any SPI core or controller driver code.

Note that although all messages to a spi_device are handled in FIFO order, messages may go to different devices in other orders. Some device might be higher priority, or have various “hard” access time requirements, for example.

On detection of any fault during the transfer, processing of the entire message is aborted, and the device is deselected. Until returning from the associated message completion callback, no other spi_message queued to that device will be processed. (This rule applies equally to all the synchronous transfer calls, which are wrappers around this core asynchronous primitive.)

Return

zero on success, else a negative error code.

int spi_async_locked(struct spi_device *spi, struct spi_message *message)¶: version of spi_async with exclusive bus usage

Parameters

struct spi_device *spi: device with which data will be exchanged
struct spi_message *message: describes the data transfers, including completion callback

Context

any (irqs may be blocked, etc)

Description

This call may be used in_irq and other contexts which can’t sleep, as well as from task contexts which can sleep.

The completion callback is invoked in a context which can’t sleep. Before that invocation, the value of message->status is undefined. When the callback is issued, message->status holds either zero (to indicate complete success) or a negative error code. After that callback returns, the driver which issued the transfer request may deallocate the associated memory; it’s no longer in use by any SPI core or controller driver code.

Note that although all messages to a spi_device are handled in FIFO order, messages may go to different devices in other orders. Some device might be higher priority, or have various “hard” access time requirements, for example.

On detection of any fault during the transfer, processing of the entire message is aborted, and the device is deselected. Until returning from the associated message completion callback, no other spi_message queued to that device will be processed. (This rule applies equally to all the synchronous transfer calls, which are wrappers around this core asynchronous primitive.)

Return

zero on success, else a negative error code.

int spi_sync(struct spi_device *spi, struct spi_message *message)¶: blocking/synchronous SPI data transfers

Parameters

struct spi_device *spi: device with which data will be exchanged
struct spi_message *message: describes the data transfers

Context

can sleep

Description

This call may only be used from a context that may sleep. The sleep is non-interruptible, and has no timeout. Low-overhead controller drivers may DMA directly into and out of the message buffers.

Note that the SPI device’s chip select is active during the message, and then is normally disabled between messages. Drivers for some frequently-used devices may want to minimize costs of selecting a chip, by leaving it selected in anticipation that the next message will go to the same chip. (That may increase power usage.)

Also, the caller is guaranteeing that the memory associated with the message will not be freed before this call returns.

Return

zero on success, else a negative error code.

int spi_sync_locked(struct spi_device *spi, struct spi_message *message)¶: version of spi_sync with exclusive bus usage

Parameters

struct spi_device *spi: device with which data will be exchanged
struct spi_message *message: describes the data transfers

Context

can sleep

Description

This call may only be used from a context that may sleep. The sleep is non-interruptible, and has no timeout. Low-overhead controller drivers may DMA directly into and out of the message buffers.

This call should be used by drivers that require exclusive access to the SPI bus. It has to be preceded by a spi_bus_lock call. The SPI bus must be released by a spi_bus_unlock call when the exclusive access is over.

Return

zero on success, else a negative error code.

int spi_bus_lock(struct spi_controller *ctlr)¶: obtain a lock for exclusive SPI bus usage

Parameters

struct spi_controller *ctlr: SPI bus master that should be locked for exclusive bus access

Context

can sleep

Description

This call may only be used from a context that may sleep. The sleep is non-interruptible, and has no timeout.

This call should be used by drivers that require exclusive access to the SPI bus. The SPI bus must be released by a spi_bus_unlock call when the exclusive access is over. Data transfer must be done by spi_sync_locked and spi_async_locked calls when the SPI bus lock is held.

Return

always zero.

int spi_bus_unlock(struct spi_controller *ctlr)¶: release the lock for exclusive SPI bus usage

Parameters

struct spi_controller *ctlr: SPI bus master that was locked for exclusive bus access

Context

can sleep

Description

This call may only be used from a context that may sleep. The sleep is non-interruptible, and has no timeout.

This call releases an SPI bus lock previously obtained by an spi_bus_lock call.

Return

always zero.

int spi_write_then_read(struct spi_device *spi, const void *txbuf, unsigned n_tx, void *rxbuf, unsigned n_rx)¶: SPI synchronous write followed by read

Parameters

struct spi_device *spi: device with which data will be exchanged
const void *txbuf: data to be written (need not be dma-safe)
unsigned n_tx: size of txbuf, in bytes
void *rxbuf: buffer into which data will be read (need not be dma-safe)
unsigned n_rx: size of rxbuf, in bytes

Context

can sleep

Description

This performs a half duplex MicroWire style transaction with the device, sending txbuf and then reading rxbuf. The return value is zero for success, else a negative errno status code. This call may only be used from a context that may sleep.

Parameters to this routine are always copied using a small buffer. Performance-sensitive or bulk transfer code should instead use spi_{async,sync}() calls with dma-safe buffers.

Return

zero on success, else a negative error code.