linux-kernel - Re: [PATCH v9 11/15] vb2: add in-fence support to QBUF

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Date:   Wed, 9 May 2018 11:35:05 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Gustavo Padovan <gustavo.padovan@...labora.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        linux-media@...r.kernel.org, kernel@...labora.com,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Pawel Osciak <pawel@...iak.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        Sakari Ailus <sakari.ailus@....fi>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 11/15] vb2: add in-fence support to QBUF

Hi,

On Tue, May 08, 2018 at 08:18:06PM -0300, Gustavo Padovan wrote:
>
>Hi Hans,
>
>On Mon, 2018-05-07 at 14:07 +0200, Hans Verkuil wrote:
>> On 04/05/18 22:06, Ezequiel Garcia wrote:
>> > From: Gustavo Padovan <gustavo.padovan@...labora.com>

[snip]

>> > diff --git a/include/media/videobuf2-core.h
>> > b/include/media/videobuf2-core.h
>> > index 364e4cb41b10..28ce8f66882e 100644
>> > --- a/include/media/videobuf2-core.h
>> > +++ b/include/media/videobuf2-core.h
>> > @@ -17,6 +17,7 @@
>> >  #include <linux/poll.h>
>> >  #include <linux/dma-buf.h>
>> >  #include <linux/bitops.h>
>> > +#include <linux/dma-fence.h>
>> >
>> >  #define VB2_MAX_FRAME	(32)
>> >  #define VB2_MAX_PLANES	(8)
>> > @@ -255,12 +256,21 @@ struct vb2_buffer {
>> >  	 * done_entry:		entry on the list that
>> > stores all buffers ready
>> >  	 *			to be dequeued to userspace
>> >  	 * vb2_plane:		per-plane information; do not
>> > change
>> > +	 * in_fence:		fence received from vb2 client
>> > to wait on before
>> > +	 *			using the buffer (queueing to
>> > the driver)
>> > +	 * fence_cb:		fence callback information
>> > +	 * fence_cb_lock:	protect callback signal/remove
>> >  	 */
>> >  	enum vb2_buffer_state	state;
>> >
>> >  	struct vb2_plane	planes[VB2_MAX_PLANES];
>> >  	struct list_head	queued_entry;
>> >  	struct list_head	done_entry;
>> > +
>> > +	struct dma_fence	*in_fence;
>> > +	struct dma_fence_cb	fence_cb;
>> > +	spinlock_t              fence_cb_lock;
>> > +
>>
>> So for the _MPLANE formats this is one fence for all planes. Which
>> makes sense, but how
>> does drm handle that? Also one fence for all planes?
>
>Yes, this is one fence for all planes.
>
>The DRM concept for planes is a totally different concept and is
>basically a representation of an user definable square on the screen,
>and to that plane there in one framebuffer attached - display hw has no
>such a multiplanar for the same image AFAICT. So you probably need some
>blit to convert the v4l2 multiplanar to a DRM framebuffer.
>

Lots of display hardware can do multi-planar formats, and there's
space in struct drm_framebuffer for up to 4 buffer handles (e.g. 3
handles are passed for Luma, Cr, and Cb when the framebuffer format is
DRM_FORMAT_YUV420) - like V4L2 MPLANE.

The V4L2 code here matches with the DRM "explicit sync"
(IN_FENCE_FD/OUT_FENCE_PTR) stuff, which is probably what we want.
The main difference is that in DRM, explicit fences aren't associated
with framebuffers, they're associated with the things using the
framebuffers - but practically it doesn't make a difference.

There can be per-buffer "implicit sync" via dma-buf reservation
objects, but I don't think this series should attempt to deal with
that.

Cheers,
-Brian

>>
>> I think there should be a comment about this somewhere.
>
>Yes, we've been over this exact discussion a few times :)
>Having entirely different things with the same name is quite confusing.
>
>Regards,
>
>Gustavo
>
>-- 
>Gustavo Padovan
>Principal Software Engineer
>Collabora Ltd.