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?
