Re: [PATCH/RFC 0/2] arm64: dts: renesas: Re-add voltages to OPP tables

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Biju,

On Tue, Oct 8, 2024 at 11:30 AM Biju Das <biju.das.jz@bp.renesas.com> wrote:
> > -----Original Message-----
> > From: Geert Uytterhoeven <geert+renesas@glider.be>
> > Subject: [PATCH/RFC 0/2] arm64: dts: renesas: Re-add voltages to OPP tables
> >
> > When CONFIG_ENERGY_MODEL=y, an error is printed on RZ/G2E and R-Car E3:
> >
> >     cpu cpu0: EM: invalid perf. state: -22
> >
> > This happens because the Operating Points Parameters tables do not list voltages, as they are all
> > identical.  Previously, it was assumed they were optional, and unused, when none of the CPU nodes is
> > tied to a regulator using the "cpu-supply" property.  This assumption turned out to be incorrect,
> > causing the reported error message.
> >
> > This RFC patch series fixes this by adding the missing voltages.
> >
> > Note that the Energy Model calculates energy efficiency by dividing the
> > (estimated) CPU power consumption by CPU core clock frequency.  When all voltages have the same value,
> > the former is proportional to clock frequency, and energy efficiency becomes a constant.  Hence all
> > operating points are considered to have the same efficiency, and the Energy Model always picks the one
> > with the highest clock rate (see also [1]).
> >
> > Alternatively, the Energy Model could be changed to silently ignore OPP tables with missing
> > frequencies.  IMHO this is not an unusual case.
>
> I guess cooling uses OPP table for frequency down scaling at higher temp. So, we need
> OPP tables for thermal governors??

Yes, you still need OPP tables for thermal governance.
But the Energy Model cannot really use it when the voltages are identical.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds