linux-kernel - Re: [PATCH] thermal/core: Emit a warning if the thermal zone is updated without ops

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Date:   Tue, 8 Dec 2020 16:19:37 +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 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

 - Add a big fat WARN when thermal_zone_device_update is called with
.get_temp == NULL because that must not happen.

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 ?





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