linux-kernel - Re: [PATCHv3 03/11] videobuf2: add V4L2_FLAG_MEMORY_NON

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Message-ID: <20200228013948.GN122464@google.com>
Date:   Fri, 28 Feb 2020 10:39:48 +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: [PATCHv3 03/11] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT
 flag

On (20/02/27 13:25), Hans Verkuil wrote:
[..]
> > +      - vb2 buffer is allocated either in consistent (it will be automatically
> 
> vb2 -> A
> 
> (vb2 is a kAPI term, and shouldn't be used in uAPI documentation)
> 
> > +	coherent between CPU and bus) or non-consistent memory. The latter
> 
> CPU and bus -> the CPU and the bus
> 
> > +	can provide performance gains, for instance CPU cache sync/flush
> 
> CPU -> the CPU
> 
> > +	operations can be avoided if the buffer is accessed by the corresponding
> > +	device only and CPU does not read/write to/from that buffer. However,
> 
> CPU -> the CPU
> 
> > +	this requires extra care from the driver -- it must guarantee memory
> > +	consistency by issuing cache flush/sync when consistency is needed.
> 
> cache -> a cache
> 
> > +	If this flag is set V4L2 will attempt to allocate vb2 buffer in
> 
> vb2 -> the
> 
> > +	non-consistent memory. The flag takes effect only if the buffer is
> > +	used for :ref:`memory mapping <mmap>` I/O and the queue reports
> 
> reports -> reports the

OK.

> > +++ b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
> > @@ -156,6 +156,13 @@ aborting or finishing any DMA in progress, an implicit
> >        - Only valid for stateless decoders. If set, then userspace can set the
> >          ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
> >  	capture buffer until the OUTPUT timestamp changes.
> > +    * - ``V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS``
> > +      - 0x00000040
> > +      - Set when the queue/buffer support memory consistency and cache
> 
> support -> supports

OK.

> > +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> > @@ -711,6 +711,8 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
> >  		*caps |= V4L2_BUF_CAP_SUPPORTS_DMABUF;
> >  	if (q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF)
> >  		*caps |= V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF;
> > +	if ((q->allow_cache_hints != 0) && (q->io_modes & VB2_MMAP))
> 
> Just say:
> 
> 	if (q->allow_cache_hints && (q->io_modes & VB2_MMAP))

OK.

	-ss