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authorHans Verkuil <hverkuil@xs4all.nl>2010-09-26 08:47:38 -0300
committerMauro Carvalho Chehab <mchehab@redhat.com>2010-10-21 01:06:14 -0200
commitee6869afc922a9849979e49bb3bbcad794872fcb (patch)
tree2266050d01da694d04b533a6509873888327108b /Documentation/video4linux
parentc29fcff3daafbf46d64a543c1950bbd206ad8c1c (diff)
downloadblackbird-op-linux-ee6869afc922a9849979e49bb3bbcad794872fcb.tar.gz
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V4L/DVB: v4l2: add core serialization lock
Drivers can optionally set a pointer to a mutex in struct video_device. The core will use that to lock before calling open, read, write, unlocked_ioctl, poll, mmap or release. Updated the documentation as well and ensure that v4l2-event knows about the lock: it will unlock it before doing a blocking wait on an event and relock it afterwards. Ensure that the 'video_is_registered' check is done when the lock is held: a typical disconnect will take the lock as well before unregistering the device nodes, so to prevent race conditions the video_is_registered check should also be done with the lock held. Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Diffstat (limited to 'Documentation/video4linux')
-rw-r--r--Documentation/video4linux/v4l2-framework.txt20
1 files changed, 20 insertions, 0 deletions
diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt
index 9b1d81c26b7d..a128e012a45c 100644
--- a/Documentation/video4linux/v4l2-framework.txt
+++ b/Documentation/video4linux/v4l2-framework.txt
@@ -453,6 +453,10 @@ You should also set these fields:
- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
(highly recommended to use this and it might become compulsory in the
future!), then set this to your v4l2_ioctl_ops struct.
+- lock: leave to NULL if you want to do all the locking in the driver.
+ Otherwise you give it a pointer to a struct mutex_lock and before any
+ of the v4l2_file_operations is called this lock will be taken by the
+ core and released afterwards.
- parent: you only set this if v4l2_device was registered with NULL as
the parent device struct. This only happens in cases where one hardware
device has multiple PCI devices that all share the same v4l2_device core.
@@ -469,6 +473,22 @@ If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or
The v4l2_file_operations struct is a subset of file_operations. The main
difference is that the inode argument is omitted since it is never used.
+v4l2_file_operations and locking
+--------------------------------
+
+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.
+
+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
+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.
+
+The implementation of a hotplug disconnect should also take the lock before
+calling v4l2_device_disconnect and video_unregister_device.
video_device registration
-------------------------
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