linux-kernel - Re: [RFC][PATCHv2 05/12] videobuf2: handle V4L2_FLAG_MEMORY_NON

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Date:   Wed, 19 Feb 2020 17:59:45 +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][PATCHv2 05/12] videobuf2: handle
 V4L2_FLAG_MEMORY_NON_CONSISTENT flag

On (20/02/19 09:25), Hans Verkuil wrote:
[..]
> > diff --git a/Documentation/media/uapi/v4l/vidioc-create-bufs.rst b/Documentation/media/uapi/v4l/vidioc-create-bufs.rst
> > index bd08e4f77ae4..68185e94b686 100644
> > --- a/Documentation/media/uapi/v4l/vidioc-create-bufs.rst
> > +++ b/Documentation/media/uapi/v4l/vidioc-create-bufs.rst
> > @@ -121,7 +121,14 @@ than the number requested.
> >  	other changes, then set ``count`` to 0, ``memory`` to
> >  	``V4L2_MEMORY_MMAP`` and ``format.type`` to the buffer type.
> >      * - __u32
> > -      - ``reserved``\ [7]
> > +      - ``flags``
> > +      - Specifies additional buffer management attributes. Valid only when
> > +	queue reports :ref:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
> > +	Old drivers and applications must set it to zero.
> 
> Drop the 'Valid only' sentence. The V4L2_FLAG_MEMORY_NON_CONSISTENT depends
> on that capability, but other flags added in the future may not.

The whole sentence, right?

> Inside add a reference to the memory flags section created in patch 3.

Sorry. Inside?

[..]
> > diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> > index 72efc1c544cd..169a8cf345ed 100644
> > --- a/include/uapi/linux/videodev2.h
> > +++ b/include/uapi/linux/videodev2.h
> > @@ -938,7 +938,10 @@ struct v4l2_requestbuffers {
> >  	__u32			type;		/* enum v4l2_buf_type */
> >  	__u32			memory;		/* enum v4l2_memory */
> >  	__u32			capabilities;
> > -	__u32			reserved[1];
> > +	union {
> > +		__u32		flags;
> > +		__u32		reserved[1];
> > +	};
> 
> How about this:
> 
> 	__u8			flags;
> 	__u8			reserved[3];
> 
> That avoids the anonymous union and allows some space for future additions.

Hmm. This way old apps, which clear out ->reserved, e.g.
memset(&x.reserved, 0x00, sizeof(x.reserved)), won't clear
out x.flags and can accidentally submit some unintended
garbage. It's not the case with anon union.

	-ss