Re: [PATCH] arm64: dts: qcom: sc7180-lite: Fix SDRAM freq for misidentified sc7180-lite boards

[Konrad Dybcio <konrad.dybcio@linaro.org>]

On 16.05.2023 02:19, Douglas Anderson wrote:
> In general, the three SKUs of sc7180 (lite, normal, and pro) are
> handled dynamically.
> 
> The cpufreq table in sc7180.dtsi includes the superset of all CPU
> frequencies. The "qcom-cpufreq-hw" driver in Linux shows that we can
> dynamically detect which frequencies are actually available on the
> currently running CPU and then we can just enable those ones.
> 
> The GPU is similarly dynamic. The nvmem has a fuse in it (see
> "gpu_speed_bin" in sc7180.dtsi) that the GPU driver can use to figure
> out which frequencies to enable.
> 
> There is one part, however, that is not so dynamic. The way SDRAM
> frequency works in sc7180 is that it's tied to cpufreq. At the busiest
> cpufreq operating points we'll pick the top supported SDRAM frequency.
> They ramp down together.
> 
> For the "pro" SKU of sc7180, we only enable one extra cpufreq step.
> That extra cpufreq step runs SDRAM at the same speed as the step
> below. Thus, for normal and pro things are OK. There is no sc7180-pro
> device tree snippet.
> 
> For the "lite" SKU if sc7180, however, things aren't so easy. The
> "lite" SKU drops 3 cpufreq entries but can still run SDRAM at max
> frequency. That messed things up with the whole scheme. This is why we
> added the "sc7180-lite" fragment in commit 8fd01e01fd6f ("arm64: dts:
> qcom: sc7180-lite: Tweak DDR/L3 scaling on SC7180-lite").
> 
> When the lite scheme came about, it was agreed that the WiFi SKUs of
> lazor would _always_ be "lite" and would, in fact, be the only "lite"
> devices. Unfortunately, this decision changed and folks didn't realize
> that it would be a problem. Specifically, some later lazor WiFi-only
> devices were built with "pro" CPUs.
> 
> Building WiFi-only lazor with "pro" CPUs isn't the end of the world.
> The SDRAM will ramp up a little sooner than it otherwise would, but
> aside from a small power hit things work OK. One problem, though, is
> that the SDRAM scaling becomes a bit quirky. Specifically, with the
> current tables we'll max out SDRAM frequency at 2.1GHz but then
> _lower_ it at 2.2GHz / 2.3GHz only to raise it back to max for 2.4GHz
> and 2.55GHz.
> 
> Let's at least fix this so that the SDRAM frequency doesn't go down in
> that quirky way. On true "lite" SKUs this change will be a no-op
> because the operating points we're touching are disabled. This change
> is only useful when a board that thinks it has a "lite" CPU actually
> has a "normal" or "pro" one stuffed.
> 
> Fixes: 8fd01e01fd6f ("arm64: dts: qcom: sc7180-lite: Tweak DDR/L3 scaling on SC7180-lite")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
I'd love it if you wrote a book detailing all the crazy stories like
this one, given it was probably not a one-off :P

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Cc: <stable@vger.kernel.org>

Konrad
> 
>  arch/arm64/boot/dts/qcom/sc7180-lite.dtsi | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi b/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
> index d8ed1d7b4ec7..4b306a59d9be 100644
> --- a/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
> +++ b/arch/arm64/boot/dts/qcom/sc7180-lite.dtsi
> @@ -16,3 +16,11 @@ &cpu6_opp11 {
>  &cpu6_opp12 {
>  	opp-peak-kBps = <8532000 23347200>;
>  };
> +
> +&cpu6_opp13 {
> +	opp-peak-kBps = <8532000 23347200>;
> +};
> +
> +&cpu6_opp14 {
> +	opp-peak-kBps = <8532000 23347200>;
> +};