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

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Date:   Thu, 10 Jun 2021 09:54:05 -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 2021-06-08 16:10, Bjorn Andersson wrote:
> On Tue 08 Jun 17:44 CDT 2021, Stephen Boyd wrote:
> 
>> Quoting Bjorn Andersson (2021-06-08 15:34:01)
>> > On Tue 08 Jun 17:29 CDT 2021, Stephen Boyd wrote:
>> >
>> > > Quoting Bjorn Andersson (2021-06-08 15:26:23)
>> > > > On Tue 08 Jun 17:15 CDT 2021, Stephen Boyd wrote:
>> > > >
>> > > > > Quoting Bjorn Andersson (2021-06-07 16:31:47)
>> > > > > > On Mon 07 Jun 12:48 CDT 2021, khsieh@...eaurora.org wrote:
>> > > > > >
>> > > > > > > Sorry about the confusion. What I meant is that even though DP controller is
>> > > > > > > in the MDSS_GDSC
>> > > > > > > power domain, DP PHY/PLL sources out of CX. The DP link clocks have a direct
>> > > > > > > impact
>> > > > > > > on the CX voltage corners. Therefore, we need to mention the CX power domain
>> > > > > > > here. And, since
>> > > > > > > we can associate only one OPP table with one device, we picked the DP link
>> > > > > > > clock over other
>> > > > > > > clocks.
>> > > > > >
>> > > > > > Thank you, that's a much more useful answer.
>> > > > > >
>> > > > > > Naturally I would think it would make more sense for the PHY/PLL driver
>> > > > > > to ensure that CX is appropriately voted for then, but I think that
>> > > > > > would result in it being the clock driver performing such vote and I'm
>> > > > > > unsure how the opp table for that would look.
>> > > > > >
>> > > > > > @Stephen, what do you say?
>> > > > > >
>> > > > >
>> > > > > Wouldn't the PHY be the one that sets some vote? So it wouldn't be the
>> > > > > clk driver, and probably not from the clk ops, but instead come from the
>> > > > > phy ops via phy_enable() and phy_configure().
>> > > > >
>> > > >
>> > > > If I understand the logic correctly *_configure_dp_phy() will both
>> > > > configure the vco clock and "request" the clock framework to change the
>> > > > rate.
>> > > >
>> > > > So I presume what you're suggesting is that that would be the place to
>> > > > cast the CX corner vote?
>> > >
>> > > Yes that would be a place to make the CX vote. The problem is then I
>> > > don't know where to drop the vote. Is that when the phy is disabled?
>> >
>> > We do pass qcom_qmp_phy_power_off() and power down the DP part as DP
>> > output is being disabled. So that sounds like a reasonable place to drop
>> > the vote for the lowest performance state.
>> >
>> 
>> So then will the corner vote be in place when the PHY isn't actually
>> powered up? That will be bad for power. The phy configure code will 
>> need
>> to know if the phy is enabled and then only put in the vote when the 
>> phy
>> is enabled, otherwise wait for enable to make the corner vote.
>> 
> 
> If we vote for a corner based on the link rate in *_configure_dp_phy()
> and put the vote for lowest corner we'd get the corner part sorted out
> afaict.
> 
> We'd still have to make sure that the PHY doesn't hang on to the cx 
> vote
> beyond that though - and implicitly in the non-DP cases...
> 
>> 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.
> 
> 
> PS. While this goes for the QMPs the DSI and eDP/DP PHYs (and PLLs)
> seems to live in MMCX.
> 
> Regards,
> Bjorn

Dp link clock rate is sourced from phy/pll (vco). However it is possible 
that different link clock rate
are sourced from same vco (phy/pll) rate. Therefore I think CX rail 
voltage level is more proper to
be decided base on link clock rate.