linux-kernel - Re: [RESEND PATCH v2 4/7] clk: qcom: gdsc: enable optional power domain support

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Message-ID: <CAPDyKFr+-qXbi4z4_wzDRaMMLKSKM7zNr55Kt-AOk97mVKM+8A@mail.gmail.com>
Date:   Fri, 9 Jul 2021 15:14:21 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Taniya Das <tdas@...eaurora.org>,
        Jonathan Marek <jonathan@...ek.ca>,
        Michael Turquette <mturquette@...libre.com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "Bryan O'Donoghue" <bryan.odonoghue@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v2 4/7] clk: qcom: gdsc: enable optional power
 domain support

On Fri, 9 Jul 2021 at 14:59, Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On Fri, 9 Jul 2021 at 15:18, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Fri, 9 Jul 2021 at 13:46, Dmitry Baryshkov
> > <dmitry.baryshkov@...aro.org> wrote:
> > >
> > > On Fri, 9 Jul 2021 at 12:33, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > >
> > > > On Fri, 9 Jul 2021 at 06:32, Dmitry Baryshkov
> > > > <dmitry.baryshkov@...aro.org> wrote:
> > > > >
> > > > > On sm8250 dispcc and videocc registers are powered up by the MMCX power
> > > > > domain. Currently we used a regulator to enable this domain on demand,
> > > > > however this has some consequences, as genpd code is not reentrant.
> > > > >
> > > > > Teach Qualcomm clock controller code about setting up power domains and
> > > > > using them for gdsc control.
> > > > >
> > > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> > > >
> > > > [...]
> > > >
> > > > > diff --git a/drivers/clk/qcom/gdsc.c b/drivers/clk/qcom/gdsc.c
> > > > > index 51ed640e527b..9401d01533c8 100644
> > > > > --- a/drivers/clk/qcom/gdsc.c
> > > > > +++ b/drivers/clk/qcom/gdsc.c
> > > > > @@ -427,6 +427,7 @@ int gdsc_register(struct gdsc_desc *desc,
> > > > >                         continue;
> > > > >                 scs[i]->regmap = regmap;
> > > > >                 scs[i]->rcdev = rcdev;
> > > > > +               scs[i]->pd.dev.parent = desc->dev;
> > > > >                 ret = gdsc_init(scs[i]);
> > > > >                 if (ret)
> > > > >                         return ret;
> > > > > @@ -439,6 +440,8 @@ int gdsc_register(struct gdsc_desc *desc,
> > > > >                         continue;
> > > > >                 if (scs[i]->parent)
> > > > >                         pm_genpd_add_subdomain(scs[i]->parent, &scs[i]->pd);
> > > > > +               else if (!IS_ERR_OR_NULL(dev->pm_domain))
> > > >
> > > > So dev_pm_domain_attach() (which calls genpd_dev_pm_attach() is being
> > > > called for gdsc platform device from the platform bus', to try to
> > > > attach the device to its corresponding PM domain.
> > > >
> > > > Looking a bit closer to genpd_dev_pm_attach(), I realize that we
> > > > shouldn't really try to attach a device to its PM domain, when its OF
> > > > node (dev->of_node) contains a "#power-domain-cells" specifier. This
> > > > is because it indicates that the device belongs to a genpd provider
> > > > itself. In this case, a "power-domains" specifier tells that it has a
> > > > parent domain.
> > > >
> > > > I will post a patch that fixes this asap.
> > >
> > > I think there is nothing to fix here. The dispcc/videocc drivers
> > > provide clocks in addition to the gdsc power domain. And provided
> > > clocks would definitely benefit from having the dispcc device being
> > > attached to the power domain which governs clock registers (MMCX in
> > > our case). Thus I think it is perfectly valid to have:
> > >
> > > rpmhpd device:
> > >  - provides MMCX domain.
> > >
> > > dispcc device:
> > >  - is attached to the MMCX domain,
> >
> > We don't need this, it's redundant and weird to me.
> >
> > Also I am kind of worried that you will hit another new path in genpd,
> > causing locking issues etc, as it has not been designed to work like
> > this (a provider device and a child domain sharing the same "parent").
>
> So, which domain should the dispcc device belong to? It's registers
> are powered by the MMCX domain. I can not attach it to the child
> (GDSC) domain either: in the case of videocc there are 4 child
> domains.

The dispcc device should *not* be attached to a PM domain.

Instead it should be registered as a genpd provider and the
corresponding PM domains it provides, should be assigned as child
domains to the MMCX domain.

This is exactly what the child/parent domain support in genpd is there
to help with.

> An alternative would be to request that all users of the provided
> clocks power on one of the child domains. However this is also not
> perfect. If some generic code (e.g. clock framework) calls into
> provided clocks (e.g. because of assigned-clock-rates), this can
> happen w/o proper power domain being powered up yet.

Issues with power on/off synchronization during genpd initializations
and genpd provider registration, certainly need to be fixed and I am
happy to help. However, my point is that I think it's a bad idea to
fix it through modelling the PM domain hierarchy in an incorrect way.

[...]

Kind regards
Uffe