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

From: Dmitry Osipenko
Date: Wed Feb 06 2019 - 12:37:44 EST

06.02.2019 17:56, Dmitry Osipenko ÐÐÑÐÑ:
> 06.02.2019 17:53, Thierry Reding ÐÐÑÐÑ:
>> 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:
>> | -------------------------
>> |
>> | 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?

Ah, sorry I probably skimmed way too quickly thorough the message. Hmm.. well, seems I was wrong.

Sowjanya, after all looks like it should be fine to use the default dma_alloc_coherent() helper. Please put it back in v14.