Re: [PATCH v2] dt-bindings: iommu: qcom_iommu: Allow 'tbu' clock

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 10/15/25 10:10 PM, Dmitry Baryshkov wrote:
> On Wed, Oct 15, 2025 at 05:48:05PM +0100, Robin Murphy wrote:
>> On 2025-10-15 5:41 pm, Konrad Dybcio wrote:
>>> From: Konrad Dybcio <konrad.dybcio@linaro.org>
>>>
>>> Some IOMMUs on some platforms (there doesn't seem to be a good denominator
> 
> It would be nice to provide some examples here.
> 
>>> for this) require the presence of a third clock, specifically for
>>> accessing the IOMMU's Translation Buffer Unit (TBU). Allow it.
>>
>> Hmmm, but isn't the only thing that accesses TBUs the consumer of the
>> qcom,tbu binding, which already has its own clock?
> 
> qcom,tbu is only defined for normal arm,mmu-500 platforms. Here Konrad
> is fixing the older and more obscure Qualcomm virtual MMU device.

(for context: this touches upon 2014-ish platforms)

I checked the address map of the physical MMU500 that lies underneath
this virtual impl and it doesn't fully expose the same registers that
the modern ones do, I only see PWR_STATUS.

The BSP kernels for those oldies don't seem to have a notion of a TBU
either, except for toggling clocks that contain "_TBU" in their name
at both the IOMMU device and some DMA-capable multimedia blocks, which
I suppose makes some sense..

Konrad