Re: [RFC] ARM: dts: omap36xx: Enable thermal throttling

[Adam Ford <aford173@gmail.com>]

On Fri, Sep 13, 2019 at 12:18 PM Daniel Lezcano
<daniel.lezcano@linaro.org> wrote:
>
> On 13/09/2019 18:51, H. Nikolaus Schaller wrote:
>
> [ ... ]
>
> >> Good news (I think)
> >>
> >> With cooling-device = <&cpu 1 2> setup, I was able to ask the max
> >> frequency and it returned 600MHz.
> >>
> >> # cat /sys/devices/virtual/thermal/thermal_zone0/temp
> >> 58500
> >> # cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies
> >> 300000 600000 800000
> >> # cat /sys/devices/system/cpu/cpufreq/policy0/scaling_m
> >> scaling_max_freq  scaling_min_freq
> >> # cat /sys/devices/system/cpu/cpufreq/policy0/scaling_max_freq
> >> 600000
> >
> > looks good!
> > But we have to understand what the <&cpu 1 2> exactly means...
> >
> > Hopefully someone reading your RFCv2 can answer...
>
Daniel,

Thank you for replying.

> I may have missed the question :)
>
> These are the states allowed for the cooling device (the one you can see
> in the /sys/class/thermal/cooling_device0/max_state. As the logic is
> inverted for cpufreq, that can be confusing.

I think that's what has be confused.

>
> If it was a fan with, let's say 5 speeds, you would use <&fan 0 5>, so
> when the mitigation begins the cooling device state is 0 and then the
> thermal governor increase the state until it sees a cooling effect.
>
> If <&fan 0 2> is set, the governor won't set a state above 2 even if the
> temperature increases.

I am not sure I know what you mean by 'state' in this context.

>
> When the cooling driver is able to return the number of states it
> supports, it is safe to set the states to THERMAL_NO_LIMIT and let the
> governor to find the balance point.

If the cooling driver is using cpufreq, is the number of supported
states equal to the number of operating points given to cpufreq?

>
> Now if the cooling device is cpufreq, the state order is inverted,
> because the cooling effects happens when decreasing the OPP.
>
> If the boards support 7 OPPs, the state 0 is 7 - 0, so no mitigation, if
> the state is 1, the cpufreq is throttle to the 6th OPP, 2 to the 5th OPP
> etc.

I am not sure how the state would be set to 2.

>
> Now the different combinations:
>
> <&cpu THERMAL_NO_LIMIT THERMAL_NO_LIMIT> the governor will use the state
> 0 to 7.
>
> <&cpu THERMAL_NO_LIMIT 2> the governor will use the state 0 to 2

What would be the difference between  <&cpu THERMAL_NO_LIMIT 2>  and
<&cpu 0 2> ?
(if there is any)

>
> <&cpu 1 2> the governor will use the state 1 and 2. That means there is
> always the cooling effect as the governor won't set it to zero thus
> stopping the mitigation.

For the purposes of the board in question, we have 4 operating points,
300MHz, 600MHz, 800MHz and 1GHz.  Once the board reaches 90C, we need
them to cease operation at 800MHz and 1GHz and only permit operation
at 300MHz and 600MHz.  I am going under the assumption that the cpu
index[0] would be for 300MHz, index[1] = 600MHz, etc.

If I am interpreting your comment correctly, I should set <&cpu
THERMAL_NO_LIMIT 2> which would allow it to either not cool and run up
to 600MHz and not exceed, is that correct?

>
>
> Does it clarify the DT spec?
>

I think your reply to my inquiry might.  If possible, it would be nice
to get this documented into the bindings doc for others in the future.
I can do it, but someone with a better understanding of the concept
maybe more qualified.  I can totally understand why some may want to
integrate this into their SoC device trees to slow the processor when
hot.

Thank you for taking the time to review this.  I appreciate it.

adam

>
>
>
> > What happens with trip point 60000?
> > (unfortunately one has to reboot in between or can you kexec between two kernel/dtb versions?)
> >
> > BR,
> > Nikolaus
> >
>
>
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