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

[Laxman Dewangan <ldewangan@nvidia.com>]

On Thursday 20 October 2016 08:26 PM, Paweł Jarosz wrote:
>
>
> W dniu 20.10.2016 o 14:40, Laxman Dewangan pisze:
>>
>> On Wednesday 19 October 2016 01:01 AM, Paweł Jarosz wrote:
>>> In current version of the driver there is error in temperature
>>> calculation.
>>>
>>> So lets fix it using proper linear function.
>>>
>>> Signed-off-by: Paweł Jarosz <paweljarosz3691@gmail.com>
>>
>> Per my calculation, existing and your equation is same as both are 
>> doing linear interpolation.
>>
>> Only think I have seen is that I have used 1000 for two consecutive 
>> temp difference and you used (t2 - t1), so your is better in this 
>> case but still the equation is same.
>>
>> The equation from you is
>>
>>     t1 + (hi -val) * factor.
>>
>> and existing one is
>>      t2 - (val -lo) * factor
>>
>>
>> factor is abs((t2-t1)/(hi-lo))
>>
>> If my analysis is not correct then can you please provide the example 
>> with calculation for better understansing?
>
> Hi
>
> For example if we try to calculate temp for val = 8, i = 1 and table:
>
> <0 9
>  2 7>
>
>     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) * 1000) / (adc_hi - adc_lo);
>
> Your method gives temp =  -498
>
>


The temp should be in the milli-centigrade in the table so it should be
<0 9
   2000 7>;

and if you relplace 1000 with the t2 -t1 where

t1  = gti->lookup_table[2*i -2]
t2 = gti->lookup_table[2 *i]

then it will work perfectly.

So your change for replacing 1000 is correct as what I said but need not 
to change whole equation altogether.

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