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

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Date:   Wed, 6 Feb 2019 15:53:43 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>, jonathanh@...dia.com,
        mkarthik@...dia.com, smohammed@...dia.com, talho@...dia.com,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org
Subject: Re: [PATCH V11 3/5] i2c: tegra: Add DMA support

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.

Thierry

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