Date:   Wed, 9 Dec 2020 13:20:24 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     rui.zhang@...el.com, Thara Gopinath <thara.gopinath@...aro.org>,
        Amit Kucheria <amitk@...nel.org>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thermal/core: Emit a warning if the thermal zone is
 updated without ops

On 09/12/2020 11:41, Lukasz Luba wrote:
> 
> 
> On 12/8/20 3:19 PM, Daniel Lezcano wrote:
>> On 08/12/2020 15:37, Lukasz Luba wrote:
>>>
>>>
>>> On 12/8/20 1:51 PM, Daniel Lezcano wrote:
>>>>
>>>> Hi Lukasz,
>>>>
>>>> On 08/12/2020 10:36, Lukasz Luba wrote:
>>>>> Hi Daniel,
>>>>
>>>> [ ... ]
>>>>
>>>>>>       static void thermal_zone_device_init(struct thermal_zone_device
>>>>>> *tz)
>>>>>> @@ -553,11 +555,9 @@ void thermal_zone_device_update(struct
>>>>>> thermal_zone_device *tz,
>>>>>>         if (atomic_read(&in_suspend))
>>>>>>             return;
>>>>>>     -    if (!tz->ops->get_temp)
>>>>>> +    if (update_temperature(tz))
>>>>>>             return;
>>>>>>     -    update_temperature(tz);
>>>>>> -
>>>>>
>>>>> I think the patch does a bit more. Previously we continued running the
>>>>> code below even when the thermal_zone_get_temp() returned an error
>>>>> (due
>>>>> to various reasons). Now we stop and probably would not schedule next
>>>>> polling, not calling:
>>>>> handle_thermal_trip() and monitor_thermal_zone()
>>>>
>>>> I agree there is a change in the behavior.
>>>>
>>>>> I would left update_temperature(tz) as it was and not check the
>>>>> return.
>>>>> The function thermal_zone_get_temp() can protect itself from missing
>>>>> tz->ops->get_temp(), so we should be safe.
>>>>>
>>>>> What do you think?
>>>>
>>>> Does it make sense to handle the trip point if we are unable to read
>>>> the
>>>> temperature?
>>>>
>>>> The lines following the update_temperature() are:
>>>>
>>>>    - thermal_zone_set_trips() which needs a correct tz->temperature
>>>>
>>>>    - handle_thermal_trip() which needs a correct tz->temperature to
>>>> compare with
>>>>
>>>>    - monitor_thermal_zone() which needs a consistent tz->passive.
>>>> This one
>>>> is updated by the governor which is in an inconsistent state because
>>>> the
>>>> temperature is not updated.
>>>>
>>>> The problem I see here is how the interrupt mode and the polling mode
>>>> are existing in the same code path.
>>>>
>>>> The interrupt mode can call thermal_notify_framework() for critical/hot
>>>> trip points without being followed by a monitoring. But for the other
>>>> trip points, the get_temp is needed.
>>>
>>> Yes, I agree that we can bail out when there is no .get_temp() callback
>>> and even not schedule next polling in such case.
>>> But I am just not sure if we can bail out and not schedule the next
>>> polling, when there is .get_temp() populated and the driver returned
>>> an error only at that moment, e.g. indicating some internal temporary,
>>> issue like send queue full, so such as -EBUSY, or -EAGAIN, etc.
>>> The thermal_zone_get_temp() would pass the error to update_temperature()
>>> but we return, losing the next try. We would not check the temperature
>>> again.
>>
>> Hmm, right. I agree with your point.
>>
>> What about the following changes:
>>
>>   - Add the new APIs:
>>
>>     thermal_zone_device_critical(struct thermal_zone_device *tz);
>>       => emergency poweroff
>>
>>     thermal_zone_device_hot(struct thermal_zone_device *tz);
>>       => userspace notification
>
> They look promising, I have to look into the existing code.
> When they would be called?

They can be called directly by the driver when there is no get_temp
callback instead of calling thermal_zone_device_update, and by the usual
code path via handle_critical_trip function.

Also that can solve the issue [1] when registering a device which is
already too hot [1] by adding the ops in the thermal zone.

[1] https://lkml.org/lkml/2020/11/28/166

>>   - Add a big fat WARN when thermal_zone_device_update is called with
>> .get_temp == NULL because that must not happen.
> 
> Good idea
> 
>>
>> If the .get_temp is NULL it is because we only have a HOT/CRITICAL
>> thermal trip points where we don't care about the temperature and
>> governor decision, right ?
>>
> 
> That is a good question. Let me dig into the code. I would say yes - we
> don't have to hassle with governor in this circumstances.


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