linux-kernel - Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

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Message-ID: <20200819135454.GA17098@lst.de>
Date:   Wed, 19 Aug 2020 15:54:55 +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 01:16:51PM +0200, Tomasz Figa wrote:
> Hi Christoph,
> 
> On Wed, Aug 19, 2020 at 8:56 AM Christoph Hellwig <hch@....de> wrote:
> >
> > The V4L2-FLAG-MEMORY-NON-CONSISTENT flag is entirely unused,
> 
> 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.