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[media] v4l2 framework doc: clarify locking

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high-latency devices.

Thanks to Hans de Goede for our discussions on this topic.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Thanks-to: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Hans Verkuil 2011-11-07 12:44:28 -03:00 committed by Mauro Carvalho Chehab
parent 0fb6ec6b1c
commit 43599f31ab
1 changed files with 11 additions and 0 deletions
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11
Documentation/video4linux/v4l2-framework.txt
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@ -612,6 +612,12 @@ You can set a pointer to a mutex_lock in struct video_device. Usually this
will be either a top-level mutex or a mutex per device node. If you want
finer-grained locking then you have to set it to NULL and do you own locking.
It is up to the driver developer to decide which method to use. However, if
your driver has high-latency operations (for example, changing the exposure
of a USB webcam might take a long time), then you might be better off with
doing your own locking if you want to allow the user to do other things with
the device while waiting for the high-latency command to finish.
If a lock is specified then all file operations will be serialized on that
lock. If you use videobuf then you must pass the same lock to the videobuf
queue initialize function: if videobuf has to wait for a frame to arrive, then
@ -619,6 +625,11 @@ it will temporarily unlock the lock and relock it afterwards. If your driver
also waits in the code, then you should do the same to allow other processes
to access the device node while the first process is waiting for something.
In the case of videobuf2 you will need to implement the wait_prepare and
wait_finish callbacks to unlock/lock if applicable. In particular, if you use
the lock in struct video_device then you must unlock/lock this mutex in
wait_prepare and wait_finish.
The implementation of a hotplug disconnect should also take the lock before
calling v4l2_device_disconnect.