linux-kernel - Re: [PATCH v2] arm64/dts/qcom/sc7180: Add Display Port dt node

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Message-ID: <b157a691a1cb8f860219ca3b2c335411@codeaurora.org>
Date:   Fri, 25 Jun 2021 08:55:41 -0700
From:   khsieh@...eaurora.org
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, robdclark@...il.com,
        sean@...rly.run, vkoul@...nel.org, agross@...nel.org,
        robh+dt@...nel.org, devicetree@...r.kernel.org,
        abhinavk@...eaurora.org, aravindh@...eaurora.org,
        freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64/dts/qcom/sc7180: Add Display Port dt node

On 2021-06-22 19:52, Bjorn Andersson wrote:
> On Tue 22 Jun 15:23 CDT 2021, Stephen Boyd wrote:
> 
>> Quoting Bjorn Andersson (2021-06-18 14:41:50)
>> > On Fri 18 Jun 15:49 CDT 2021, Stephen Boyd wrote:
>> >
>> > > Quoting khsieh@...eaurora.org (2021-06-10 09:54:05)
>> > > > On 2021-06-08 16:10, Bjorn Andersson wrote:
>> > > > > On Tue 08 Jun 17:44 CDT 2021, Stephen Boyd wrote:
>> > > > >
>> > > > >> Honestly I suspect the DP PHY is _not_ in the CX domain as CX is for
>> > > > >> digital logic. Probably the PLL is the hardware that has some minimum
>> > > > >> CX
>> > > > >> requirement, and that flows down into the various display clks like
>> > > > >> the
>> > > > >> link clk that actually clock the DP controller hardware. The mdss_gdsc
>> > > > >> probably gates CX for the display subsystem (mdss) so if we had proper
>> > > > >> corner aggregation logic we could indicate that mdss_gdsc is a child
>> > > > >> of
>> > > > >> the CX domain and then make requests from the DP driver for particular
>> > > > >> link frequencies on the mdss_gdsc and then have that bubble up to CX
>> > > > >> appropriately. I don't think any of that sort of code is in place
>> > > > >> though, right?
>> > > > >
>> > > > > I haven't checked sc7180, but I'm guessing that it's following the
>> > > > > other
>> > > > > modern platforms, where all the MDSS related pieces (including e.g.
>> > > > > dispcc) lives in the MMCX domain, which is separate from CX.
>> > > > >
>> > > > > So the parent of MDSS_GDSC should be MMCX, while Kuogee's answer (and
>> > > > > the dp-opp-table) tells us that the PLL lives in the CX domain.
>> > >
>> > > Isn't MMCX a "child" of CX? At least my understanding is that MMCX is
>> > > basically a GDSC that clamps all of multimedia hardware block power
>> > > logic so that the leakage is minimized when multimedia isn't in use,
>> > > i.e. the device is suspended. In terms of bumping up the voltage we have
>> > > to pin that on CX though as far as I know because that's the only power
>> > > domain that can actually change voltage, while MMCX merely gates that
>> > > voltage for multimedia.
>> > >
>> >
>> > No, MMCX is a separate rail from CX, which powers the display blocks and
>> > is parent of MDSS_GDSC. But I see in rpmhpd that sc7180 is not one of
>> > these platforms, so I presume this means that the displayport controller
>> > thereby sits in MDSS_GDSC parented by CX.
>> >
>> > But in line with what you're saying, the naming of the supplies to the
>> > QMP indicates that the power for the PLLs is static. As such the only
>> > moving things would be the clock rates in the DP controller and as such
>> > that's what needs to scale the voltage.
>> >
>> > So if the resources we're scaling is the clocks in the DP controller
>> > then the gist of the patch is correct. The only details I see is that
>> > the DP controller actually sits in MDSS_GDSC - while it should control
>> > the level of its parent (CX). Not sure if we can describe that in a
>> > simple way.
>> 
>> Right. I'm not sure things could be described any better right now. If
>> we need to change this to be MDSS_GDSC power domain and control the
>> level of the parent then I suppose we'll have to make some sort of DT
>> change and pair that with a driver change. Maybe if that happens we 
>> can
>> just pick a new compatible and leave the old code in place.
>> 
> 
> I would prefer that we stay away from making up a new compatible for
> that, but let's see when we get there.
> 
>> Are you happy enough with this current patch?
>> 
> 
> Yes, I think this looks good.
> 
>> >
>> >
>> > PS. Why does the node name of the opp-table have to be globally unique?
>> 
>> Presumably the opp table node name can be 'opp-table' as long as it
>> lives under the node that's using it. If the opp table is at / or /soc
>> then it will need to be unique. I'd prefer just 'opp-table' if 
>> possible.
> 
> I asked the same question (if it has to be globally unique) in the 
> patch
> adding sdhci nodes for sc7280 and I didn't get a sufficient answer...
> 
> So now I do want to know why "opp-table" wouldn't be sufficient name 
> for
> these device-internal nodes.
> 
my opinion is dp_opp_table is more consistency with mdp and dsi.
Either one is fine. Please let me know asap.
> Regards,
> Bjorn