linux-kernel - Re: [PATCH v3 10/15] [media] vb2: add 'ordered' property to queues

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Date:   Mon, 2 Oct 2017 14:43:50 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Gustavo Padovan <gustavo@...ovan.org>
Cc:     linux-media@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Jonathan.Chai@....com
Subject: Re: [PATCH v3 10/15] [media] vb2: add 'ordered' property to queues

Hi,

On Thu, Sep 07, 2017 at 03:42:21PM -0300, Gustavo Padovan wrote:
>From: Gustavo Padovan <gustavo.padovan@...labora.com>
>
>For explicit synchronization (and soon for HAL3/Request API) we need
>the v4l2-driver to guarantee the ordering in which the buffers were queued
>by userspace. This is already true for many drivers, but we never needed
>to say it.
>
>Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.com>
>---
> include/media/videobuf2-core.h | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
>index 5ed8d3402474..20099dc22f26 100644
>--- a/include/media/videobuf2-core.h
>+++ b/include/media/videobuf2-core.h
>@@ -508,6 +508,9 @@ struct vb2_buf_ops {
>  * @last_buffer_dequeued: used in poll() and DQBUF to immediately return if the
>  *		last decoded buffer was already dequeued. Set for capture queues
>  *		when a buffer with the V4L2_BUF_FLAG_LAST is dequeued.
>+ * @ordered: if the driver can guarantee that the queue will be ordered or not.
>+ *		The default is not ordered unless the driver sets this flag. It
>+ *		is mandatory for using explicit fences.

If it's mandatory for fences (why is that?), then I guess this patch
should come before any of the fence implementation?

But it's not entirely clear to me what this flag means - ordered with
respect to what? Ordered such that the order in which the buffers are
queued in the driver are the same order that they will be dequeued by
userspace?

I think the order they are queued from userspace can still be
different from both the order they are queued in the driver (because
the in-fences can signal in any order), and dequeued again in
userspace, so "ordered" seems a bit ambiguous.

I think it should be more clear.

Cheers
-Brian

>  * @fileio:	file io emulator internal data, used only if emulator is active
>  * @threadio:	thread io internal data, used only if thread is active
>  */
>@@ -560,6 +563,7 @@ struct vb2_queue {
> 	unsigned int			is_output:1;
> 	unsigned int			copy_timestamp:1;
> 	unsigned int			last_buffer_dequeued:1;
>+	unsigned int			ordered:1;
>
> 	struct vb2_fileio_data		*fileio;
> 	struct vb2_threadio_data	*threadio;
>-- 
>2.13.5
>