Re: [PATCH 0/3] arm64: dts: qcom: sa8775p: Add interconnect to SMMU

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

On 2023-06-09 15:52, Konrad Dybcio wrote:
> 
> 
> On 9.06.2023 16:45, Robin Murphy wrote:
>> On 2023-06-09 13:56, Parikshit Pareek wrote:
>>> On Fri, Jun 09, 2023 at 10:52:26AM +0200, Konrad Dybcio wrote:
>>>>
>>>>
>>>> On 9.06.2023 07:41, Parikshit Pareek wrote:
>>>>> Some qcom SoCs have SMMUs, which need the interconnect bandwidth to be
>>>>> This series introduce the due support for associated interconnect, and
>>>>> setting of the due interconnect-bandwidth. Setting due interconnect
>>>>> bandwidth is needed to avoid the issues like [1], caused by not having
>>>>> due clock votes(indirectly dependent upon interconnect bandwidth).
>>>>
>>>> [1] ???
>>>
>>> My bad. Intended to mention following:
>>> https://lore.kernel.org/linux-arm-msm/20230418165224.vmok75fwcjqdxspe@echanude/
>>
>> This sounds super-dodgy - do you really have to rely on configuration of the interconnect path from the SMMU's pagetable walker to RAM to keep a completely different interconnect path clocked for the CPU to access SMMU registers? You can't just request the programming interface clock directly like on other SoCs?
> On Qualcomm platforms, particularly so with the more recent ones, some
> clocks are managed by various remote cores. Half of what the interconnect
> infra does on these SoCs is telling one such core to change the internally
> managed clock's rate based on the requested bw.

That much I get, it just seems like an arse-backwards design decision if 
it's really necessary to pretend the SMMU needs to access memory in 
order for the CPU to be able to access the SMMU. The respective SMMU 
interfaces are functionally independent of each other - even if it is 
the case in the integration that both interfaces and/or the internal TCU 
clock do happen to be driven synchronously from the same parent clock - 
and in any sane interconnect the CPU->SMMU and SMMU->RAM routes would be 
completely different and not intersect at all.

Thanks,
Robin.