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Mon, 19 Oct 2026 20:09:37 -0700 (PDT) Received: from unknown748F3CBA5068 (dynamic-2a02-3100-9c92-6501-cda9-90cd-4e49-75d9.310.pool.telefonica.de. [2a02:3100:9c92:6501:cda9:90cd:4e49:75d9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e1d817d75sm3304578f8f.25.2026.10.19.20.09.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 20:09:36 -0700 (PDT) Date: Tue, 20 Oct 2026 05:09:29 +0200 From: Karl Mehltretter To: Christian =?utf-8?B?S8O2bmln?= Cc: Laurent Pinchart , linux-media@vger.kernel.org, Tomasz Figa , Marek Szyprowski , Mauro Carvalho Chehab , Hans de Goede , Hans Verkuil , Benjamin Gaignard , Tomasz Stanislawski , Sumit Semwal , dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/4] media: synchronize CPU access to imported DMA-BUFs Message-ID: References: <20261019124433.GU595565@killaraus.ideasonboard.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hi Christian, On Mon, Oct 19, 2026 at 04:48:13PM +0100, Christian König wrote: > > What the drivers do more likely is that they use the vmap interface to access the DMA-buf with the CPU inside the kernel. Correct? > Yes. UVC and the tested vivid vb2-vmalloc path write the imported DMA-BUF through a kernel vmap. Userspace mmaps the DMA-BUF and brackets each read with DMA_BUF_IOCTL_SYNC. > In this case dma_buf_begin_cpu_access() and dma_buf_end_cpu_access() should be called to properly frame that access. > > That seems to be added in patch #2, but I absolutely can't judge if that is correct or not since at the same time you speak about userptr access and the code really looks a bit like spaghetti. USERPTR is not part of the CPU-access bracketing. The cover letter mentions it only as a regression test. An earlier version changed q->dma_dir to DMA_BIDIRECTIONAL and made read-only output USERPTR fail PREPARE_BUF with EFAULT. Patch 2 leaves q->dma_dir unchanged and passes DMA_BIDIRECTIONAL only to attach_dmabuf() for imported DMA-BUFs. Thanks, Karl