Re: [PATCH v2 2/3] arm64: tegra: Add GPCDMA support for Tegra I2C

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Oct 07, 2022 at 03:20:37PM +0200, Thierry Reding wrote:
> On Tue, Sep 06, 2022 at 08:17:15PM +0530, Akhil R wrote:
> > Add dma properties to support GPCDMA for I2C in Tegra 186 and later
> > chips
> > 
> > Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
> > ---
> >  arch/arm64/boot/dts/nvidia/tegra186.dtsi | 32 ++++++++++++++++++++++++
> >  arch/arm64/boot/dts/nvidia/tegra194.dtsi | 32 ++++++++++++++++++++++++
> >  arch/arm64/boot/dts/nvidia/tegra234.dtsi | 32 ++++++++++++++++++++++++
> >  3 files changed, 96 insertions(+)
> > 
> > diff --git a/arch/arm64/boot/dts/nvidia/tegra186.dtsi b/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > index 59a10fb184f8..3580fbf99091 100644
> > --- a/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > +++ b/arch/arm64/boot/dts/nvidia/tegra186.dtsi
> > @@ -672,6 +672,10 @@
> >  		clock-names = "div-clk";
> >  		resets = <&bpmp TEGRA186_RESET_I2C1>;
> >  		reset-names = "i2c";
> > +		iommus = <&smmu TEGRA186_SID_GPCDMA_0>;
> > +		dma-coherent;
> 
> I wonder: why do we need the iommus and dma-coherent properties here?
> The I2C controllers are not directly accessing memory, instead it's the
> GPCDMA via the dmas/dma-names properties. The GPCDMA already has these
> properties set, so they seem to be useless here.

Looking at this some more, the reason why we need these is so that the
struct device backing these I2C controllers is attached to an IOMMU and
the DMA ops are set up correspondingly. Without these, the DMA memory
allocated by the I2C controllers will not be mapped through the IOMMU
and cause faults because the GPCDMA is the one that needs to access
those.

I do recall that we have a similar case for audio where the "sound card"
needs to have an iommus property to make sure it allocates memory
through the same IOMMU domain as the ADMA, which is the device that ends
up doing the actual memory accesses.

Rob, Robin, Will, do you know of a good way other than the DT workaround
here to address this? I think ideally we would want to obtain the "DMA
parent" of these devices so that we allocate memory for that parent
instead of the child. We do have some existing infrastructure for this
type of relationship with the __of_get_dma_parent() function as well as
the interconnects property, but I wonder if that's really the right way
to represent this.

Adding "interconnects" properties would also duplicate the "dmas"
properties we already use to obtain the TX and RX DMA channels. One
simple way to more accurately do this would be to reach into the DMA
engine internals (dma_chan->device->dev) and pass that to dma_alloc_*()
to make sure we allocate for the correct device. For audio that could be
a bit complicated because most of that code is shared across multiple
vendors. I couldn't find any examples where a driver would reach into
DMA channels to allocate for the parent, so I'm wondering what other
people do to solve this issue. Or if anyone else even has the same
issue.

Adding Lars-Peter for the sound/dmaengine helpers and Vinod for general
dmaengine. Perhaps they have some thoughts on or experience with this.

Thierry

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