Re: [PATCH] thermal: generic-adc: Fix linear temperature approximation

[Laxman Dewangan <ldewangan@nvidia.com>]

On Thursday 20 October 2016 10:56 PM, Paweł Jarosz wrote:
>
>> It is Negative Coefficient Thermistor (NCT) and so adc reads are 
>> different.
>> Lower temp gives higher value.
>>
>> So table should be
>>
>> <7000 10
>> 9000 8>;
>>
>> temp1 = gti->lookup_table[2 * i];
>> temp2 = gti->lookup_table[2 * i - 2];
>>
>> and then use in following equation.
>>
>>>
>>>     adc_hi = gti->lookup_table[2 * i - 1];
>>>     adc_lo = gti->lookup_table[2 * i + 1];
>>>     temp = gti->lookup_table[2 * i];
>>>     temp -= (val - adc_lo) * (temp1 - temp2) / (adc_hi - adc_lo);
>
> val = 9
> adc_hi = 10
> adc_lo = 8
> temp = 9000 - (9 - 8)*(7000 - 9000)/(10 - 8)
> temp = 9000 - (1)*(-2000)/(2)
>
> temp = 9000 - (-1000) = 10000
>
> still wrong ... temp for val = 9 should be 8000


Your temp1 and temp2 calculation is something wrong.

temp1 = gti->lookup_table[2 * i];
temp2 = gti->lookup_table[2 * i - 2];
so temp1 = 9000, temp2 = 7000

temp -= (val - adc_lo) * (temp1 - temp2) / (adc_hi - adc_lo);

temp = 9000 - (9 - 8)*(9000 - 7000)/(10 - 8)
temp = 9000 - (1)*(2000)/(2) = 8000 which is expected.



>
> Maybe we could leave this patch as it is?
>


temp = gti->lookup_table[2 * i];
+ tlow = gti->lookup_table[2 * i - 2];
(modify) temp -= (val - adc_lo) * (temp - tlow) / (adc_hi - adc_lo);

This will gives you correct result.

Let me know if you can send the updated patch. Else I will send.

Thanks,
Laxman
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