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

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Message-ID: <5e66803c-b13b-cb00-e2ff-d096a1650612@gmail.com>
Date:   Wed, 6 Feb 2019 17:56:02 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...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

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?