On Tuesday 16 November 2010 22:56:45 Hans Verkuil wrote:
> The BKL replacement mutex had some serious performance side-effects on
> V4L drivers. It is replaced by a better heuristic that works around the
> worst of the side-effects.
>
> Read the v4l2-dev.c comments for the whole sorry story. This is a
> temporary measure only until we can convert all v4l drivers to use
> unlocked_ioctl.
>
> Signed-off-by: Hans Verkuil <hverkuil@xxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> drivers/media/video/v4l2-dev.c | 37 ++++++++++++++++++++++++++++++++++---
> drivers/media/video/v4l2-device.c | 1 +
> include/media/v4l2-dev.h | 2 +-
> include/media/v4l2-device.h | 2 ++
> 4 files changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-dev.c
> index 8eb0756..59ef642 100644
> --- a/drivers/media/video/v4l2-dev.c
> +++ b/drivers/media/video/v4l2-dev.c
> @@ -258,11 +258,42 @@ static long v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> if (vdev->lock)
> mutex_unlock(vdev->lock);
> } else if (vdev->fops->ioctl) {
> - /* TODO: convert all drivers to unlocked_ioctl */
> - lock_kernel();
> + /* This code path is a replacement for the BKL. It is a major
> + * hack but it will have to do for those drivers that are not
> + * yet converted to use unlocked_ioctl.
> + *
> + * There are two options: if the driver implements struct
> + * v4l2_device, then the lock defined there is used to
> + * serialize the ioctls. Otherwise the v4l2 core lock defined
> + * below is used. This lock is really bad since it serializes
> + * completely independent devices.
> + *
> + * Both variants suffer from the same problem: if the driver
> + * sleeps, then it blocks all ioctls since the lock is still
> + * held. This is very common for VIDIOC_DQBUF since that
> + * normally waits for a frame to arrive. As a result any other
> + * ioctl calls will proceed very, very slowly since each call
> + * will have to wait for the VIDIOC_QBUF to finish. Things that
> + * should take 0.01s may now take 10-20 seconds.
> + *
> + * The workaround is to *not* take the lock for VIDIOC_DQBUF.
> + * This actually works OK for videobuf-based drivers, since
> + * videobuf will take its own internal lock.
> + */
> + static DEFINE_MUTEX(v4l2_ioctl_mutex);
> + struct mutex *m = vdev->v4l2_dev ?
> + &vdev->v4l2_dev->ioctl_lock : &v4l2_ioctl_mutex;
> +
> + if (cmd != VIDIOC_DQBUF) {
> + int res = mutex_lock_interruptible(m);
> +
> + if (res)
> + return res;
> + }
> if (video_is_registered(vdev))
> ret = vdev->fops->ioctl(filp, cmd, arg);
> - unlock_kernel();
> + if (cmd != VIDIOC_DQBUF)
> + mutex_unlock(m);
> } else
> ret = -ENOTTY;
>
> diff --git a/drivers/media/video/v4l2-device.c b/drivers/media/video/v4l2-device.c
> index 0b08f96..7fe6f92 100644
> --- a/drivers/media/video/v4l2-device.c
> +++ b/drivers/media/video/v4l2-device.c
> @@ -35,6 +35,7 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
>
> INIT_LIST_HEAD(&v4l2_dev->subdevs);
> spin_lock_init(&v4l2_dev->lock);
> + mutex_init(&v4l2_dev->ioctl_lock);
> v4l2_dev->dev = dev;
> if (dev == NULL) {
> /* If dev == NULL, then name must be filled in by the caller */
> diff --git a/include/media/v4l2-dev.h b/include/media/v4l2-dev.h
> index 15802a0..59dec5a 100644
> --- a/include/media/v4l2-dev.h
> +++ b/include/media/v4l2-dev.h
> @@ -39,7 +39,7 @@ struct v4l2_file_operations {
> ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
> ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
> unsigned int (*poll) (struct file *, struct poll_table_struct *);
> - long (*ioctl) (struct file *, unsigned int, unsigned long);
> + long (*ioctl __deprecated) (struct file *, unsigned int, unsigned long);
> long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
> int (*mmap) (struct file *, struct vm_area_struct *);
> int (*open) (struct file *);
> diff --git a/include/media/v4l2-device.h b/include/media/v4l2-device.h
> index 6648036..b16f307 100644
> --- a/include/media/v4l2-device.h
> +++ b/include/media/v4l2-device.h
> @@ -51,6 +51,8 @@ struct v4l2_device {
> unsigned int notification, void *arg);
> /* The control handler. May be NULL. */
> struct v4l2_ctrl_handler *ctrl_handler;
> + /* BKL replacement mutex. Temporary solution only. */
> + struct mutex ioctl_lock;
> };
>
> /* Initialize v4l2_dev and make dev->driver_data point to v4l2_dev.
>
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