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

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Message-ID: <dd5f9f97-ab30-5bb0-1211-66d211035968@arm.com>
Date:   Tue, 8 Dec 2020 14:37:26 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
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 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.

> 
> IMHO, we should return if update_temperature() is failing.
> 
> Perhaps, it would make sense to simply prevent to register a thermal
> zone if the get_temp ops is not defined.
> 
> AFAICS, if the interrupt mode without get_temp callback are for hot and
> critical trip points which can be directly invoked from the sensor via a
> specified callback, no thermal zone would be needed in this case.
> 
> 
>