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[209.85.128.47]) by smtp.gmail.com with ESMTPSA id t3sm19138142eje.82.2020.08.19.07.04.12 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 Aug 2020 07:04:13 -0700 (PDT) Received: by mail-wm1-f47.google.com with SMTP id k8so2341008wma.2 for ; Wed, 19 Aug 2020 07:04:12 -0700 (PDT) X-Received: by 2002:a1c:5581:: with SMTP id j123mr5137250wmb.11.1597845489930; Wed, 19 Aug 2020 06:58:09 -0700 (PDT) MIME-Version: 1.0 References: <20200819065555.1802761-1-hch@lst.de> <20200819065555.1802761-6-hch@lst.de> <20200819135454.GA17098@lst.de> In-Reply-To: <20200819135454.GA17098@lst.de> From: Tomasz Figa Date: Wed, 19 Aug 2020 15:57:53 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT To: Christoph Hellwig Cc: Mauro Carvalho Chehab , Thomas Bogendoerfer , "James E.J. Bottomley" , Joonyoung Shim , Seung-Woo Kim , Kyungmin Park , Ben Skeggs , Pawel Osciak , Marek Szyprowski , Matt Porter , "list@263.net:IOMMU DRIVERS , Joerg Roedel ," , Tom Lendacky , alsa-devel@alsa-project.org, linux-samsung-soc , linux-ia64@vger.kernel.org, linux-scsi@vger.kernel.org, linux-parisc@vger.kernel.org, Linux Doc Mailing List , nouveau@lists.freedesktop.org, Linux Kernel Mailing List , linux-nvme@lists.infradead.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org, "list@263.net:IOMMU DRIVERS , Joerg Roedel ," , Linux Media Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 19, 2020 at 3:55 PM Christoph Hellwig wrote: > > 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 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. 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. 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. Best regards, Tomasz