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Date:   Thu, 21 Jul 2022 11:34:36 +0200
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Subject: Re: [PATCH v3 3/4] thermal/core: Build ascending ordered indexes for
 the trip points
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To:     Zhang Rui <rui.zhang@intel.com>, rafael@kernel.org
Cc:     quic_manafm@quicinc.com, amitk@kernel.org, lukasz.luba@arm.com,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220715210911.714479-1-daniel.lezcano@linaro.org>
 <20220715210911.714479-3-daniel.lezcano@linaro.org>
 <6d08939a167870ff7c1c83bb254fda5939f1d648.camel@intel.com>
 <04bd27a8-5148-c8eb-5687-83ef511618df@linaro.org>
 <7ee301b76beb679f89e946dfb5921a2d853bd8dc.camel@intel.com>
 <c0d3e523-d75a-d837-313f-bf46f3fab3b4@linaro.org>
 <e3bfb483aa6ce09756b38b3a568fac819b154beb.camel@intel.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <e3bfb483aa6ce09756b38b3a568fac819b154beb.camel@intel.com>
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On 19/07/2022 16:17, Zhang Rui wrote:
> On Tue, 2022-07-19 at 09:22 +0200, Daniel Lezcano wrote:
>> On 19/07/2022 03:14, Zhang Rui wrote:
>>> On Mon, 2022-07-18 at 15:21 +0200, Daniel Lezcano wrote:
>>>>
>>>> Hi Zhang,
>>>>
>>>> thanks for the review
>>>>
>>>> On 18/07/2022 07:28, Zhang Rui wrote:
>>>>> On Fri, 2022-07-15 at 23:09 +0200, Daniel Lezcano wrote:
>>>>
>>>> [ ... ]
>>>>
>>>>>> Instead of taking the risk of breaking the existing
>>>>>> platforms,
>>>>>> use an
>>>>>> array of temperature ordered trip identifiers and make it
>>>>>> available
>>>>>> for the code needing to browse the trip points in an ordered
>>>>>> way.
>>>>>>
>>>>>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>>>>>> ---
>>>>
>>>> [ ... ]
>>>>
>>>>>> +static void sort_trips_indexes(struct thermal_zone_device
>>>>>> *tz)
>>>>>> +{
>>>>>> +       int i, j;
>>>>>> +
>>>>>> +       for (i = 0; i < tz->trips; i++)
>>>>>> +               tz->trips_indexes[i] = i;
>>>>>> +
>>>>>> +       for (i = 0; i < tz->trips; i++) {
>>>>>> +               for (j = i + 1; j < tz->trips; j++) {
>>>>>> +                       int t1, t2;
>>>>>> +
>>>>>> +                       tz->ops->get_trip_temp(tz, tz-
>>>>>>> trips_indexes[i], &t1);
>>>>>
>>>>> This line can be moved to the upper loop.
>>>>
>>>> Right, thanks!
>>>>
>>>>>> +                       tz->ops->get_trip_temp(tz, tz-
>>>>>>> trips_indexes[j], &t2);
>>>>>> +
>>>>>
>>>>> what about the disabled trip points?
>>>>>
>>>>> we should ignore those trip points and check the return value
>>>>> to
>>>>> make
>>>>> sure we're comparing the valid trip_temp values.
>>>>
>>>> We don't have to care about, whatever the position, the
>>>> corresponding
>>>> trip id will be disabled by the trip init function before calling
>>>> this
>>>> one and ignored in the handle_thermal_trip() function
>>>
>>> hah, I missed this one and replied to your latest reply directly.
>>>
>>> The thing I'm concerning is that if we don't check the return
>>> value,
>>> for a disabled trip point, the trip_temp (t1/t2) returned is some
>>> random value, it all depends on the previous value set by last
>>> successful .get_trip_temp(), and this may screw up the sorting.
>>
>> The indexes array is the same size as the trip array, that makes the
>> code much less prone to errors.
>>
>> To have the same number of trip points, the index of the disabled
>> trip
>> must be inserted also in the array. We don't care about its position
>> in
>> the indexes array because it is discarded in the handle_trip_point()
>> function anyway. For this reason, the random temperature of the
>> disabled
>> trip point and the resulting position in the sorting is harmless.
>>
>> It is made on purpose to ignore the return value, so we have a
>> simpler code.
>>
> Let's take below case for example,
> say, we have three trip points 0, 1, 2, and trip point 1 is broken and
> disabled.
> 
> trip temp for trip point 0 is 10 and for trip point 2 is 20.
> .get_trip_temp(tz, 1, &t) fails, and t is an uninitialized random value
> 
> 
> Initial:
>     trip_indexes[0]=0,trip_indexes[1]=1,trip_indexes[2]=2
> step1:
>     i=0,j=1
>     get trip temp for trip point trip_indexes[0]=0 and trip_indexes[1]=1
>     trip point 1 returns trip temp 5, and it swaps with trip point 0
>     so
>     trip_indexes[0]=1,trip_indexes[1]=0,trip_indexes[2]=2
> step2:
>     i=0,j=2
>     get trip temp for trip point trip_indexes[0]=1 and trip_indexes[2]=2
>     trip point 1 returns trip temp 25, and it swaps with trip point 2
>     so
>     trip_indexes[0]=2,trip_indexes[1]=0,trip_indexes[2]=1
> 
> And the sorting is broken now.
> 
> please correct me if I'm missing anything.

Oh, nice! Thanks for the detailed explanation.

We can initialize t1 and t2 to INT_MAX, so if the get_trip_temp() fails, 
they will be set to the maximum temperature and it will be at the end of 
the array.

Alternatively, we check the disabled bit and set the temperature to INT_MAX.


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