On Wednesday 17 March 2010 15:29:48 Pawel Osciak wrote:
> Hello,
>
> during the V4L2 brainstorm meeting in Norway we have concluded that streaming
> error handling in dqbuf is lacking a bit and might result in the application
> losing video buffers.
>
> V4L2 specification states that DQBUF should set errno to EIO in such cases:
>
>
> "EIO
>
> VIDIOC_DQBUF failed due to an internal error. Can also indicate temporary
> problems like signal loss. Note the driver might dequeue an (empty) buffer
> despite returning an error, or even stop capturing."
>
> There is a problem with this though. v4l2-ioctl.c code does not copy back
> v4l2_buffer fields to userspace on a failed ioctl invocation, i.e. when
> __video_do_ioctl() does not return 0, it jumps over the copy_to_user()
> code:
>
> /* ... */
> err = __video_do_ioctl(file, cmd, parg);
> /* ... */
> if (err < 0)
> goto out;
> /* ... */
> if (copy_to_user((void __user *)arg, parg, _IOC_SIZE(cmd)))
> err = -EFAULT;
> /* ... */
> out:
>
>
> This is fine in general, but in the case of DQBUF errors, the v4l2_buffer
> fields are not copied back. Because of that, the application does not have any
> means of discovering on which buffer the operation failed. So it cannot reuse
> that buffer, even despite the fact that the spec allows such behavior.
>
>
> This RFC proposes a modification to the DQBUF behavior in cases of internal
> (recoverable) errors to allow recovery from such situations.
>
> We propose a new flag for the v4l2_buffer "flags" field,
> "V4L2_BUF_FLAG_ERROR".
> There already exists a "V4L2_BUF_FLAG_DONE" flag, so to support older
> applications, the new flag should always be set together with it.
>
> Applications unaware of the new flag would simply display a corrupted frame,
> but
> we believe it is still a better solution than failing altogether. Old EIO
> behavior remains so the change is backwards compatible.
>
> I will post relevant V4L2 documentation updates after (if) this change is
> accepted.
>
>
> This series is rebased onto my recent videobuf clean-up and poll behavior
> patches.
>
> The series contains:
> [PATCH 1/2] v4l: Add a new ERROR flag for DQBUF after recoverable streaming
> errors
> [PATCH 2/2] v4l: videobuf: Add support for V4L2_BUF_FLAG_ERROR
Reviewed-by: Hans Verkuil <hverk...@xs4all.nl>
I think this is a very sensible change. After all, DQBUF succeeds, even though
the
buffer itself contains errors. But that is not the fault of DQBUF. It is enough
to
flag that the buffer does have an error. Without this you actually loose the
buffer completely from the point of view of the application. And that's really
nasty.
Regards,
Hans
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG
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