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Date: Thu, 20 Aug 2020 06:43:47 +0200 From: Christoph Hellwig <hch@....de> To: Tomasz Figa <tfiga@...omium.org> Cc: Christoph Hellwig <hch@....de>, Mauro Carvalho Chehab <mchehab@...nel.org>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>, Joonyoung Shim <jy0922.shim@...sung.com>, Seung-Woo Kim <sw0312.kim@...sung.com>, Kyungmin Park <kyungmin.park@...sung.com>, Ben Skeggs <bskeggs@...hat.com>, Pawel Osciak <pawel@...iak.com>, Marek Szyprowski <m.szyprowski@...sung.com>, Matt Porter <mporter@...nel.crashing.org>, "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>, Tom Lendacky <thomas.lendacky@....com>, alsa-devel@...a-project.org, linux-samsung-soc <linux-samsung-soc@...r.kernel.org>, linux-ia64@...r.kernel.org, linux-scsi@...r.kernel.org, linux-parisc@...r.kernel.org, Linux Doc Mailing List <linux-doc@...r.kernel.org>, nouveau@...ts.freedesktop.org, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-nvme@...ts.infradead.org, linux-mips@...r.kernel.org, linux-mm@...ck.org, netdev@...r.kernel.org, "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>, Linux Media Mailing List <linux-media@...r.kernel.org> Subject: Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT 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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