Re: [PATCH V2] arm64: dts: qcom: sc7280: Add nodes for eMMC and SD card

[Doug Anderson <dianders@google.com>]

Hi,

On Tue, Apr 20, 2021 at 10:21 AM <sbhanu@codeaurora.org> wrote:
>
> On 2021-04-15 01:55, Doug Anderson wrote:
> > Hi,
> >
> > On Tue, Apr 13, 2021 at 3:59 AM <sbhanu@codeaurora.org> wrote:
> >>
> >> >> >>> +                                       required-opps =
> >> >> >>> <&rpmhpd_opp_low_svs>;
> >> >> >>> +                                       opp-peak-kBps = <1200000
> >> >> >>> 76000>;
> >> >> >>> +                                       opp-avg-kBps = <1200000
> >> >> >>> 50000>;
> >> >> >> Why are the kBps numbers so vastly different than the ones on sc7180
> >> >> >> for the same OPP point. That implies:
> >> >> >>
> >> >> >> a) sc7180 is wrong.
> >> >> >>
> >> >> >> b) This patch is wrong.
> >> >> >>
> >> >> >> c) The numbers are essentially random and don't really matter.
> >> >> >>
> >> >> >> Can you identify which of a), b), or c) is correct, or propose an
> >> >> >> alternate explanation of the difference?
> >> >> >>
> >> >>
> >> >> We calculated bus votes values for both sc7180 and sc7280 with ICB
> >> >> tool,
> >> >> above mentioned values we got for sc7280.
> >> >
> >> > I don't know what an ICB tool is. Please clarify.
> >> >
> >> > Also: just because a tool spits out numbers that doesn't mean it's
> >> > correct. Presumably the tool could be wrong or incorrectly configured.
> >> > We need to understand why these numbers are different.
> >> >
> >> we checked with ICB tool team on this they conformed as Rennell &
> >> Kodiak
> >> are different chipsets,
> >> we might see delta in ib/ab values due to delta in scaling factors.
> >
> > ...but these numbers are in kbps, aren't they? As I understand it
> > these aren't supposed to be random numbers spit out by a tool but are
> > supposed to be understandable by how much bandwidth an IP block (like
> > MMC) needs from the busses it's connected to. Since the MMC IP block
> > on sc7180 and sc7280 is roughly the same there shouldn't be a big
> > difference in numbers.
> >
> > Something smells wrong.
> >
> > Adding a few people who understand interconnects better than I do,
> > though.
> >
>
> ICB team has re-checked the Rennell ICB tool and they confirmed that
> some configs were wrong in Rennell ICB tool and they corrected it.With
> the new updated Rennell ICB tool below are the values :
>
>
> Rennell LC:(Sc7180)
>
> opp-384000000 {
>               opp-hz = /bits/ 64 <384000000>;
>               required-opps = <&rpmhpd_opp_nom>;
>               opp-peak-kBps = <5400000 490000>;
>               opp-avg-kBps = <6600000 300000>;
> };
>
>
> And now, these values are near to Kodaik LC values:
>
> Kodaik LC:(SC7280)
>
> opp-384000000 {
>             opp-hz = /bits/ 64 <384000000>;
>             required-opps = <&rpmhpd_opp_nom>;
>             opp-peak-kBps = <5400000 399000>;
>             opp-avg-kBps = <6000000 300000>;
> };

This still isn't making sense to me.

* sc7180 and sc7280 are running at the same speed. I'm glad the
numbers are closer now, but I would have thought they'd be exactly the
same.

* Aren't these supposed to be sensible? This is eMMC that does max
transfer rates of 400 megabytes / second to the external device. You
have bandwidths listed here of 5,400,000 kBps = 5,400,000 kilobytes /
second = 5400 megabytes / second. I can imagine there being some
overhead where an internal bus might need to be faster but that seems
excessive. This is 13.5x!

* I can't see how it can make sense that "average" values are higher
than "peak" values.

It still feels like there's a misconfiguration somewhere.

-Doug