Buffer Sharing and Synchronization¶
The dma-buf subsystem provides the framework for sharing buffers for hardware (DMA) access across multiple device drivers and subsystems, and for synchronizing asynchronous hardware access.
This is used, for example, by drm “prime” multi-GPU support, but is of course not limited to GPU use cases.
The three main components of this are: (1) dma-buf, representing a sg_table and exposed to userspace as a file descriptor to allow passing between devices, (2) fence, which provides a mechanism to signal when one device has finished access, and (3) reservation, which manages the shared or exclusive fence(s) associated with the buffer.
Reservation Objects¶
The reservation object provides a mechanism to manage shared and exclusive fences associated with a buffer. A reservation object can have attached one exclusive fence (normally associated with write operations) or N shared fences (read operations). The RCU mechanism is used to protect read access to fences from locked write-side updates.
Parameters
struct dma_resv *obj- the reservation object
Parameters
struct dma_resv *obj- the reservation object
Reserve space to add shared fences to a dma_resv.
Parameters
struct dma_resv *obj- reservation object
unsigned int num_fences- number of fences we want to add
Description
Should be called before dma_resv_add_shared_fence(). Must
be called with obj->lock held.
RETURNS Zero for success, or -errno
Add a fence to a shared slot
Parameters
struct dma_resv *obj- the reservation object
struct dma_fence *fence- the shared fence to add
Description
Add a fence to a shared slot, obj->lock must be held, and
dma_resv_reserve_shared() has been called.
-
void
dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)¶ Add an exclusive fence.
Parameters
struct dma_resv *obj- the reservation object
struct dma_fence *fence- the shared fence to add
Description
Add a fence to the exclusive slot. The obj->lock must be held.
-
int
dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)¶ Copy all fences from src to dst.
Parameters
struct dma_resv *dst- the destination reservation object
struct dma_resv *src- the source reservation object
Description
Copy all fences from src to dst. dst-lock must be held.
-
int
dma_resv_get_fences_rcu(struct dma_resv *obj, struct dma_fence **pfence_excl, unsigned *pshared_count, struct dma_fence ***pshared)¶ Get an object’s shared and exclusive fences without update side lock held
Parameters
struct dma_resv *obj- the reservation object
struct dma_fence **pfence_excl- the returned exclusive fence (or NULL)
unsigned *pshared_count- the number of shared fences returned
struct dma_fence ***pshared- the array of shared fence ptrs returned (array is krealloc’d to the required size, and must be freed by caller)
Description
Retrieve all fences from the reservation object. If the pointer for the exclusive fence is not specified the fence is put into the array of the shared fences as well. Returns either zero or -ENOMEM.
-
long
dma_resv_wait_timeout_rcu(struct dma_resv *obj, bool wait_all, bool intr, unsigned long timeout)¶ Wait on reservation’s objects shared and/or exclusive fences.
Parameters
struct dma_resv *obj- the reservation object
bool wait_all- if true, wait on all fences, else wait on just exclusive fence
bool intr- if true, do interruptible wait
unsigned long timeout- timeout value in jiffies or zero to return immediately
Description
RETURNS Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or greater than zer on success.
-
bool
dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)¶ Test if a reservation object’s fences have been signaled.
Parameters
struct dma_resv *obj- the reservation object
bool test_all- if true, test all fences, otherwise only test the exclusive fence
Description
RETURNS true if all fences signaled, else false
-
struct
dma_resv_list¶ a list of shared fences
Definition
struct dma_resv_list {
struct rcu_head rcu;
u32 shared_count, shared_max;
struct dma_fence __rcu *shared[];
};
Members
rcu- for internal use
shared_count- table of shared fences
shared_max- for growing shared fence table
shared- shared fence table
-
struct
dma_resv¶ a reservation object manages fences for a buffer
Definition
struct dma_resv {
struct ww_mutex lock;
seqcount_ww_mutex_t seq;
struct dma_fence __rcu *fence_excl;
struct dma_resv_list __rcu *fence;
};
Members
lock- update side lock
seq- sequence count for managing RCU read-side synchronization
fence_excl- the exclusive fence, if there is one currently
fence- list of current shared fences
-
struct dma_resv_list *
dma_resv_get_list(struct dma_resv *obj)¶ get the reservation object’s shared fence list, with update-side lock held
Parameters
struct dma_resv *obj- the reservation object
Description
Returns the shared fence list. Does NOT take references to the fence. The obj->lock must be held.
Parameters
struct dma_resv *obj- the reservation object
struct ww_acquire_ctx *ctx- the locking context
Description
Locks the reservation object for exclusive access and modification. Note, that the lock is only against other writers, readers will run concurrently with a writer under RCU. The seqlock is used to notify readers if they overlap with a writer.
As the reservation object may be locked by multiple parties in an undefined order, a #ww_acquire_ctx is passed to unwind if a cycle is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation object may be locked by itself by passing NULL as ctx.
-
int
dma_resv_lock_interruptible(struct dma_resv *obj, struct ww_acquire_ctx *ctx)¶ lock the reservation object
Parameters
struct dma_resv *obj- the reservation object
struct ww_acquire_ctx *ctx- the locking context
Description
Locks the reservation object interruptible for exclusive access and modification. Note, that the lock is only against other writers, readers will run concurrently with a writer under RCU. The seqlock is used to notify readers if they overlap with a writer.
As the reservation object may be locked by multiple parties in an undefined order, a #ww_acquire_ctx is passed to unwind if a cycle is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation object may be locked by itself by passing NULL as ctx.
-
void
dma_resv_lock_slow(struct dma_resv *obj, struct ww_acquire_ctx *ctx)¶ slowpath lock the reservation object
Parameters
struct dma_resv *obj- the reservation object
struct ww_acquire_ctx *ctx- the locking context
Description
Acquires the reservation object after a die case. This function
will sleep until the lock becomes available. See dma_resv_lock() as
well.
-
int
dma_resv_lock_slow_interruptible(struct dma_resv *obj, struct ww_acquire_ctx *ctx)¶ slowpath lock the reservation object, interruptible
Parameters
struct dma_resv *obj- the reservation object
struct ww_acquire_ctx *ctx- the locking context
Description
Acquires the reservation object interruptible after a die case. This function
will sleep until the lock becomes available. See
dma_resv_lock_interruptible() as well.
Parameters
struct dma_resv *obj- the reservation object
Description
Tries to lock the reservation object for exclusive access and modification. Note, that the lock is only against other writers, readers will run concurrently with a writer under RCU. The seqlock is used to notify readers if they overlap with a writer.
Also note that since no context is provided, no deadlock protection is possible.
Returns true if the lock was acquired, false otherwise.
Parameters
struct dma_resv *obj- the reservation object
Description
Returns true if the mutex is locked, false if unlocked.
-
struct ww_acquire_ctx *
dma_resv_locking_ctx(struct dma_resv *obj)¶ returns the context used to lock the object
Parameters
struct dma_resv *obj- the reservation object
Description
Returns the context used to lock a reservation object or NULL if no context was used or the object is not locked at all.
Parameters
struct dma_resv *obj- the reservation object
Description
Unlocks the reservation object following exclusive access.
-
struct dma_fence *
dma_resv_get_excl(struct dma_resv *obj)¶ get the reservation object’s exclusive fence, with update-side lock held
Parameters
struct dma_resv *obj- the reservation object
Description
Returns the exclusive fence (if any). Does NOT take a reference. Writers must hold obj->lock, readers may only hold a RCU read side lock.
RETURNS The exclusive fence or NULL
-
struct dma_fence *
dma_resv_get_excl_rcu(struct dma_resv *obj)¶ get the reservation object’s exclusive fence, without lock held.
Parameters
struct dma_resv *obj- the reservation object
Description
If there is an exclusive fence, this atomically increments it’s reference count and returns it.
RETURNS The exclusive fence or NULL if none
DMA Fences¶
DMA fences, represented by struct dma_fence, are the kernel internal
synchronization primitive for DMA operations like GPU rendering, video
encoding/decoding, or displaying buffers on a screen.
A fence is initialized using dma_fence_init() and completed using
dma_fence_signal(). Fences are associated with a context, allocated through
dma_fence_context_alloc(), and all fences on the same context are
fully ordered.
Since the purposes of fences is to facilitate cross-device and cross-application synchronization, there’s multiple ways to use one:
- Individual fences can be exposed as a
sync_file, accessed as a file descriptor from userspace, created by callingsync_file_create(). This is called explicit fencing, since userspace passes around explicit synchronization points. - Some subsystems also have their own explicit fencing primitives, like
drm_syncobj. Compared tosync_file, adrm_syncobjallows the underlying fence to be updated. - Then there’s also implicit fencing, where the synchronization points are
implicitly passed around as part of shared
dma_bufinstances. Such implicit fences are stored instruct dma_resvthrough thedma_buf.resvpointer.
DMA Fence Cross-Driver Contract¶
Since dma_fence provide a cross driver contract, all drivers must follow the
same rules:
- Fences must complete in a reasonable time. Fences which represent kernels and shaders submitted by userspace, which could run forever, must be backed up by timeout and gpu hang recovery code. Minimally that code must prevent further command submission and force complete all in-flight fences, e.g. when the driver or hardware do not support gpu reset, or if the gpu reset failed for some reason. Ideally the driver supports gpu recovery which only affects the offending userspace context, and no other userspace submissions.
- Drivers may have different ideas of what completion within a reasonable time means. Some hang recovery code uses a fixed timeout, others a mix between observing forward progress and increasingly strict timeouts. Drivers should not try to second guess timeout handling of fences from other drivers.
- To ensure there’s no deadlocks of
dma_fence_wait()against other locks drivers should annotate all code required to reachdma_fence_signal(), which completes the fences, withdma_fence_begin_signalling()anddma_fence_end_signalling(). - Drivers are allowed to call
dma_fence_wait()while holdingdma_resv_lock(). This means any code required for fence completion cannot acquire adma_resvlock. Note that this also pulls in the entire established locking hierarchy arounddma_resv_lock()anddma_resv_unlock(). - Drivers are allowed to call
dma_fence_wait()from theirshrinkercallbacks. This means any code required for fence completion cannot allocate memory with GFP_KERNEL. - Drivers are allowed to call
dma_fence_wait()from theirmmu_notifierrespectivelymmu_interval_notifiercallbacks. This means any code required for fence completeion cannot allocate memory with GFP_NOFS or GFP_NOIO. Only GFP_ATOMIC is permissible, which might fail.
Note that only GPU drivers have a reasonable excuse for both requiring
mmu_interval_notifier and shrinker callbacks at the same time as having to
track asynchronous compute work using dma_fence. No driver outside of
drivers/gpu should ever call dma_fence_wait() in such contexts.
DMA Fence Signalling Annotations¶
Proving correctness of all the kernel code around dma_fence through code
review and testing is tricky for a few reasons:
- It is a cross-driver contract, and therefore all drivers must follow the same rules for lock nesting order, calling contexts for various functions and anything else significant for in-kernel interfaces. But it is also impossible to test all drivers in a single machine, hence brute-force N vs. N testing of all combinations is impossible. Even just limiting to the possible combinations is infeasible.
- There is an enormous amount of driver code involved. For render drivers
there’s the tail of command submission, after fences are published,
scheduler code, interrupt and workers to process job completion,
and timeout, gpu reset and gpu hang recovery code. Plus for integration
with core mm with have
mmu_notifier, respectivelymmu_interval_notifier, andshrinker. For modesetting drivers there’s the commit tail functions between when fences for an atomic modeset are published, and when the corresponding vblank completes, including any interrupt processing and related workers. Auditing all that code, across all drivers, is not feasible. - Due to how many other subsystems are involved and the locking hierarchies
this pulls in there is extremely thin wiggle-room for driver-specific
differences.
dma_fenceinteracts with almost all of the core memory handling through page fault handlers viadma_resv,dma_resv_lock()anddma_resv_unlock(). On the other side it also interacts through all allocation sites throughmmu_notifierandshrinker.
Furthermore lockdep does not handle cross-release dependencies, which means
any deadlocks between dma_fence_wait() and dma_fence_signal() can’t be caught
at runtime with some quick testing. The simplest example is one thread
waiting on a dma_fence while holding a lock:
lock(A);
dma_fence_wait(B);
unlock(A);
while the other thread is stuck trying to acquire the same lock, which prevents it from signalling the fence the previous thread is stuck waiting on:
lock(A);
unlock(A);
dma_fence_signal(B);
By manually annotating all code relevant to signalling a dma_fence we can
teach lockdep about these dependencies, which also helps with the validation
headache since now lockdep can check all the rules for us:
cookie = dma_fence_begin_signalling();
lock(A);
unlock(A);
dma_fence_signal(B);
dma_fence_end_signalling(cookie);
For using dma_fence_begin_signalling() and dma_fence_end_signalling() to
annotate critical sections the following rules need to be observed:
- All code necessary to complete a
dma_fencemust be annotated, from the point where a fence is accessible to other threads, to the point wheredma_fence_signal()is called. Un-annotated code can contain deadlock issues, and due to the very strict rules and many corner cases it is infeasible to catch these just with review or normal stress testing. struct dma_resvdeserves a special note, since the readers are only protected by rcu. This means the signalling critical section starts as soon as the new fences are installed, even beforedma_resv_unlock()is called.- The only exception are fast paths and opportunistic signalling code, which
calls
dma_fence_signal()purely as an optimization, but is not required to guarantee completion of adma_fence. The usual example is a wait IOCTL which callsdma_fence_signal(), while the mandatory completion path goes through a hardware interrupt and possible job completion worker. - To aid composability of code, the annotations can be freely nested, as long
as the overall locking hierarchy is consistent. The annotations also work
both in interrupt and process context. Due to implementation details this
requires that callers pass an opaque cookie from
dma_fence_begin_signalling()todma_fence_end_signalling(). - Validation against the cross driver contract is implemented by priming
lockdep with the relevant hierarchy at boot-up. This means even just
testing with a single device is enough to validate a driver, at least as
far as deadlocks with
dma_fence_wait()againstdma_fence_signal()are concerned.
DMA Fences Functions Reference¶
Parameters
void- no arguments
Description
Return a stub fence which is already signaled.
-
u64
dma_fence_context_alloc(unsigned num)¶ allocate an array of fence contexts
Parameters
unsigned num- amount of contexts to allocate
Description
This function will return the first index of the number of fence contexts
allocated. The fence context is used for setting dma_fence.context to a
unique number by passing the context to dma_fence_init().
-
bool
dma_fence_begin_signalling(void)¶ begin a critical DMA fence signalling section
Parameters
void- no arguments
Description
Drivers should use this to annotate the beginning of any code section
required to eventually complete dma_fence by calling dma_fence_signal().
The end of these critical sections are annotated with
dma_fence_end_signalling().
Opaque cookie needed by the implementation, which needs to be passed to
dma_fence_end_signalling().
Return
-
void
dma_fence_end_signalling(bool cookie)¶ end a critical DMA fence signalling section
Parameters
bool cookie- opaque cookie from
dma_fence_begin_signalling()
Description
Closes a critical section annotation opened by dma_fence_begin_signalling().
Parameters
struct dma_fence *fence- the fence to signal
Description
Signal completion for software callbacks on a fence, this will unblock
dma_fence_wait() calls and run all the callbacks added with
dma_fence_add_callback(). Can be called multiple times, but since a fence
can only go from the unsignaled to the signaled state and not back, it will
only be effective the first time.
Unlike dma_fence_signal(), this function must be called with dma_fence.lock
held.
Returns 0 on success and a negative error value when fence has been signalled already.
Parameters
struct dma_fence *fence- the fence to signal
Description
Signal completion for software callbacks on a fence, this will unblock
dma_fence_wait() calls and run all the callbacks added with
dma_fence_add_callback(). Can be called multiple times, but since a fence
can only go from the unsignaled to the signaled state and not back, it will
only be effective the first time.
Returns 0 on success and a negative error value when fence has been signalled already.
-
signed long
dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)¶ sleep until the fence gets signaled or until timeout elapses
Parameters
struct dma_fence *fence- the fence to wait on
bool intr- if true, do an interruptible wait
signed long timeout- timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
Description
Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the remaining timeout in jiffies on success. Other error values may be returned on custom implementations.
Performs a synchronous wait on this fence. It is assumed the caller directly or indirectly (buf-mgr between reservation and committing) holds a reference to the fence, otherwise the fence might be freed before return, resulting in undefined behavior.
See also dma_fence_wait() and dma_fence_wait_any_timeout().
-
void
dma_fence_release(struct kref *kref)¶ default relese function for fences
Parameters
struct kref *krefdma_fence.recfount
Description
This is the default release functions for dma_fence. Drivers shouldn’t call
this directly, but instead call dma_fence_put().
Parameters
struct dma_fence *fence- fence to release
Description
This is the default implementation for dma_fence_ops.release. It calls
kfree_rcu() on fence.
Parameters
struct dma_fence *fence- the fence to enable
Description
This will request for sw signaling to be enabled, to make the fence
complete as soon as possible. This calls dma_fence_ops.enable_signaling
internally.
-
int
dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb, dma_fence_func_t func)¶ add a callback to be called when the fence is signaled
Parameters
struct dma_fence *fence- the fence to wait on
struct dma_fence_cb *cb- the callback to register
dma_fence_func_t func- the function to call
Description
cb will be initialized by dma_fence_add_callback(), no initialization
by the caller is required. Any number of callbacks can be registered
to a fence, but a callback can only be registered to one fence at a time.
Note that the callback can be called from an atomic context. If fence is already signaled, this function will return -ENOENT (and not call the callback).
Add a software callback to the fence. Same restrictions apply to
refcount as it does to dma_fence_wait(), however the caller doesn’t need to
keep a refcount to fence afterward dma_fence_add_callback() has returned:
when software access is enabled, the creator of the fence is required to keep
the fence alive until after it signals with dma_fence_signal(). The callback
itself can be called from irq context.
Returns 0 in case of success, -ENOENT if the fence is already signaled and -EINVAL in case of error.
Parameters
struct dma_fence *fence- the dma_fence to query
Description
This wraps dma_fence_get_status_locked() to return the error status
condition on a signaled fence. See dma_fence_get_status_locked() for more
details.
Returns 0 if the fence has not yet been signaled, 1 if the fence has been signaled without an error condition, or a negative error code if the fence has been completed in err.
-
bool
dma_fence_remove_callback(struct dma_fence *fence, struct dma_fence_cb *cb)¶ remove a callback from the signaling list
Parameters
struct dma_fence *fence- the fence to wait on
struct dma_fence_cb *cb- the callback to remove
Description
Remove a previously queued callback from the fence. This function returns true if the callback is successfully removed, or false if the fence has already been signaled.
WARNING: Cancelling a callback should only be done if you really know what you’re doing, since deadlocks and race conditions could occur all too easily. For this reason, it should only ever be done on hardware lockup recovery, with a reference held to the fence.
Behaviour is undefined if cb has not been added to fence using
dma_fence_add_callback() beforehand.
-
signed long
dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)¶ default sleep until the fence gets signaled or until timeout elapses
Parameters
struct dma_fence *fence- the fence to wait on
bool intr- if true, do an interruptible wait
signed long timeout- timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
Description
Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the remaining timeout in jiffies on success. If timeout is zero the value one is returned if the fence is already signaled for consistency with other functions taking a jiffies timeout.
-
signed long
dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count, bool intr, signed long timeout, uint32_t *idx)¶ sleep until any fence gets signaled or until timeout elapses
Parameters
struct dma_fence **fences- array of fences to wait on
uint32_t count- number of fences to wait on
bool intr- if true, do an interruptible wait
signed long timeout- timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
uint32_t *idx- used to store the first signaled fence index, meaningful only on positive return
Description
Returns -EINVAL on custom fence wait implementation, -ERESTARTSYS if interrupted, 0 if the wait timed out, or the remaining timeout in jiffies on success.
Synchronous waits for the first fence in the array to be signaled. The caller needs to hold a reference to all fences in the array, otherwise a fence might be freed before return, resulting in undefined behavior.
See also dma_fence_wait() and dma_fence_wait_timeout().
-
void
dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, spinlock_t *lock, u64 context, u64 seqno)¶ Initialize a custom fence.
Parameters
struct dma_fence *fence- the fence to initialize
const struct dma_fence_ops *ops- the dma_fence_ops for operations on this fence
spinlock_t *lock- the irqsafe spinlock to use for locking this fence
u64 context- the execution context this fence is run on
u64 seqno- a linear increasing sequence number for this context
Description
Initializes an allocated fence, the caller doesn’t have to keep its
refcount after committing with this fence, but it will need to hold a
refcount again if dma_fence_ops.enable_signaling gets called.
context and seqno are used for easy comparison between fences, allowing
to check which fence is later by simply using dma_fence_later().
-
struct
dma_fence¶ software synchronization primitive
Definition
struct dma_fence {
spinlock_t *lock;
const struct dma_fence_ops *ops;
union {
struct list_head cb_list;
ktime_t timestamp;
struct rcu_head rcu;
};
u64 context;
u64 seqno;
unsigned long flags;
struct kref refcount;
int error;
};
Members
lock- spin_lock_irqsave used for locking
ops- dma_fence_ops associated with this fence
{unnamed_union}- anonymous
cb_list- list of all callbacks to call
timestamp- Timestamp when the fence was signaled.
rcu- used for releasing fence with kfree_rcu
context- execution context this fence belongs to, returned by
dma_fence_context_alloc() seqno- the sequence number of this fence inside the execution context, can be compared to decide which fence would be signaled later.
flags- A mask of DMA_FENCE_FLAG_* defined below
refcount- refcount for this fence
error- Optional, only valid if < 0, must be set before calling dma_fence_signal, indicates that the fence has completed with an error.
Description
the flags member must be manipulated and read using the appropriate atomic ops (bit_*), so taking the spinlock will not be needed most of the time.
DMA_FENCE_FLAG_SIGNALED_BIT - fence is already signaled DMA_FENCE_FLAG_TIMESTAMP_BIT - timestamp recorded for fence signaling DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT - enable_signaling might have been called DMA_FENCE_FLAG_USER_BITS - start of the unused bits, can be used by the implementer of the fence for its own purposes. Can be used in different ways by different fence implementers, so do not rely on this.
Since atomic bitops are used, this is not guaranteed to be the case. Particularly, if the bit was set, but dma_fence_signal was called right before this bit was set, it would have been able to set the DMA_FENCE_FLAG_SIGNALED_BIT, before enable_signaling was called. Adding a check for DMA_FENCE_FLAG_SIGNALED_BIT after setting DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT closes this race, and makes sure that after dma_fence_signal was called, any enable_signaling call will have either been completed, or never called at all.
-
struct
dma_fence_cb¶ callback for
dma_fence_add_callback()
Definition
struct dma_fence_cb {
struct list_head node;
dma_fence_func_t func;
};
Members
node- used by
dma_fence_add_callback()to append this struct to fence::cb_list func- dma_fence_func_t to call
Description
This struct will be initialized by dma_fence_add_callback(), additional
data can be passed along by embedding dma_fence_cb in another struct.
-
struct
dma_fence_ops¶ operations implemented for fence
Definition
struct dma_fence_ops {
bool use_64bit_seqno;
const char * (*get_driver_name)(struct dma_fence *fence);
const char * (*get_timeline_name)(struct dma_fence *fence);
bool (*enable_signaling)(struct dma_fence *fence);
bool (*signaled)(struct dma_fence *fence);
signed long (*wait)(struct dma_fence *fence, bool intr, signed long timeout);
void (*release)(struct dma_fence *fence);
void (*fence_value_str)(struct dma_fence *fence, char *str, int size);
void (*timeline_value_str)(struct dma_fence *fence, char *str, int size);
};
Members
use_64bit_seqno- True if this dma_fence implementation uses 64bit seqno, false otherwise.
get_driver_nameReturns the driver name. This is a callback to allow drivers to compute the name at runtime, without having it to store permanently for each fence, or build a cache of some sort.
This callback is mandatory.
get_timeline_nameReturn the name of the context this fence belongs to. This is a callback to allow drivers to compute the name at runtime, without having it to store permanently for each fence, or build a cache of some sort.
This callback is mandatory.
enable_signalingEnable software signaling of fence.
For fence implementations that have the capability for hw->hw signaling, they can implement this op to enable the necessary interrupts, or insert commands into cmdstream, etc, to avoid these costly operations for the common case where only hw->hw synchronization is required. This is called in the first
dma_fence_wait()ordma_fence_add_callback()path to let the fence implementation know that there is another driver waiting on the signal (ie. hw->sw case).This function can be called from atomic context, but not from irq context, so normal spinlocks can be used.
A return value of false indicates the fence already passed, or some failure occurred that made it impossible to enable signaling. True indicates successful enabling.
dma_fence.errormay be set in enable_signaling, but only when false is returned.Since many implementations can call
dma_fence_signal()even when before enable_signaling has been called there’s a race window, where thedma_fence_signal()might result in the final fence reference being released and its memory freed. To avoid this, implementations of this callback should grab their own reference usingdma_fence_get(), to be released when the fence is signalled (through e.g. the interrupt handler).This callback is optional. If this callback is not present, then the driver must always have signaling enabled.
signaledPeek whether the fence is signaled, as a fastpath optimization for e.g.
dma_fence_wait()ordma_fence_add_callback(). Note that this callback does not need to make any guarantees beyond that a fence once indicates as signalled must always return true from this callback. This callback may return false even if the fence has completed already, in this case information hasn’t propogated throug the system yet. See alsodma_fence_is_signaled().May set
dma_fence.errorif returning true.This callback is optional.
waitCustom wait implementation, defaults to
dma_fence_default_wait()if not set.The dma_fence_default_wait implementation should work for any fence, as long as enable_signaling works correctly. This hook allows drivers to have an optimized version for the case where a process context is already available, e.g. if enable_signaling for the general case needs to set up a worker thread.
Must return -ERESTARTSYS if the wait is intr = true and the wait was interrupted, and remaining jiffies if fence has signaled, or 0 if wait timed out. Can also return other error values on custom implementations, which should be treated as if the fence is signaled. For example a hardware lockup could be reported like that.
This callback is optional.
release- Called on destruction of fence to release additional resources.
Can be called from irq context. This callback is optional. If it is
NULL, then
dma_fence_free()is instead called as the default implementation. fence_value_strCallback to fill in free-form debug info specific to this fence, like the sequence number.
This callback is optional.
timeline_value_str- Fills in the current value of the timeline as a string, like the sequence number. Note that the specific fence passed to this function should not matter, drivers should only use it to look up the corresponding timeline structures.
Parameters
struct dma_fence *fence- fence to reduce refcount of
Parameters
struct dma_fence *fence- fence to increase refcount of
Description
Returns the same fence, with refcount increased by 1.
-
struct dma_fence *
dma_fence_get_rcu(struct dma_fence *fence)¶ get a fence from a dma_resv_list with rcu read lock
Parameters
struct dma_fence *fence- fence to increase refcount of
Description
Function returns NULL if no refcount could be obtained, or the fence.
-
struct dma_fence *
dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)¶ acquire a reference to an RCU tracked fence
Parameters
struct dma_fence __rcu **fencep- pointer to fence to increase refcount of
Description
Function returns NULL if no refcount could be obtained, or the fence. This function handles acquiring a reference to a fence that may be reallocated within the RCU grace period (such as with SLAB_TYPESAFE_BY_RCU), so long as the caller is using RCU on the pointer to the fence.
An alternative mechanism is to employ a seqlock to protect a bunch of
fences, such as used by struct dma_resv. When using a seqlock,
the seqlock must be taken before and checked after a reference to the
fence is acquired (as shown here).
The caller is required to hold the RCU read lock.
-
bool
dma_fence_is_signaled_locked(struct dma_fence *fence)¶ Return an indication if the fence is signaled yet.
Parameters
struct dma_fence *fence- the fence to check
Description
Returns true if the fence was already signaled, false if not. Since this
function doesn’t enable signaling, it is not guaranteed to ever return
true if dma_fence_add_callback(), dma_fence_wait() or
dma_fence_enable_sw_signaling() haven’t been called before.
This function requires dma_fence.lock to be held.
See also dma_fence_is_signaled().
-
bool
dma_fence_is_signaled(struct dma_fence *fence)¶ Return an indication if the fence is signaled yet.
Parameters
struct dma_fence *fence- the fence to check
Description
Returns true if the fence was already signaled, false if not. Since this
function doesn’t enable signaling, it is not guaranteed to ever return
true if dma_fence_add_callback(), dma_fence_wait() or
dma_fence_enable_sw_signaling() haven’t been called before.
It’s recommended for seqno fences to call dma_fence_signal when the operation is complete, it makes it possible to prevent issues from wraparound between time of issue and time of use by checking the return value of this function before calling hardware-specific wait instructions.
See also dma_fence_is_signaled_locked().
-
bool
__dma_fence_is_later(u64 f1, u64 f2, const struct dma_fence_ops *ops)¶ return if f1 is chronologically later than f2
Parameters
u64 f1- the first fence’s seqno
u64 f2- the second fence’s seqno from the same context
const struct dma_fence_ops *ops- dma_fence_ops associated with the seqno
Description
Returns true if f1 is chronologically later than f2. Both fences must be from the same context, since a seqno is not common across contexts.
-
bool
dma_fence_is_later(struct dma_fence *f1, struct dma_fence *f2)¶ return if f1 is chronologically later than f2
Parameters
struct dma_fence *f1- the first fence from the same context
struct dma_fence *f2- the second fence from the same context
Description
Returns true if f1 is chronologically later than f2. Both fences must be from the same context, since a seqno is not re-used across contexts.
-
struct dma_fence *
dma_fence_later(struct dma_fence *f1, struct dma_fence *f2)¶ return the chronologically later fence
Parameters
struct dma_fence *f1- the first fence from the same context
struct dma_fence *f2- the second fence from the same context
Description
Returns NULL if both fences are signaled, otherwise the fence that would be signaled last. Both fences must be from the same context, since a seqno is not re-used across contexts.
Parameters
struct dma_fence *fence- the dma_fence to query
Description
Drivers can supply an optional error status condition before they signal
the fence (to indicate whether the fence was completed due to an error
rather than success). The value of the status condition is only valid
if the fence has been signaled, dma_fence_get_status_locked() first checks
the signal state before reporting the error status.
Returns 0 if the fence has not yet been signaled, 1 if the fence has been signaled without an error condition, or a negative error code if the fence has been completed in err.
Parameters
struct dma_fence *fence- the dma_fence
int error- the error to store
Description
Drivers can supply an optional error status condition before they signal the fence, to indicate that the fence was completed due to an error rather than success. This must be set before signaling (so that the value is visible before any waiters on the signal callback are woken). This helper exists to help catching erroneous setting of #dma_fence.error.
Parameters
struct dma_fence *fence- the fence to wait on
bool intr- if true, do an interruptible wait
Description
This function will return -ERESTARTSYS if interrupted by a signal, or 0 if the fence was signaled. Other error values may be returned on custom implementations.
Performs a synchronous wait on this fence. It is assumed the caller directly or indirectly holds a reference to the fence, otherwise the fence might be freed before return, resulting in undefined behavior.
See also dma_fence_wait_timeout() and dma_fence_wait_any_timeout().
Seqno Hardware Fences¶
Parameters
struct dma_fence *fence- fence to cast to a seqno_fence
Description
Returns NULL if the fence is not a seqno_fence, or the seqno_fence otherwise.
-
void
seqno_fence_init(struct seqno_fence *fence, spinlock_t *lock, struct dma_buf *sync_buf, uint32_t context, uint32_t seqno_ofs, uint32_t seqno, enum seqno_fence_condition cond, const struct dma_fence_ops *ops)¶ initialize a seqno fence
Parameters
struct seqno_fence *fence- seqno_fence to initialize
spinlock_t *lock- pointer to spinlock to use for fence
struct dma_buf *sync_buf- buffer containing the memory location to signal on
uint32_t context- the execution context this fence is a part of
uint32_t seqno_ofs- the offset within sync_buf
uint32_t seqno- the sequence # to signal on
enum seqno_fence_condition cond- fence wait condition
const struct dma_fence_ops *ops- the fence_ops for operations on this seqno fence
Description
This function initializes a struct seqno_fence with passed parameters, and takes a reference on sync_buf which is released on fence destruction.
A seqno_fence is a dma_fence which can complete in software when enable_signaling is called, but it also completes when (s32)((sync_buf)[seqno_ofs] - seqno) >= 0 is true
The seqno_fence will take a refcount on the sync_buf until it’s destroyed, but actual lifetime of sync_buf may be longer if one of the callers take a reference to it.
Certain hardware have instructions to insert this type of wait condition in the command stream, so no intervention from software would be needed. This type of fence can be destroyed before completed, however a reference on the sync_buf dma-buf can be taken. It is encouraged to re-use the same dma-buf for sync_buf, since mapping or unmapping the sync_buf to the device’s vm can be expensive.
It is recommended for creators of seqno_fence to call dma_fence_signal()
before destruction. This will prevent possible issues from wraparound at
time of issue vs time of check, since users can check dma_fence_is_signaled()
before submitting instructions for the hardware to wait on the fence.
However, when ops.enable_signaling is not called, it doesn’t have to be
done as soon as possible, just before there’s any real danger of seqno
wraparound.
DMA Fence Array¶
-
struct dma_fence_array *
dma_fence_array_create(int num_fences, struct dma_fence **fences, u64 context, unsigned seqno, bool signal_on_any)¶ Create a custom fence array
Parameters
int num_fences- [in] number of fences to add in the array
struct dma_fence **fences- [in] array containing the fences
u64 context- [in] fence context to use
unsigned seqno- [in] sequence number to use
bool signal_on_any- [in] signal on any fence in the array
Description
Allocate a dma_fence_array object and initialize the base fence with
dma_fence_init().
In case of error it returns NULL.
The caller should allocate the fences array with num_fences size
and fill it with the fences it wants to add to the object. Ownership of this
array is taken and dma_fence_put() is used on each fence on release.
If signal_on_any is true the fence array signals if any fence in the array signals, otherwise it signals when all fences in the array signal.
-
bool
dma_fence_match_context(struct dma_fence *fence, u64 context)¶ Check if all fences are from the given context
Parameters
struct dma_fence *fence- [in] fence or fence array
u64 context- [in] fence context to check all fences against
Description
Checks the provided fence or, for a fence array, all fences in the array against the given context. Returns false if any fence is from a different context.
-
struct
dma_fence_array_cb¶ callback helper for fence array
Definition
struct dma_fence_array_cb {
struct dma_fence_cb cb;
struct dma_fence_array *array;
};
Members
cb- fence callback structure for signaling
array- reference to the parent fence array object
-
struct
dma_fence_array¶ fence to represent an array of fences
Definition
struct dma_fence_array {
struct dma_fence base;
spinlock_t lock;
unsigned num_fences;
atomic_t num_pending;
struct dma_fence **fences;
struct irq_work work;
};
Members
base- fence base class
lock- spinlock for fence handling
num_fences- number of fences in the array
num_pending- fences in the array still pending
fences- array of the fences
work- internal irq_work function
Parameters
struct dma_fence *fence- fence to test
Description
Return true if it is a dma_fence_array and false otherwise.
-
struct dma_fence_array *
to_dma_fence_array(struct dma_fence *fence)¶ cast a fence to a dma_fence_array
Parameters
struct dma_fence *fence- fence to cast to a dma_fence_array
Description
Returns NULL if the fence is not a dma_fence_array, or the dma_fence_array otherwise.
DMA Fence uABI/Sync File¶
Parameters
struct dma_fence *fence- fence to add to the sync_fence
Description
Creates a sync_file containg fence. This function acquires and additional
reference of fence for the newly-created sync_file, if it succeeds. The
sync_file can be released with fput(sync_file->file). Returns the
sync_file or NULL in case of error.
Parameters
int fd- sync_file fd to get the fence from
Description
Ensures fd references a valid sync_file and returns a fence that represents all fence in the sync_file. On error NULL is returned.
-
struct
sync_file¶ sync file to export to the userspace
Definition
struct sync_file {
struct file *file;
char user_name[32];
#ifdef CONFIG_DEBUG_FS;
struct list_head sync_file_list;
#endif;
wait_queue_head_t wq;
unsigned long flags;
struct dma_fence *fence;
struct dma_fence_cb cb;
};
Members
file- file representing this fence
user_name- Name of the sync file provided by userspace, for merged fences. Otherwise generated through driver callbacks (in which case the entire array is 0).
sync_file_list- membership in global file list
wq- wait queue for fence signaling
flags- flags for the sync_file
fence- fence with the fences in the sync_file
cb- fence callback information
Description
flags: POLL_ENABLED: whether userspace is currently poll()’ing or not
Indefinite DMA Fences¶
At various times struct dma_fence with an indefinite time until dma_fence_wait()
finishes have been proposed. Examples include:
- Future fences, used in HWC1 to signal when a buffer isn’t used by the display any longer, and created with the screen update that makes the buffer visible. The time this fence completes is entirely under userspace’s control.
- Proxy fences, proposed to handle &drm_syncobj for which the fence has not yet been set. Used to asynchronously delay command submission.
- Userspace fences or gpu futexes, fine-grained locking within a command buffer that userspace uses for synchronization across engines or with the CPU, which are then imported as a DMA fence for integration into existing winsys protocols.
- Long-running compute command buffers, while still using traditional end of batch DMA fences for memory management instead of context preemption DMA fences which get reattached when the compute job is rescheduled.
Common to all these schemes is that userspace controls the dependencies of these fences and controls when they fire. Mixing indefinite fences with normal in-kernel DMA fences does not work, even when a fallback timeout is included to protect against malicious userspace:
- Only the kernel knows about all DMA fence dependencies, userspace is not aware of dependencies injected due to memory management or scheduler decisions.
- Only userspace knows about all dependencies in indefinite fences and when exactly they will complete, the kernel has no visibility.
Furthermore the kernel has to be able to hold up userspace command submission for memory management needs, which means we must support indefinite fences being dependent upon DMA fences. If the kernel also support indefinite fences in the kernel like a DMA fence, like any of the above proposal would, there is the potential for deadlocks.
Indefinite Fencing Dependency Cycle
This means that the kernel might accidentally create deadlocks through memory management dependencies which userspace is unaware of, which randomly hangs workloads until the timeout kicks in. Workloads, which from userspace’s perspective, do not contain a deadlock. In such a mixed fencing architecture there is no single entity with knowledge of all dependencies. Thefore preventing such deadlocks from within the kernel is not possible.
The only solution to avoid dependencies loops is by not allowing indefinite fences in the kernel. This means:
- No future fences, proxy fences or userspace fences imported as DMA fences, with or without a timeout.
- No DMA fences that signal end of batchbuffer for command submission where userspace is allowed to use userspace fencing or long running compute workloads. This also means no implicit fencing for shared buffers in these cases.