1.6. V4L2 File handlers¶
struct v4l2_fh provides a way to easily keep file handle specific
data that is used by the V4L2 framework.
Attention
New drivers must use struct v4l2_fh
since it is also used to implement priority handling
(ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY).
The users of v4l2_fh (in the V4L2 framework, not the driver) know
whether a driver uses v4l2_fh as its file->private_data pointer
by testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags.
This bit is set whenever v4l2_fh_init() is called.
struct v4l2_fh is allocated as a part of the driver’s own file handle
structure and file->private_data is set to it in the driver’s open()
function by the driver.
In many cases the struct v4l2_fh will be embedded in a larger
structure. In that case you should call:
v4l2_fh_init()andv4l2_fh_add()inopen()v4l2_fh_del()andv4l2_fh_exit()inrelease()
Drivers can extract their own file handle structure by using the container_of macro.
Example:
struct my_fh {
int blah;
struct v4l2_fh fh;
};
...
int my_open(struct file *file)
{
struct my_fh *my_fh;
struct video_device *vfd;
int ret;
...
my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
...
v4l2_fh_init(&my_fh->fh, vfd);
...
file->private_data = &my_fh->fh;
v4l2_fh_add(&my_fh->fh);
return 0;
}
int my_release(struct file *file)
{
struct v4l2_fh *fh = file->private_data;
struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
...
v4l2_fh_del(&my_fh->fh);
v4l2_fh_exit(&my_fh->fh);
kfree(my_fh);
return 0;
}
Below is a short description of the v4l2_fh functions used:
v4l2_fh_init
(fh, vdev)
- Initialise the file handle. This MUST be performed in the driver’s
v4l2_file_operations->open() handler.
- Add a
v4l2_fhtovideo_devicefile handle list. Must be called once the file handle is completely initialized.
- Unassociate the file handle from
video_device. The file handle exit function may now be called.
- Uninitialise the file handle. After uninitialisation the
v4l2_fhmemory can be freed.
If struct v4l2_fh is not embedded, then you can use these helper functions:
v4l2_fh_open
(struct file *filp)
- This allocates a
struct v4l2_fh, initializes it and adds it to thestruct video_deviceassociated with the file struct.
v4l2_fh_release
(struct file *filp)
- This deletes it from the
struct video_deviceassociated with the file struct, uninitialised thev4l2_fhand frees it.
These two functions can be plugged into the v4l2_file_operation’s open()
and release() ops.
Several drivers need to do something when the first file handle is opened and
when the last file handle closes. Two helper functions were added to check
whether the v4l2_fh struct is the only open filehandle of the
associated device node:
- Returns 1 if the file handle is the only open file handle, else 0.
v4l2_fh_is_singular_file
(struct file *filp)
- Same, but it calls v4l2_fh_is_singular with filp->private_data.
1.6.1. V4L2 fh functions and data structures¶
-
struct
v4l2_fh¶ Describes a V4L2 file handler
Definition
struct v4l2_fh {
struct list_head list;
struct video_device *vdev;
struct v4l2_ctrl_handler *ctrl_handler;
enum v4l2_priority prio;
wait_queue_head_t wait;
struct mutex subscribe_lock;
struct list_head subscribed;
struct list_head available;
unsigned int navailable;
u32 sequence;
struct v4l2_m2m_ctx *m2m_ctx;
};
Members
list- list of file handlers
vdev- pointer to
struct video_device ctrl_handler- pointer to
struct v4l2_ctrl_handler prio- priority of the file handler, as defined by
enum v4l2_priority wait- event’ s wait queue
subscribe_lock- serialise changes to the subscribed list; guarantee that the add and del event callbacks are orderly called
subscribed- list of subscribed events
available- list of events waiting to be dequeued
navailable- number of available events at available list
sequence- event sequence number
m2m_ctx- pointer to
struct v4l2_m2m_ctx
-
void
v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)¶ Initialise the file handle.
Parameters
struct v4l2_fh *fh- pointer to
struct v4l2_fh struct video_device *vdev- pointer to
struct video_device
Description
Parts of the V4L2 framework using the
file handles should be initialised in this function. Must be called
from driver’s v4l2_file_operations->open() handler if the driver
uses struct v4l2_fh.
Parameters
struct v4l2_fh *fh- pointer to
struct v4l2_fh
Description
Note
The fh file handle must be initialised first.
-
int
v4l2_fh_open(struct file *filp)¶ Ancillary routine that can be used as the open() op of v4l2_file_operations.
Parameters
struct file *filp- pointer to struct file
Description
It allocates a v4l2_fh and inits and adds it to the struct video_device
associated with the file pointer.
Parameters
struct v4l2_fh *fh- pointer to
struct v4l2_fh
Description
On error filp->private_data will be NULL, otherwise it will point to
the struct v4l2_fh.
Note
Must be called in v4l2_file_operations->release() handler if the driver
uses struct v4l2_fh.
Parameters
struct v4l2_fh *fh- pointer to
struct v4l2_fh
Description
Parts of the V4L2 framework using the v4l2_fh must release their resources here, too.
Note
Must be called in v4l2_file_operations->release() handler if the
driver uses struct v4l2_fh.
-
int
v4l2_fh_release(struct file *filp)¶ Ancillary routine that can be used as the release() op of v4l2_file_operations.
Parameters
struct file *filp- pointer to struct file
Description
It deletes and exits the v4l2_fh associated with the file pointer and
frees it. It will do nothing if filp->private_data (the pointer to the
v4l2_fh struct) is NULL.
This function always returns 0.
-
int
v4l2_fh_is_singular(struct v4l2_fh *fh)¶ Returns 1 if this filehandle is the only filehandle opened for the associated video_device.
Parameters
struct v4l2_fh *fh- pointer to
struct v4l2_fh
Description
If fh is NULL, then it returns 0.
-
int
v4l2_fh_is_singular_file(struct file *filp)¶ Returns 1 if this filehandle is the only filehandle opened for the associated video_device.
Parameters
struct file *filp- pointer to struct file
Description
This is a helper function variant of v4l2_fh_is_singular() with uses
struct file as argument.
If filp->private_data is NULL, then it will return 0.