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SubjectRe: [PATCH 1/6] v4l2-mem2mem: return CAPTURE buffer first
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Le mercredi 11 mars 2020 à 14:42 -0300, Ezequiel Garcia a écrit :
> When the request API is used, typically an OUTPUT (src) buffer
> will be part of a request. A userspace process will be typically
> blocked, waiting on the request file descriptor.
>
> Returning the OUTPUT (src) buffer will wake-up such processes,
> who will immediately attempt to dequeue the CAPTURE buffer,
> only to find it's still unavailable.
>
> Therefore, change v4l2_m2m_buf_done_and_job_finish returning
> the CAPTURE (dst) buffer first, to avoid signalling the request
> file descriptor prematurely, i.e. before the CAPTURE buffer is done.
>
> When the request API is not used, this change should have
> no impact.
>
> Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>

This was tested with upcoming GStreamer element with O_NONBLOCK flag passed when
opening the video node. Before this change, EAGAIN would from time to time be
returned when DQBUF(CAPTURE) was called.

gst-launch-1.0 filesrc location=somefile.mp4 ! parsebin ! v4l2slh264dec ! fakevideosink
https://gitlab.freedesktop.org/ndufresne/gst-plugins-bad/-/blob/v4l2codecs-hantro-v3/sys/v4l2codecs/gstv4l2decoder.c#L139

Tested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>

> ---
> drivers/media/v4l2-core/v4l2-mem2mem.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-
> core/v4l2-mem2mem.c
> index 8986c31176e9..62ac9424c92a 100644
> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> @@ -504,12 +504,21 @@ void v4l2_m2m_buf_done_and_job_finish(struct
> v4l2_m2m_dev *m2m_dev,
>
> if (WARN_ON(!src_buf || !dst_buf))
> goto unlock;
> - v4l2_m2m_buf_done(src_buf, state);
> dst_buf->is_held = src_buf->flags & V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
> if (!dst_buf->is_held) {
> v4l2_m2m_dst_buf_remove(m2m_ctx);
> v4l2_m2m_buf_done(dst_buf, state);
> }
> + /*
> + * If the request API is being used, returning the OUTPUT
> + * (src) buffer will wake-up any process waiting on the
> + * request file descriptor.
> + *
> + * Therefore, return the CAPTURE (dst) buffer first,
> + * to avoid signalling the request file descriptor
> + * before the CAPTURE buffer is done.
> + */
> + v4l2_m2m_buf_done(src_buf, state);
> schedule_next = _v4l2_m2m_job_finish(m2m_dev, m2m_ctx);
> unlock:
> spin_unlock_irqrestore(&m2m_dev->job_spinlock, flags);

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