Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30 thermal sensor

From: Thara Gopinath
Date: Tue Jun 15 2021 - 22:50:49 EST



On 6/15/21 3:32 PM, Dmitry Osipenko wrote:
15.06.2021 19:18, Daniel Lezcano пишет:
On 15/06/2021 15:01, Dmitry Osipenko wrote:
15.06.2021 13:26, Viresh Kumar пишет:
On 15-06-21, 12:03, Daniel Lezcano wrote:

[Cc Viresh]

On 29/05/2021 19:09, Dmitry Osipenko wrote:
All NVIDIA Tegra30 SoCs have a two-channel on-chip sensor unit which
monitors temperature and voltage of the SoC. Sensors control CPU frequency
throttling, which is activated by hardware once preprogrammed temperature
level is breached, they also send signal to Power Management controller to
perform emergency shutdown on a critical overheat of the SoC die. Add
driver for the Tegra30 TSENSOR module, exposing it as a thermal sensor
and a cooling device.

IMO it does not make sense to expose the hardware throttling mechanism
as a cooling device because it is not usable anywhere from the thermal
framework.

Moreover, that will collide with the thermal / cpufreq framework
mitigation (hardware sets the frequency but the software thinks the freq
is different), right ?

H/w mitigation is additional and should be transparent to the software
mitigation. The software mitigation is much more flexible, but it has
latency. Software also could crash and hang.

Hardware mitigation doesn't have latency and it will continue to work
regardless of the software state.

Yes, I agree. Both solutions have their pros and cons. However, I don't
think they can co-exist sanely.

The CCF driver is aware about the h/w cooling status [1], hence software
sees the actual frequency.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit?id=344d5df34f5abd468267daa98f041abf90b2f660

Ah interesting, thanks for the pointer.

What I'm worried about is the consistency with cpufreq.

Probably cpufreq_update_limits() should be called from the interrupt
handler.

IIUC, the cpufreq already should be prepared for the case where firmware
may override frequency. Viresh, could you please clarify what are the
possible implications of the frequency overriding?

I am not even sure what the cooling device is doing here:

tegra_tsensor_set_cur_state() is not implemented and it says hardware
changed it by itself. What is the benefit you are getting out of the
cooling device here ?

It allows userspace to check whether hardware cooling is active via the
cooling_device sysfs. Otherwise we don't have ability to check whether
h/w cooling is active, I think it's a useful information. It's also
interesting to see the cooling_device stats, showing how many times h/w
mitigation was active.

Actually the stats are for software mitigation. For the driver, create a
debugfs entry like what do the other drivers or a module parameter with
the stats.

Okay

The hardware limiter should let know the cpufreq framework about the
frequency change.

https://lkml.org/lkml/2021/6/8/1792

May be post the sensor without the hw limiter for now and address that
in a separate series ?


I wasn't aware about existence of the thermal pressure, thank you for
pointing at it. At a quick glance it should be possible to benefit from
the information about the additional pressure.

Seems the current thermal pressure API assumes that there is only one
user of the API. So it's impossible to aggregate the pressure from
different sources, like software cpufreq pressure + h/w freq pressure.
Correct? If yes, then please let me know yours thoughts about the best
approach of supporting the aggregation.

Hi,

Thermal pressure is letting scheduler know that the max capacity available for a cpu to schedule tasks is reduced due to a thermal event.
So you cannot have a h/w thermal pressure and s/w thermal pressure. There is eventually only one capping applied at h/w level and the frequency corresponding to this capping should be used for thermal pressure.

Ideally you should not be having both s/w and h/w trying to throttle at the same time. Why is this a scenario and what prevents you from disabling s/w throttling when h/w throttling is enabled. Now if there has to a aggregation for whatever reason this should be done at the thermal driver level and passed to scheduler.


That is a good question. IMO, first step would be to call
cpufreq_update_limits().

Right

[ Cc Thara who implemented the thermal pressure ]

May be Thara has an idea about how to aggregate both? There is another
series floating around with hardware limiter [1] and the same problematic.

[1] https://lkml.org/lkml/2021/6/8/1791

Thanks, it indeed looks similar.

I guess the common thermal pressure update code could be moved out into
a new special cpufreq thermal QoS handler (policy->thermal_constraints),
where handler will select the frequency constraint and set up the
pressure accordingly. So there won't be any races in the code.

It was a conscious decision to keep thermal pressure update out of qos max freq update because there are platforms that don't use the qos framework. For eg acpi uses cpufreq_update_policy.
But you are right. We have two platforms now applying h/w throttling and cpufreq_cooling applying s/w throttling. So it does make sense to have one api doing all the computation to update thermal pressure. I am not sure how exactly/where exactly this will reside.

So for starters, I think you should replicate the update of thermal pressure in your h/w driver when you know that h/w is throttling/throttled the frequency. You can refer to cpufreq_cooling.c to see how it is done.

Moving to a common api can be done as a separate patch series.

--
Warm Regards
Thara (She/Her/Hers)