linux-kernel - Re: [PATCH] PM: domains: Don't attach a device to genpd that corresponds to a provider

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Date:   Fri, 9 Jul 2021 15:57:56 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Kevin Hilman <khilman@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM: domains: Don't attach a device to genpd that
 corresponds to a provider

Hi Ulf,

On Fri, Jul 9, 2021 at 3:48 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> On Fri, 9 Jul 2021 at 15:35, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Fri, Jul 9, 2021 at 3:23 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > On Fri, 9 Jul 2021 at 15:07, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > On Fri, Jul 9, 2021 at 2:56 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > > > According to the common power domain DT bindings, a power domain provider
> > > > > must have a "#power-domain-cells" property in its OF node. Additionally, if
> > > > > a provider has a "power-domains" property, it means that it has a parent
> > > > > domain.
> > > >
> > > > OK.
> > > >
> > > > > It has turned out that some OF nodes that represents a genpd provider may
> > > > > also be compatible with a regular platform device. This leads to, during
> > > > > probe, genpd_dev_pm_attach(), genpd_dev_pm_attach_by_name() and
> > > > > genpd_dev_pm_attach_by_id() tries to attach the corresponding struct device
> > > > > to the genpd provider's parent domain, which is wrong. Instead the genpd
> > > >
> > > > Why is that wrong?
> > >
> > > It may lead to that the struct device that corresponds to a genpd
> > > provider may be attached to the parent domain. In other words, the
> > > parent domain will not only be controlled by a child domain
> > > (corresponding to the provider), but also through the provider's
> > > struct device. As far as I can tell, this has never been the intent
> > > for how things should work in genpd.
> >
> > Ah, you're worried about the case where the subdomain is a child of
> > the parent domain, but the actual subdomain controller (represented
> > by the platform device) isn't?
>
> Well, even if the platform device represents a subdomain controller,
> should it really be attached to the parent domain?

That's what the presence of the "power-domains" property means,
isn't it?
If the subdomain controller itself is not part of the parent power
domain, there should not be a "power-domains" property.  So perhaps
we need a new property ("power-domain-parent"?) to indicate what is
the parent domain for the subdomains in this case?

> In any case, it means that the provider needs to manage runtime PM,
> etc for its struct device to not prevent the parent domain from being
> powered off.

Shouldn't all drivers for devices that can be somewhere in a PM Domain
hierarchy do that anyway? :-)  See e.g. commit 3a611e26e958b037
("net/smsc911x: Add minimal runtime PM support").

If "simple-bus" would do that, we could get rid of "simple-pm-bus"...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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