Re: [PATCH V11 3/5] i2c: tegra: Add DMA support

[Dmitry Osipenko <digetx@gmail.com>]

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?