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Date: Thu, 7 Feb 2019 15:45:05 +0000 From: Ørjan Eide <Orjan.Eide@....com> To: Christoph Hellwig <hch@...radead.org> CC: Brian Starkey <Brian.Starkey@....com>, Liam Mark <lmark@...eaurora.org>, "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>, "tkjos@...roid.com" <tkjos@...roid.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>, "arve@...roid.com" <arve@...roid.com>, "joel@...lfernandes.org" <joel@...lfernandes.org>, nd <nd@....com>, "maco@...roid.com" <maco@...roid.com>, "christian@...uner.io" <christian@...uner.io> Subject: Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory On Wed, Feb 06, 2019 at 11:31:04PM -0800, Christoph Hellwig wrote: > The CPU may only access DMA mapped memory if ownership has been > transferred back to the CPU using dma_sync_{single,sg}_to_cpu, and then > before the device can access it again ownership needs to be transferred > back to the device using dma_sync_{single,sg}_to_device. > > > I've run some testing, and this patch does indeed fix the crash in > > dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg > > list. > > > > Tested-by: Ørjan Eide <orjan.eide@....com> > > > > I tested this on an older kernel, v4.14, since the dma-mapping code > > moved, in v4.19, to ignore the dma_address and instead use sg_phys() to > > get a valid address from the page, which is always valid in the ion sg > > lists. While this wouldn't crash on newer kernels, it's still good to > > avoid the unnecessary work when no CMO is needed. > > Can you also test is with CONFIG_DMA_API_DEBUG enabled, as that should > catch all the usual mistakes in DMA API usage, including the one found? I checked again with CONFIG_DMA_API_DEBUG=y, both with and without this patch, and I didn't get any dma-mapping errors. The issue I hit, without this patch, is when a CPU access starts after a device have attached, which caused ion to create a copy of the buffer's sg list with dma_address zeroed, but before the device have mapped the buffer. -- Ørjan
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