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V4L/DVB: V4L: File handles: Add documentation

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Add documentation on using V4L2 file handles (v4l2_fh) in V4L2 drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Sakari Ailus 2010-03-20 18:28:48 -03:00 committed by Mauro Carvalho Chehab
parent 1babcb460f
commit 6cd84b78ed
1 changed files with 72 additions and 0 deletions
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72
Documentation/video4linux/v4l2-framework.txt
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@ -608,3 +608,75 @@ scatter/gather method (videobuf-dma-sg), DMA with linear access
Please see Documentation/video4linux/videobuf for more information on how
to use the videobuf layer.
struct v4l2_fh
--------------
struct v4l2_fh provides a way to easily keep file handle specific data
that is used by the V4L2 framework. Using v4l2_fh is optional for
drivers.
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.
Useful functions:
- v4l2_fh_init()
Initialise the file handle. This *MUST* be performed in the driver's
v4l2_file_operations->open() handler.
- v4l2_fh_add()
Add a v4l2_fh to video_device file handle list. May be called after
initialising the file handle.
- v4l2_fh_del()
Unassociate the file handle from video_device(). The file handle
exit function may now be called.
- v4l2_fh_exit()
Uninitialise the file handle. After uninitialisation the v4l2_fh
memory can be freed.
struct v4l2_fh is allocated as a part of the driver's own file handle
structure and is set to file->private_data in the driver's open
function by the driver. 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;
...
ret = v4l2_fh_init(&my_fh->fh, vfd);
if (ret)
return ret;
v4l2_fh_add(&my_fh->fh);
file->private_data = &my_fh->fh;
...
}
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);
...
}