linux-kernel - Re: [RFC][PATCH 12/15] videobuf2: add begin/end cpu

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Date:   Tue, 4 Feb 2020 11:50:21 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        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 Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 12/15] videobuf2: add begin/end cpu_access callbacks
 to dma-sg

On (20/02/03 19:04), Tomasz Figa wrote:
[..]
> > I very much agree with that. But this should be very clearly documented.
> > Should V4L2_CAP_MEMORY_NON_CONSISTENT always be set in this case?
> >
> 
> Yes, IMHO that would make sense. My understanding is that currently
> the consistency of allocated memory is unspecified, so it can be
> either. With V4L2_FLAG_MEMORY_NON_CONSISTENT, the userspace can
> explicitly ask for inconsistent memory.
> 
> Moreover, I'd vote for setting V4L2_CAP_MEMORY_NON_CONSISTENT when
> V4L2_FLAG_MEMORY_NON_CONSISTENT is guaranteed to return inconsistent
> memory to avoid "optional" features or "hints" without guaranteed
> behavior.

Documentation/DMA-attributes.txt says the following

  DMA_ATTR_NON_CONSISTENT
  -----------------------

  DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
  consistent or non-consistent memory as it sees fit.  By using this API,
  you are guaranteeing to the platform that you have all the correct and
  necessary sync points for this memory in the driver.

	-ss