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Date:   Thu, 20 Aug 2020 07:20:04 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     alsa-devel@alsa-project.org, linux-ia64@vger.kernel.org,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
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        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        linux-mm@kvack.org, Christoph Hellwig <hch@lst.de>,
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        Joonyoung Shim <jy0922.shim@samsung.com>,
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        Kyungmin Park <kyungmin.park@samsung.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Matt Porter <mporter@kernel.crashing.org>,
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Subject: Re: [PATCH 05/28] media/v4l2: remove
 V4L2-FLAG-MEMORY-NON-CONSISTENT
Message-ID: <20200820052004.GA5305@lst.de>
References: <20200819065555.1802761-1-hch@lst.de> <20200819065555.1802761-6-hch@lst.de> <CAAFQd5COLxjydDYrfx47ht8tj-aNPiaVnC+WyQA7nvpW4gs=ww@mail.gmail.com> <20200819135454.GA17098@lst.de> <CAAFQd5BuXP7t3d-Rwft85j=KTyXq7y4s24mQxLr=VoY9krEGZw@mail.gmail.com> <20200820044347.GA4533@lst.de>
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On Thu, Aug 20, 2020 at 06:43:47AM +0200, Christoph Hellwig wrote:
> 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.

So I took a deeper look.  I don't really think it qualifies as a UAPI
in our traditional sense.  For one it only appeared in 5.9-rc1, so we
can trivially expedite the patch into 5.9-rc and not actually make it
show up in any released kernel version.  And even as of the current
Linus' tree the only user is a test driver.  So I really think the best
way to go ahead is to just revert it ASAP as the design wasn't thought
out at all.