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

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Message-ID: <4e3e4485-ba22-eb47-fb95-e8d626160bc6@gmail.com>
Date:   Thu, 22 Dec 2022 18:46:54 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sumit Gupta <sumitg@...dia.com>, treding@...dia.com,
        krzysztof.kozlowski@...aro.org, dmitry.osipenko@...labora.com,
        viresh.kumar@...aro.org, rafael@...nel.org, jonathanh@...dia.com,
        robh+dt@...nel.org, linux-kernel@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org
Cc:     sanjayc@...dia.com, ksitaraman@...dia.com, ishah@...dia.com,
        bbasu@...dia.com
Subject: Re: [Patch v1 08/10] cpufreq: tegra194: add OPP support and set
 bandwidth

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.