From mboxrd@z Thu Jan  1 00:00:00 1970
From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Subject: Re: [PATCHv2 3/9] v4l: add buffer exporting via dmabuf
Date: Wed, 01 Aug 2012 13:35:03 +0200
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To: =?ISO-8859-1?Q?R=E9mi?= Denis-Courmont <remi@remlab.net>
Cc: Hans Verkuil <hverkuil@xs4all.nl>, Tomasz Stanislawski <t.stanislaws@samsung.com>, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, airlied@redhat.com, m.szyprowski@samsung.com, kyungmin.park@samsung.com, sumit.semwal@ti.com, daeinki@gmail.com, daniel.vetter@ffwll.ch, robdclark@gmail.com, pawel@osciak.com, linaro-mm-sig@lists.linaro.org, subashrp@gmail.com, mchehab@redhat.com, g.liakhovetski@gmx.de
List-Id: dri-devel@lists.freedesktop.org

Hi R=E9mi,

On Wednesday 01 August 2012 10:37:02 R=E9mi Denis-Courmont wrote:
> On Tue, 31 Jul 2012 23:52:35 +0200, Laurent Pinchart wrote:
> >> I want to receive the video buffers in user space for processing.
> >> Typically "processing" is software encoding or conversion. That's =
what
> >> virtually any V4L application does on the desktop...
> >=20
> > But what prevents you from using MMAP ?
>=20
> As I wrote several times earlier, MMAP uses fixed number of buffers. =
In
> some tightly controlled media pipeline with low latency, it might wor=
k.

Sorry about making you repeat.

> But in general, the V4L element in the pipeline does not know how fas=
t the
> downstream element(s) will consume the buffers. Thus it has to copy f=
rom
> the MMAP buffers into anonymous user memory pending processing. Then =
any
> dequeued buffer can be requeued as soon as possible. In theory, it mi=
ght
> also be that, even though the latency is known, the number of require=
d
> buffers exceeds the maximum MMAP buffers count of the V4L device. Eit=
her
> way, user space ends up doing memory copy from MMAP to custom buffers=
=2E
>=20
> This problem does not exist with USERBUF - the V4L2 element can simpl=
y
> allocate a new buffer for each dequeued buffer.

What about using the CREATE_BUFS ioctl to add new MMAP buffers at runti=
me ?

> By the way, this was already discussed a few months ago for the exact=
 same
> DMABUF patch series...

--=20
Regards,

Laurent Pinchart