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

[Sumit Gupta]

On 22/12/22 21:16, Dmitry Osipenko wrote:
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>
> 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@xxxxxxxxxxxxx/

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.