linux-kernel - Re: [RFC PATCH 4/9] opp: core: Don't warn if required OPP device does not exist

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Date:   Fri, 15 Oct 2021 13:26:35 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Sven Peter <sven@...npeter.dev>, Marc Zyngier <maz@...nel.org>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 4/9] opp: core: Don't warn if required OPP device does
 not exist

On Thu, 14 Oct 2021 at 19:02, Hector Martin <marcan@...can.st> wrote:
>
> On 14/10/2021 21.55, Ulf Hansson wrote:
> > On Thu, 14 Oct 2021 at 13:43, Hector Martin <marcan@...can.st> wrote:
> >> I was poking around and noticed the OPP core can already integrate with
> >> interconnect requirements, so perhaps the memory controller can be an
> >> interconnect provider, and the CPU nodes can directly reference it as a
> >> consumer? This seems like a more accurate model of what the hardware
> >> does, and I think I saw some devices doing this already.
> >
> > Yeah, that could work too. And, yes, I agree, it may be a better
> > description of the HW.
> >
> >>
> >> (only problem is I have no idea of the actual bandwidth numbers involved
> >> here... I'll have to run some benchmarks to make sure this isn't just
> >> completely dummy data)
> >>
>
> So... I tried getting bandwidth numbers and failed. It seems these
> registers don't actually affect peak performance in any measurable way.
> I'm also getting almost the same GeekBench scores on macOS with and
> without this mechanism enabled, although there is one subtest that seems
> to show a measurable difference.
>
> My current guess is this is something more subtle (latencies? idle
> timers and such?) than a performance state. If that is the case, do you
> have any ideas as to the best way to model it in Linux? Should we even
> bother if it mostly has a minimal performance gain for typical workloads?

For latency constraints, we have dev_pm_qos. This will make the genpd
governor, to prevent deeper idle states for the device and its
corresponding PM domain (genpd). But that doesn't sound like a good
fit here.

If you are right, it rather sounds like there is some kind of
quiescence mode of the memory controller that can be prevented. But I
have no clue, of course. :-)

>
> I'll try to do some latency tests, see if I can make sense of what it's
> actually doing.
>

Kind regards
Uffe