linux-kernel - Re: [PATCH v2 1/3] dt-bindings: usb: qcom,dwc3: Add multi-pd bindings for dwc3 qcom

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Message-ID: <CAE-0n530M3eft-o0qB+yEzGjZgCLMgY==ZgdvwiVCwqqCAVxxA@mail.gmail.com>
Date:   Mon, 25 Oct 2021 15:41:05 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Sandeep Maheswaram <quic_c_sanm@...cinc.com>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Felipe Balbi <balbi@...nel.org>,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_pkondeti@...cinc.com, quic_ppratap@...cinc.com
Subject: Re: [PATCH v2 1/3] dt-bindings: usb: qcom,dwc3: Add multi-pd bindings
 for dwc3 qcom

Quoting Bjorn Andersson (2021-10-25 14:43:23)
> On Mon 25 Oct 13:17 PDT 2021, Stephen Boyd wrote:
>
> > Quoting Bjorn Andersson (2021-10-25 12:10:35)
> > > On Mon 25 Oct 02:07 PDT 2021, Sandeep Maheswaram wrote:
> > >
> > > > Add multi pd bindings to set performance state for cx domain
> > > > to maintain minimum corner voltage for USB clocks.
> > > >
> > > > Signed-off-by: Sandeep Maheswaram <quic_c_sanm@...cinc.com>
> > > > ---
> > > > v2:
> > > > Make cx domain mandatory.
> > > >
> > > >  Documentation/devicetree/bindings/usb/qcom,dwc3.yaml | 8 +++++++-
> > > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/usb/qcom,dwc3.yaml b/Documentation/devicetree/bindings/usb/qcom,dwc3.yaml
> > > > index 2bdaba0..fd595a8 100644
> > > > --- a/Documentation/devicetree/bindings/usb/qcom,dwc3.yaml
> > > > +++ b/Documentation/devicetree/bindings/usb/qcom,dwc3.yaml
> > > > @@ -42,7 +42,13 @@ properties:
> > > >
> > > >    power-domains:
> > > >      description: specifies a phandle to PM domain provider node
> > > > -    maxItems: 1
> > > > +    minItems: 2
> > > > +    items:
> > > > +      - description: cx power domain
> > > > +      - description: USB gdsc power domain
> > > > +
> > > > +  required-opps:
> > > > +    description: specifies the performance state to power domain
> > >
> > > I'm still worried about the fact that we can't just rely on the USB GDSC
> > > being a subdomin of CX in order to just "turn on" CX.
> > >
> > > Afaict accepting this path forward means that for any device that sits
> > > in a GDSC power domain we will have to replicate this series for the
> > > related driver.
> > >
> >
> > I suspect the problem is that it's not just "turn on" but wanting to
> > turn it on and then set the performance state to some value based on the
> > clk frequency.
>
> I don't see an opp-table involved, just the required-opps for the
> purpose of turning CX on a little bit more. Perhaps I'm missing
> something here though.

Indeed. There's only one clk frequency for USB so only one performance
state/required-opps is used. In general that isn't the case and so we'll
eventually need to map some GDSC on/off state to the clk frequency of
whatever clk domain is associated with CX for a device.

>
> > Maybe the simplest version of that could be supported
> > somehow by having dev_pm_opp_set_rate() figure out that the 'level'
> > applies to the parent power domain instead of the child one?
>
> Having the performance_state request cascade up through the GDSC sounds
> like a nice solution; I've not looked at the code to see if this is
> feasible though.

When the binding was introduced I recall we punted on the parent child
conversion stuff. One problem at a time. There's also the possibility
for a power domain to be parented by multiple power domains so
translation tables need to account for that.

>
> > Or we may need to make another part of the OPP binding to indicate the
> > relationship between the power domain and the OPP and the parent of
> > the power domain.
>
> I suspect this would be useful if a power-domain provider needs to
> translate a performance_state into a different supply-performance_state.
> Not sure if we have such case currently; these examples are all an
> adjustable power-domain with "gating" subdomains.

Even for this case, we should be able to have the GDSC map the on state
to some performance state in the parent domain. Maybe we need to add
some code to the gdsc.c file to set a performance state on the parent
domain when it is turned on. I'm not sure where the value for that perf
state comes from. I guess we can hardcode it in the driver for now and
if it needs to be multiple values based on the clk frequency we can push
it out to an OPP table or something like that.

>
>
> PS. I think we have the same problem in the display subsystem, the
> sub-blocks are powered by MDSS_GDSC, which is a subdomain of MMCX. We
> trust the parent mdss node to keep the GDSC powered and specify MMCX as
> the power-domain for the children, so that we can affect their levels by
> respective opp-table.
>

Yes, a GDSC is really a gate on a parent power domain like CX or MMCX,
etc. Is the display subsystem an example of different clk frequencies
wanting to change the perf state of CX? If so it's a good place to work
out the translation scheme for devices that aren't listing the CX power
domain in DT.