linux-kernel - Re: [PATCH v8 01/34] opp: Add dev_pm_opp

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Date:   Wed, 18 Aug 2021 10:29:40 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Peter Chen <peter.chen@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Richard Weinberger <richard@....at>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Lucas Stach <dev@...xeye.de>, Stefan Agner <stefan@...er.ch>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        linux-staging@...ts.linux.dev, linux-spi@...r.kernel.org,
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        Linux Media Mailing List <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        DTML <devicetree@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v8 01/34] opp: Add dev_pm_opp_sync() helper

On Wed, 18 Aug 2021 at 08:27, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 18-08-21, 09:22, Dmitry Osipenko wrote:
> > 18.08.2021 08:58, Viresh Kumar пишет:
> > > What about calling dev_pm_opp_set_rate(dev, clk_get_rate(dev)) here
> > > instead ? That will work, right ? The advantage is it works without
> > > any special routine to do so.
> >
> > It will work, but a dedicated helper is nicer.
> >
> > > I also wonder looking at your gr3d.c changes, you set a set-opp
> > > helper, but the driver doesn't call set_opp_rate at all. Who calls it
> > > ?
> >
> > dev_pm_opp_sync() calls it from _set_opp().
>
> Okay, please use dev_pm_opp_set_rate() instead then. New helper just
> adds to the confusion and isn't doing anything special apart from
> doing clk_get_rate() for you.
>
> > > And if it is all about just syncing the genpd core, then can the genpd
> > > core do something like what clk framework does? i.e. allow a new
> > > optional genpd callback, get_performance_state() (just like
> > > set_performance_state()), which can be called initially by the core to
> > > get the performance to something other than zero. opp-set-rate is
> > > there to set the performance state and enable the stuff as well.
> > > That's why it looks incorrect in your case, where the function was
> > > only required to be called once, and you are ending up calling it on
> > > each resume. Limiting that with another local variable is bad as well.
> >
> > We discussed variant with get_performance_state() previously and Ulf
> > didn't like it either since it still requires to touch 'internals' of GENPD.
>
> Hmm, I wonder if that would be a problem since only genpd core is
> going to call that routine anyway.

Me and Dmitry discussed adding a new genpd callback for this. I agreed
that it seems like a reasonable thing to add, if he insists.

The intent was to invoke the new callback from __genpd_dev_pm_attach()
when the device has been attached to its genpd. This allows the
callback, to invoke clk_get_rate() and then dev_pm_opp_set_rate(), to
update the vote according to the current state of the HW.

I am not sure if/why that approach seemed insufficient?

Another option to solve the problem, I think, is simply to patch
drivers to let them call dev_pm_opp_set_rate() during ->probe(), this
should synchronize the HW state too.

Dmitry, can you please elaborate on this?

Kind regards
Uffe