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

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Date:   Thu, 6 Feb 2020 17:51:46 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        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 Tue, Feb 4, 2020 at 11:50 AM Sergey Senozhatsky
<senozhatsky@...omium.org> wrote:
>
> 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.

Good point. And I also realized that some platforms just have no way
to make the memory inconsistent, because they may have hardware
coherency.

Then we need to keep it a hint only.

Best regards,
Tomasz