From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dmitry Osipenko Subject: Re: [PATCH V11 3/5] i2c: tegra: Add DMA support Date: Wed, 6 Feb 2019 17:56:02 +0300 Message-ID: <5e66803c-b13b-cb00-e2ff-d096a1650612@gmail.com> References: <1549356985-25726-1-git-send-email-skomatineni@nvidia.com> <1549356985-25726-3-git-send-email-skomatineni@nvidia.com> <20190206145343.GA3651@ulmo> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20190206145343.GA3651@ulmo> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Thierry Reding Cc: Sowjanya Komatineni , jonathanh@nvidia.com, mkarthik@nvidia.com, smohammed@nvidia.com, talho@nvidia.com, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org, linux-i2c@vger.kernel.org List-Id: linux-tegra@vger.kernel.org 06.02.2019 17:53, Thierry Reding пишет: > On Tue, Feb 05, 2019 at 05:26:25PM +0300, Dmitry Osipenko wrote: > [...] >> Oh, another important moment is that physically contiguous dma_buf >> allocation isn't guaranteed by the DMA API. This may become a problem >> for T186+ that can transfer up to 64K. We need to enforce the >> contiguous-allocation requirement by using >> dma_alloc_attrs(DMA_ATTR_FORCE_CONTIGUOUS) instead of the >> dma_alloc_coherent(), please see my other comment below. > > Actually I don't think that's necessary here. DMA_ATTR_FORCE_CONTIGUOUS > only seems relevant if you've got an IOMMU attached to the device to > make sure the physical memory is also contiguous. > > See this extract from Documentation/DMA-attributes.txt: > > | DMA_ATTR_FORCE_CONTIGUOUS > | ------------------------- > | > | By default DMA-mapping subsystem is allowed to assemble the buffer > | allocated by dma_alloc_attrs() function from individual pages if it can > | be mapped as contiguous chunk into device dma address space. By > | specifying this attribute the allocated buffer is forced to be contiguous > | also in physical memory. > > We don't have an IOMMU attached to I2C or APBDMA, so this can't happen > and even if we had an IOMMU attached, all we care about is the device's > DMA address space, which means IOVA space, and that would still be > guaranteed to be contiguous, according to the above. Yes, but doesn't T186+ have IOMMU support for the DMA controller?