linux-kernel - Re: [RFC][PATCH 05/15] videobuf2: handle V4L2_FLAG_MEMORY_NON

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Message-ID: <20200122021805.GE149602@google.com>
Date:   Wed, 22 Jan 2020 11:18:05 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Pawel Osciak <posciak@...omium.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 05/15] videobuf2: handle
 V4L2_FLAG_MEMORY_NON_CONSISTENT in REQBUFS

On (20/01/10 10:55), Hans Verkuil wrote:
> On 12/17/19 4:20 AM, Sergey Senozhatsky wrote:
> > This patch lets user-space to request a non-consistent memory
> > allocation during REQBUFS ioctl call. We use one bit of a
> > ->reserved[1] member of struct v4l2_requestbuffers, which is
> > now renamed to ->flags.
> > 
> > There is just 1 four-byte reserved area in v4l2_requestbuffers
> > struct, therefore for backward compatibility ->reserved and
> > ->flags were put into anonymous union.
> > 
> > Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> > ---
> >  Documentation/media/uapi/v4l/vidioc-reqbufs.rst | 14 ++++++++++++--
> >  drivers/media/common/videobuf2/videobuf2-v4l2.c | 14 ++++++++++++--
> >  drivers/media/v4l2-core/v4l2-ioctl.c            |  3 ---
> >  include/uapi/linux/videodev2.h                  |  5 ++++-
> >  4 files changed, 28 insertions(+), 8 deletions(-)
> > 
> > diff --git a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> > index d0c643db477a..9b69a61d9fd4 100644
> > --- a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> > +++ b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> > @@ -112,10 +112,20 @@ aborting or finishing any DMA in progress, an implicit
> >  	``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will
> >  	free any previously allocated buffers, so this is typically something
> >  	that will be done at the start of the application.
> > -    * - __u32
> > +    * - union
> > +      - (anonymous)
> > +    * -
> > +      - __u32
> > +      - ``flags``\ [1]
> > +      - Specifies additional buffer management attributes. E.g. when
> > +        ``V4L2_FLAG_MEMORY_NON_CONSISTENT`` set vb2 backends may be allocated
> > +        in non-consistent memory.
> 
> This should link to the table with these memory flags, rather than
> effectively documenting V4L2_FLAG_MEMORY_NON_CONSISTENT again.

OK.

> You also probably meant "vb2 buffers" rather than "vb2 backends".

Thanks.

> 
> > +    * -
> > +      - __u32
> >        - ``reserved``\ [1]
> >        - A place holder for future extensions. Drivers and applications
> > -	must set the array to zero.
> > +	must set the array to zero, unless application wants to specify
> > +        buffer management ``flags``.
> 
> I think support for this flag should be signaled as a V4L2_BUF_CAP capability.
> If the capability is not set, then vb2 should set 'flags' to 0 to preserve the
> old 'Drivers and applications must set the array to zero' behavior.

The patch set adds V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS towards the end of the
series, I guess I can shuffle the patches and change the wording here.

> The documentation for 'reserved[1]' should be changed to something like:
> 
> 	Kept for backwards compatibility. Use ``flags`` instead.

OK.

	-ss