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Date:   Thu, 20 Aug 2020 06:43:47 +0200
From:   Christoph Hellwig <>
To:     Tomasz Figa <>
Cc:     Christoph Hellwig <>,
        Mauro Carvalho Chehab <>,
        Thomas Bogendoerfer <>,
        "James E.J. Bottomley" <>,
        Joonyoung Shim <>,
        Seung-Woo Kim <>,
        Kyungmin Park <>,
        Ben Skeggs <>,
        Pawel Osciak <>,
        Marek Szyprowski <>,
        Matt Porter <>,
        " DRIVERS <>, Joerg
        Roedel <>," <>,
        Tom Lendacky <>,,
        linux-samsung-soc <>,,,,
        Linux Doc Mailing List <>,,
        Linux Kernel Mailing List <>,,,,,
        " DRIVERS <>, Joerg
        Roedel <>," <>,
        Linux Media Mailing List <>
Subject: Re: [PATCH 05/28] media/v4l2: remove

On Wed, Aug 19, 2020 at 03:57:53PM +0200, Tomasz Figa wrote:
> > > Could you explain what makes you think it's unused? It's a feature of
> > > the UAPI generally supported by the videobuf2 framework and relied on
> > > by Chromium OS to get any kind of reasonable performance when
> > > accessing V4L2 buffers in the userspace.
> >
> > Because it doesn't do anything except on PARISC and non-coherent MIPS,
> > so by definition it isn't used by any of these media drivers.
> It's still an UAPI feature, so we can't simply remove the flag, it
> must stay there as a no-op, until the problem is resolved.

Ok, I'll switch to just ignoring it for the next version.

> Also, it of course might be disputable as an out-of-tree usage, but
> selecting CONFIG_DMA_NONCOHERENT_CACHE_SYNC makes the flag actually do
> something on other platforms, including ARM64.

It isn't just disputable, but by kernel policies simply is not relevant.

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