Re: [Patch v1 08/10] cpufreq: tegra194: add OPP support and set bandwidth

[Sumit Gupta <sumitg@nvidia.com>]

On 22/12/22 21:16, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
> 
> 
> 20.12.2022 19:02, Sumit Gupta пишет:
>> Add support to use OPP table from DT in Tegra194 cpufreq driver.
>> Tegra SoC's receive the frequency lookup table (LUT) from BPMP-FW.
>> Cross check the OPP's present in DT against the LUT from BPMP-FW
>> and enable only those DT OPP's which are present in LUT also.
>>
>> The OPP table in DT has CPU Frequency to bandwidth mapping where
>> the bandwidth value is per MC channel. DRAM bandwidth depends on the
>> number of MC channels which can vary as per the boot configuration.
>> This per channel bandwidth from OPP table will be later converted by
>> MC driver to final bandwidth value by multiplying with number of
>> channels before sending the request to BPMP-FW.
>>
>> If OPP table is not present in DT, then use the LUT from BPMP-FW directy
>> as the frequency table and not do the DRAM frequency scaling which is
>> same as the current behavior.
>>
>> Now, as the CPU Frequency table is being controlling through OPP table
>> in DT. Keeping fewer entries in the table will create less frequency
>> steps and scale fast to high frequencies if required.
> 
> It's not exactly clear what you're doing here. Are you going to scale
> memory BW based on CPU freq? If yes, then this is wrong because CPU freq
> is independent from the memory subsystem.
> 
> All Tegra30+ SoCs have ACTMON hardware unit that monitors CPU memory
> activity and CPU memory BW should be scaled based on CPU memory events
> counter. We have ACTMON devfreq driver for older SoCs. I have no clue
> how ACTMON can be accessed on T186+, perhaps there should be a BPMP FW
> API for that.
> 

Yes, scaling the memory BW based on CPU freq.
Referred below patch set for previous generation of Tegra Soc's which 
you mentioned and tried to trace the history.
 
https://patchwork.ozlabs.org/project/linux-tegra/patch/1418719298-25314-3-git-send-email-tomeu.vizoso@collabora.com/

In new Tegra Soc's, actmon counter control and usage has been moved to 
BPMP-FW where only 'MCALL' counter is used and 'MCCPU is not being used.
Using the actmon counter was a reactive way to scale the frequency which 
is less effective due to averaging over a time period.
We are now using the proactive way where clients tell their bandwidth
needs to help achieve better performance.