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

Ulf Hansson
2021-10-28T10:32:28Z

On Wed, 27 Oct 2021 at 17:09, Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> On Wed 27 Oct 07:24 PDT 2021, Ulf Hansson wrote:
>
> > On Wed, 27 Oct 2021 at 06:55, Bjorn Andersson
> > <bjorn.andersson@linaro.org> wrote:
> > >
> > > On Tue 26 Oct 19:48 CDT 2021, Stephen Boyd wrote:
> > >
> > > > +Rajendra
> > > >
> > > > Quoting Bjorn Andersson (2021-10-25 19:48:02)
> > > > > On Mon 25 Oct 15:41 PDT 2021, Stephen Boyd wrote:
> > > > >
> > > > > >
> > > > > > 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.
> > > > > >
> > > > >
> > > > > But for this case - and below display case - the subdomain (the device's
> > > > > power-domain) is just a dumb gate. So there is no translation, the given
> > > > > performance_state applies to the parent. Or perhaps such implicitness
> > > > > will come back and bite us?
> > > >
> > > > In the gate case I don't see how the implicitness will ever be a
> > > > problem.
> > > >
> > > > >
> > > > > I don't think we allow a power-domain to be a subdomain of two
> > > > > power-domains - and again it's not applicable to USB or display afaict.
> > > >
> > > > Ah maybe. I always confuse power domains and genpd.
> > > >
> > > > >
> > > > > > >
> > > > > > > > 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.
> > > > > >
> > > > >
> > > > > For the GDSC I believe we only have 1:1 mapping, so implementing
> > > > > set_performance_state to just pass that on to the parent might do the
> > > > > trick (although I haven't thought this through).
> > > > >
> > > > > Conceptually I guess this would be like calling clk_set_rate() on a
> > > > > clock gate, relying on it being propagated upwards. The problem here is
> > > > > that the performance_state is just a "random" integer without a well
> > > > > defined unit.
> > > > >
> > > >
> > > > Right. Ideally it would be in the core code somehow so that if there
> > > > isn't a set_performance_state function we go to the parent or some
> > > > special return value from the function says "call it on my parent". The
> > > > translation scheme could come later so we can translate the "random"
> > > > integer between parent-child domains.
> > >
> > > As a proof of concept it should be sufficient to just add an
> > > implementation of sc->pd.set_performance_state in gdsc.c. But I agree
> > > that it would be nice to push this into some framework code, perhaps
> > > made opt-in by some GENPD_FLAG_xyz.
> > >
> > > > At the end of the day the device
> > > > driver wants to set a frequency or runtime pm get the device and let the
> > > > OPP table or power domain code figure out what the level is supposed to
> > > > be.
> > > >
> > >
> > > Yes and this is already working for the non-nested case - where the
> > > single power-domain jumps between performance states as the opp code
> > > switches from one opp to another.
> > >
> > > So if we can list only the child power-domain (i.e. the GDSC) and have
> > > the performance_stat requests propagate up to the parent rpmhpd resource
> > > I think we're good.
> > >
> > >
> > > Let's give this a spin and confirm that this is the case...
> > >
> > > > >
> > > > >
> > > > > The one case where I believe we talked about having different mapping
> > > > > between the performance_state levels was in the relationship between CX
> > > > > and MX. But I don't think we ever did anything about that...
> > > >
> > > > Hmm alright. I think there's a constraint but otherwise nobody really
> > > > wants to change both at the same time.
> > > >
> > > > > >
> > > > > > 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.
> > > > >
> > > > > Yes, the various display components sits in MDSS_GDSC but the opp-tables
> > > > > needs to change the performance_state of MDSS_GDSC->parent (i.e. CX or
> > > > > MMCX, depending on platform).
> > > > >
> > > > > As I said, today we hack this by trusting that the base drm/msm driver
> > > > > will keep MDSS_GDSC on and listing MMCX (or CX) as power-domain for each
> > > > > of these components.
> > > > >
> > > > >
> > > > > So if we solve this, then that seems to directly map to the static case
> > > > > for USB as well.
> > > > >
> > > >
> > > > 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.
> >
>
> Thanks for jumping in Ulf.
>
> > First, one domain can have two parent domains. Both from DT and from
> > genpd point of view, just to make this clear.
> >
>
> I was under the impression that the only such configuration we supported
> was that we can explicitly attach and control multiple PDs from a
> driver. I didn't think we could say that a given genpd is a subdomain of
> multiple other genpds...
>
> That said, it's better if we can ignore this, as it doesn't apply to our
> problem at hand.
>
> > 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.
> >
>
> So to clarify, we have the following situation:
>
> +---------------+
> | CX |
> | +-----------+ |
> | | USB_GDSC | |
> | | +-------+ | |
> | | | dwc3 | | |
> | | +-------+ | |
> | +-----------+ |
> +---------------+
>
> CX can operate at different performance_states, USB_GDSC can be toggled
> on/off and hence dwc3 needs CX to operate at a performance_state meeting
> its needs.
>
> The proposed patch is to list both CX and USB_GDSC as power-domains for
> dwc3, in order for the required-opp in the dwc3 to affect CX.

Okay. Then I need to point out that this looks wrong to me.

We should be able to support the needs for dwc3, by letting CX to
become the parent domain for USB_GDSC.

If there is something missing from the genpd point of view, for
example, let's fix that!

>
> > From the performance states point of view, genpd supports propagating
> > performance states to parent domains, via a 1:1 mapping of the
> > performance state. Note that, we have also quite recently made genpd's
> > ->set_performance_state() callback to be optional. A vote for a
> > performance state will be propagated to the parent domain, even if the
> > child domain would lack the ->set_performance_state() callback. This
> > should be useful, where a child domain relies on its parent domain for
> > performance state management, which seems to be the case for the USB
> > GDSC/CX power domains, right?
> >
>
> I presume you're referring to the first half of
> _genpd_set_performance_state(). This looks to be exactly what Stephen
> and I discussed implementing.

Yes.

>
> I had a rather messy tree when I looked at this last time, presumably
> missing something else to hide this propagation.
>
>
> For the USB_GDSC we today don't describe that as a subdomain of CX, but
> per your guidance and the recently introduced 3652265514f5 ("clk: qcom:
> gdsc: enable optional power domain support") we should be fairly close
> to the solution.

Great!

>
>
> The one "problem" I can see is that I believe that some of the GDSCs in
> GCC should be subdomains of MX, so the above referenced patch would then
> need to be extended to allow specifying which of the multiple
> power-domains each GDSC should be a subdomain of - something Dmitry and
> I did discuss, but wasn't needed for the display GDSC.
> Perhaps I'm just misinformed regarding this need though.

I didn't quite follow all of this.

But, perhaps using "#power-domain-cells = <2>" for the power-domain
provider can help to specify this for the consumer/child-domain?

>
> > In regards to the parsing of the "required-opps" DT binding for a
> > device node, I think that should work for cases like these, too. Or is
> > there something missing around this?
> >
>
> Given that Sandeep's proposed patch solves his problem without touching
> the framework those patches (required-opps) must already have been
> picked up.

Right!

Kind regards
Uffe