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

[Robin Murphy <robin.murphy@arm.com>]

On 2022-10-07 16:17, Thierry Reding wrote:
> On Fri, Oct 07, 2022 at 03:53:21PM +0100, Robin Murphy wrote:
>> On 2022-10-07 15:34, Thierry Reding wrote:
>>> 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.
>>
>> As far as I'm aware that's the correct approach, i.e. if a driver is using
>> an external dmaengine then it's responsible for making DMA mappings for the
>> correct DMA channel device. We ended up being a bit asymmetrical in that the
>> dmaengine driver itself has to take care of its own mapping for the
>> non-memory end of a transfer when an IOMMU is involved - that's what
>> dma_map_resource() was created for, see pl330 and rcar-dmac for examples.
>>
>> The only driver I have first-hand experience with in this context is
>> amba-pl011, using pl330 through an SMMU on the Arm Juno board, but that
>> definitely works fine without DT hacks.
> 
> Oh yeah, I see. That's exactly what I had in mind. And now that I'm
> using the right regex I can also find more occurrences of this. Thanks
> for those pointers. Looks like that's the right approach then. I'll try
> to make corresponding changes and see if there's any fallout.

Eww, from a quick look the DMA API usage in i2c-tegra is worse than just 
this. Calling dma_sync_* on a buffer from dma_alloc_coherent() is both 
egregiously wrong and semantically pointless - the fundamental purpose 
of a coherent allocation is that both CPU and device can access it at 
any time without any explicit synchronisation. If the driver's doing its 
own bounce-buffering for reasons of data alignment then the syncs should 
just go; otherwise get rid of the coherent buffer and replace the syncs 
with proper map/unmap. Either way, be sure to throw CONFIG_DMA_API_DEBUG 
at it, hard.

Cheers,
Robin.