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

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Message-ID: <20190207071633.GA25149@infradead.org>
Date:   Wed, 6 Feb 2019 23:16:33 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        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 Wed, Feb 06, 2019 at 05:56:02PM +0300, Dmitry Osipenko wrote:
> > 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?

Even if it did, why would that matter?  The device would only see
IOVA space, the CPU for a normal driver only sees kernel virtual
space.

Forced contiguous space only makes sense when you have access by CPU
physical address, which should not happen for "normal" device drivers.