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

From: Dragan Simic
Date: Fri Jan 26 2024 - 04:04:15 EST

On 2024-01-26 08:30, Alexey Charkov wrote:
On Fri, Jan 26, 2024 at 11:05 AM Dragan Simic <dsimic@xxxxxxxxxxx> wrote:
On 2024-01-26 07:44, Alexey Charkov wrote:
> On Fri, Jan 26, 2024 at 10:32 AM Dragan Simic <dsimic@xxxxxxxxxxx>
> wrote:
>> 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

Let me point out that the emphasis is again on "shorter period". :)
Yes, when the CPU load is bursty, having multiple same-voltage OPPs
almost surely won't help us at all, as I already noted. However,
the things will surely change when the CPU cores are loaded for
longer amounts of time and, as a result, the defined thermal trips
are reached, because the cooling system gets saturated.

> 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 :)

As I already noted above, the things are quite different when the CPU
load isn't bursty. Once the cooling setup is saturated, the heat no
longer gets transferred effectively to the surroundings, while the CPU
cores keep producing the heat, which cannot continue indefinitely. As
a result, the CPU cores need to run slower and "spread" the total amount
of joules over time, but they still should run as fast as possible.

Wouldn't in this "non-bursty" case the total thermal production be
driven by how fast the system generates tasks, which is independent of
the thermal and frequency state? If joules per task are constant
(under steady state load), then it shouldn't matter whether they are
generated in short bursts or in a slower continuous flow - as long as
the time to dissipate the heat is longer than the time between tasks
at the high frequency state, in which case we are back to the "bursty"
scenario.

You're right, if there's enough time for the generated heat to dissipate,
between scheduled CPU tasks, there should be no significant OPP dips,
and how bursty (or not) the CPU load is shouldn't matter much. Actually,
if you agree, exactly this might be our definition of bursty CPU load.

The things change when there isn't enough time.

Another option is to introduce active cooling, which also comes with
its own set of limits, but the initial assumption is that the cooling
setup remains unchanged.

In the end, if all that weren't the case, we wouldn't need CPU thermal
throttling at all, or not as much. :)

Throttling would also lower the voltage at some point, which cools it
down much faster!

Of course, but the key is not to cool (and slow down) the CPU cores too
much, but just enough to stay within the available thermal envelope,
which is where the same-voltage, lower-frequency OPPs should shine.

When the CPU load isn't bursty but steady and high, we don't race to
idle, but run a marathon instead, so to speak. :)