From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 05/28] media/v4l2: remove
 V4L2-FLAG-MEMORY-NON-CONSISTENT
Date: Thu, 20 Aug 2020 06:43:47 +0200
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To: Tomasz Figa <tfiga@chromium.org>
Cc: Christoph Hellwig <hch@lst.de>, Mauro Carvalho Chehab <mchehab@kernel.org>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>, Joonyoung Shim <jy0922.shim@samsung.com>, Seung-Woo Kim <sw0312.kim@samsung.com>, Kyungmin Park <kyungmin.park@samsung.com>, Ben Skeggs <bskeggs@redhat.com>, Pawel Osciak <pawel@osciak.com>, Marek Szyprowski <m.szyprowski@samsung.com>, Matt Porter <mporter@kernel.crashing.org>, "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
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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.