Re: [PATCH 4/4] arm64: dts: rockchip: Add OPP data for CPU cores on RK3588

From: Alexey Charkov
Date: Fri Jan 26 2024 - 03:51:13 EST

On Fri, Jan 26, 2024 at 10:32 AM Dragan Simic <dsimic@xxxxxxxxxxx> wrote:
>
> Hello Daniel,
>
> On 2024-01-25 10:30, Daniel Lezcano wrote:
> > On 24/01/2024 21:30, Alexey Charkov wrote:
> >> By default the CPUs on RK3588 start up in a conservative performance
> >> mode. Add frequency and voltage mappings to the device tree to enable
> >> dynamic scaling via cpufreq
> >>
> >> Signed-off-by: Alexey Charkov <alchark@xxxxxxxxx>
> >> ---
> >> arch/arm64/boot/dts/rockchip/rk3588s.dtsi | 209
> >> ++++++++++++++++++++++++++++++
> >> 1 file changed, 209 insertions(+)
> >>
> >> diff --git a/arch/arm64/boot/dts/rockchip/rk3588s.dtsi
> >> b/arch/arm64/boot/dts/rockchip/rk3588s.dtsi
> >> index 131b9eb21398..e605be531a0f 100644
> >> --- a/arch/arm64/boot/dts/rockchip/rk3588s.dtsi
> >> +++ b/arch/arm64/boot/dts/rockchip/rk3588s.dtsi
> >> @@ -97,6 +97,7 @@ cpu_l0: cpu@0 {
> >> clocks = <&scmi_clk SCMI_CLK_CPUL>;
> >> assigned-clocks = <&scmi_clk SCMI_CLK_CPUL>;
> >> assigned-clock-rates = <816000000>;
> >> + operating-points-v2 = <&cluster0_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <32768>;
> >> i-cache-line-size = <64>;
> >> @@ -116,6 +117,7 @@ cpu_l1: cpu@100 {
> >> enable-method = "psci";
> >> capacity-dmips-mhz = <530>;
> >> clocks = <&scmi_clk SCMI_CLK_CPUL>;
> >> + operating-points-v2 = <&cluster0_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <32768>;
> >> i-cache-line-size = <64>;
> >> @@ -135,6 +137,7 @@ cpu_l2: cpu@200 {
> >> enable-method = "psci";
> >> capacity-dmips-mhz = <530>;
> >> clocks = <&scmi_clk SCMI_CLK_CPUL>;
> >> + operating-points-v2 = <&cluster0_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <32768>;
> >> i-cache-line-size = <64>;
> >> @@ -154,6 +157,7 @@ cpu_l3: cpu@300 {
> >> enable-method = "psci";
> >> capacity-dmips-mhz = <530>;
> >> clocks = <&scmi_clk SCMI_CLK_CPUL>;
> >> + operating-points-v2 = <&cluster0_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <32768>;
> >> i-cache-line-size = <64>;
> >> @@ -175,6 +179,7 @@ cpu_b0: cpu@400 {
> >> clocks = <&scmi_clk SCMI_CLK_CPUB01>;
> >> assigned-clocks = <&scmi_clk SCMI_CLK_CPUB01>;
> >> assigned-clock-rates = <816000000>;
> >> + operating-points-v2 = <&cluster1_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <65536>;
> >> i-cache-line-size = <64>;
> >> @@ -194,6 +199,7 @@ cpu_b1: cpu@500 {
> >> enable-method = "psci";
> >> capacity-dmips-mhz = <1024>;
> >> clocks = <&scmi_clk SCMI_CLK_CPUB01>;
> >> + operating-points-v2 = <&cluster1_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <65536>;
> >> i-cache-line-size = <64>;
> >> @@ -215,6 +221,7 @@ cpu_b2: cpu@600 {
> >> clocks = <&scmi_clk SCMI_CLK_CPUB23>;
> >> assigned-clocks = <&scmi_clk SCMI_CLK_CPUB23>;
> >> assigned-clock-rates = <816000000>;
> >> + operating-points-v2 = <&cluster2_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <65536>;
> >> i-cache-line-size = <64>;
> >> @@ -234,6 +241,7 @@ cpu_b3: cpu@700 {
> >> enable-method = "psci";
> >> capacity-dmips-mhz = <1024>;
> >> clocks = <&scmi_clk SCMI_CLK_CPUB23>;
> >> + operating-points-v2 = <&cluster2_opp_table>;
> >> cpu-idle-states = <&CPU_SLEEP>;
> >> i-cache-size = <65536>;
> >> i-cache-line-size = <64>;
> >> @@ -348,6 +356,207 @@ l3_cache: l3-cache {
> >> };
> >> };
> >> + cluster0_opp_table: opp-table-cluster0 {
> >> + compatible = "operating-points-v2";
> >> + opp-shared;
> >> +
> >> + opp-408000000 {
> >> + opp-hz = /bits/ 64 <408000000>;
> >> + opp-microvolt = <675000 675000 950000>;
> >> + clock-latency-ns = <40000>;
> >> + };
> >> + opp-600000000 {
> >> + opp-hz = /bits/ 64 <600000000>;
> >> + opp-microvolt = <675000 675000 950000>;
> >> + clock-latency-ns = <40000>;
> >> + };
> >> + opp-816000000 {
> >> + opp-hz = /bits/ 64 <816000000>;
> >> + opp-microvolt = <675000 675000 950000>;
> >> + clock-latency-ns = <40000>;
> >> + };
> >> + opp-1008000000 {
> >> + opp-hz = /bits/ 64 <1008000000>;
> >> + opp-microvolt = <675000 675000 950000>;
> >> + clock-latency-ns = <40000>;
> >> + };
> >
> > It is not useful to introduce OPP with the same voltage. There is no
> > gain in terms of energy efficiency as the compute capacity is linearly
> > tied with power consumption (P=CxFxV²) in this case.
> >
> > For example, opp-408 consumes 2 bogoWatts and opp-816 consumes 4
> > bogoWatts (because of the same voltage).
> >
> > For a workload, opp-408 takes 10 sec and opp-816 takes 5 sec because
> > it is twice faster.
> >
> > The energy consumption is:
> >
> > opp-408 = 10 x 2 = 20 BogoJoules
> > opp-816 = 5 x 4 = 20 BogoJoules
>
> I'd respectfully disagree that including multiple OPPs with the same
> voltage
> but different frequencies isn't useful. Please allow me to explain.
>
> See, the total amount of consumed energy is, in general, the same for
> such
> OPPs and the same CPU task(s), if we ignore the static leakage current
> and
> such stuff, which isn't important here. Though, the emphasis here is on
> "total", i.e. without taking into account the actual amount of time
> required
> for the exemplified CPU task(s) to complete. If the total amount of
> time
> is quite short, we aren't going to heat up the package and the board
> enough
> to hit the CPU thermal throttling; this approach is also sometimes
> referred
> to as "race to idle", which is actually quite effective for
> battery-powered
> mobile devices that tend to load their CPU cores in bursts, while
> remaining
> kind of inactive for the remaining time.
>
> However, if the CPU task(s) last long enough to actually saturate the
> thermal
> capacities of the package and the board or the device, we're getting
> into the
> CPU throttling territory, in which running the CPU cores slower, but
> still as
> fast as possible, may actually be beneficial for the overall CPU
> performance.
> By running the CPU cores slower, we're lowering the power and
> "spreading" the
> total energy consumption over time, i.e. we're making some time to allow
> the
> generated heat to dissipate into the surroundings. As we know, having
> more
> energy consumed by the SoC means more heat generated by the SoC, but the
> resulting temperature of the SoC depends on how fast the energy is
> consumed,
> which equals to how fast the CPUs run; of course, all that is valid
> under
> the reasonable assumption that the entire cooling setup, including the
> board
> surroundings, remains unchanged all the time.

On the other hand, convective heat dissipation is approximately
proportional to the temperature differential, therefore heating up the
core to a higher temperature over a shorter period of time would let
it dissipate the same joule amount faster. Given that total joules
generated for a particular load are approximately the same for
different frequencies as long as voltage remains the same (as Daniel
pointed out), higher frequency seems to lead to better heat transfer
to the environment for the same load. And also the task completes
sooner, which is probably always good, ceteris paribus.

Not sure how that all changes when throttling enters the game though :)

Best regards,
Alexey