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Date:   Thu, 28 Oct 2021 09:53:14 -0700
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Rajendra Nayak <rnayak@codeaurora.org>
Cc:     Ulf Hansson <ulf.hansson@linaro.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Sandeep Maheswaram <quic_c_sanm@quicinc.com>,
        Rob Herring <robh+dt@kernel.org>,
        Andy Gross <agross@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Felipe Balbi <balbi@kernel.org>,
        Doug Anderson <dianders@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>,
        devicetree@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        quic_pkondeti@quicinc.com, quic_ppratap@quicinc.com
Subject: Re: [PATCH v2 1/3] dt-bindings: usb: qcom,dwc3: Add multi-pd
 bindings for dwc3 qcom
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On Thu 28 Oct 03:46 PDT 2021, Rajendra Nayak wrote:

> 
> On 10/28/2021 4:05 PM, Ulf Hansson wrote:
> > [...]
> > 
> > > > > > Got it. So in this case we could have the various display components
> > > > > > that are in the mdss gdsc domain set their frequency via OPP and then
> > > > > > have that translate to a level in CX or MMCX. How do we parent the power
> > > > > > domains outside of DT? I'm thinking that we'll need to do that if MMCX
> > > > > > is parented by CX or something like that and the drivers for those two
> > > > > > power domains are different. Is it basic string matching?
> > > > > 
> > > > > In one way or another we need to invoke pm_genpd_add_subdomain() to link
> > > > > the two power-domains (actually genpds) together, like what was done in
> > > > > 3652265514f5 ("clk: qcom: gdsc: enable optional power domain support").
> > > > > 
> > > > > In the case of MMCX and CX, my impression of the documentation is that
> > > > > they are independent - but if we need to express that CX is parent of
> > > > > MMCX, they are both provided by rpmhpd which already supports this by
> > > > > just specifying .parent on mmcx to point to cx.
> > > > 
> > > > I was trying to follow the discussion, but it turned out to be a bit
> > > > complicated to catch up and answer all things. In any case, let me
> > > > just add a few overall comments, perhaps that can help to move things
> > > > forward.
> > > > 
> > > > First, one domain can have two parent domains. Both from DT and from
> > > > genpd point of view, just to make this clear.
> > > > 
> > > > Although, it certainly looks questionable to me, to hook up the USB
> > > > device to two separate power domains, one to control power and one to
> > > > control performance. Especially, if it's really the same piece of HW
> > > > that is managing both things.
> > > []..
> > > > Additionally, if it's correct to model
> > > > the USB GDSC power domain as a child to the CX power domain from HW
> > > > point of view, we should likely do that.
> > > 
> > > I think this would still require a few things in genpd, since
> > > CX and USB GDSC are power domains from different providers.
> > > Perhaps a pm_genpd_add_subdomain_by_name()?
> > > 
> > 
> > I think of_genpd_add_subdomain() should help to address this. No?
> 
> We only describe the provider nodes in DT and not the individual power domains.
> For instance GCC is the power domain provider which is in DT, and USB GDSC is one
> of the many power domains it supports, similarly RPMHPD is the provider node in
> DT and CX is one of the many power domains it supports.
> So we would need some non-DT way of hooking up power domains from two different
> providers as parent/child.
> 

See 266e5cf39a0f ("arm64: dts: qcom: sm8250: remove mmcx regulator") and
3652265514f5 ("clk: qcom: gdsc: enable optional power domain support")

MMCX is declared as power-domain for the dispcc (which is correct
in itself) and the gdsc code will register GDSCs as subdomains of
the same power-domain.


To ensure this code path is invoked the clock driver itself needed this
6158b94ec807 ("clk: qcom: dispcc-sm8250: use runtime PM for the clock
controller")

So at least in theory, considering only USB the minimum would be to
pm_runtime_enable() gcc-7280 and add power-domains = <CX> to the gcc
node.


The "problem" I described would be if there are GDSCs that are
subdomains of MX - which I've seen hinted in some documentation. If so
we should to specify both CX and MX as power-domains for &gcc and the
gdsc implementation needs to be extended to allow us to select between
the two.

For this I believe a combination of genpd_dev_pm_attach_by_name() and
of_genpd_add_subdomain() would do the trick.

That is, if there actually are GDSCs exposed by gcc that are not
subdomains of CX - otherwise none of this is needed.

Regards,
Bjorn