Re: [PATCH] arm64: dts: qcom: sc7180: Add 'sustainable_power' for CPU thermal zones

[Matthias Kaehlcke <mka@chromium.org>]

On Thu, Aug 13, 2020 at 11:30:33AM -0700, Matthias Kaehlcke wrote:
> The 'sustainable_power' attribute provides an estimate of the sustained
> power that can be dissipated at the desired control temperature. One
> could argue that this value is not necessarily the same for all devices
> with the same SoC, which may have different form factors or thermal
> designs. However there are reasons to specify a (default) value at SoC
> level for SC7180: most importantly, if no value is specified at all the
> power_allocator thermal governor (aka 'IPA') estimates a value, using the
> minimum power of all cooling devices of the zone, which can result in
> overly aggressive thermal throttling. For most devices an approximate
> conservative value should be more useful than the minimum guesstimate
> of power_allocator. Devices that need a different value can overwrite
> it in their <device>.dts. Also the thermal zones for SC7180 have a high
> level of granularity (essentially one for each function block), which
> makes it more likely that the default value just works for many devices.
> 
> The values correspond to 1901 MHz for the big cores, and 1804 MHz for
> the small cores. The values were determined by limiting the CPU
> frequencies to different max values and launching a bunch of processes
> that cause high CPU load ('while true; do true; done &' is simple and
> does a good job). A frequency is deemed sustainable if the CPU
> temperatures don't rise (consistently) above the second trip point
> ('control temperature', 95 degC in this case). Once the highest
> sustainable frequency is found, the sustainable power can be calculated
> by multiplying the energy consumption per core at this frequency (which
> can be found in /sys/kernel/debug/energy_model/) with the number of
> cores that are specified as cooling devices.
> 
> The sustainable frequencies were determined at room temperature
> on a device without heat sink or other passive cooling elements.
> 
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> If maintainers think 'sustainable_power' should be specified at
> device level (with which I conceptually agree) I'm fine with
> doing that, just seemed it could be useful to have a reasonable
> 'default' at SoC level in this case.

Any comments on this?