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From: Guenter Roeck <linux@roeck-us.net>
To: Jonathan Cameron <jic23@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>
Cc: Jean Delvare <jdelvare@suse.com>,
	linux-hwmon@vger.kernel.org, Peter Rosin <peda@axentia.se>,
	Chris Lesiak <chris.lesiak@licor.com>,
	linux-iio@vger.kernel.org
Subject: Re: [PATCH] hwmon: (ntc_thermistor): Use library interpolation
Date: Sun, 4 Jul 2021 10:50:13 -0700	[thread overview]
Message-ID: <eecc85ba-0a8b-969b-3979-82304198437c@roeck-us.net> (raw)
In-Reply-To: <20210704171406.61235987@jic23-huawei>

On 7/4/21 9:14 AM, Jonathan Cameron wrote:
> On Sat,  3 Jul 2021 20:01:31 +0200
> Linus Walleij <linus.walleij@linaro.org> wrote:
> 
>> The kernel has a helper function for linear interpolation so
>> use it. It incidentally makes the code easier to read as well.
>>
>> Cc: Peter Rosin <peda@axentia.se>
>> Cc: Chris Lesiak <chris.lesiak@licor.com>
>> Cc: linux-iio@vger.kernel.org
>> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> 
> Potential precision problem because the multiplication by 1000 is now done
> post division whereas before it was before it?
> 
> I'm only eyeballing the code so no idea if it makes a practical difference.
> 

I think so, since the code now only reports temperatures in full degrees C
which wasn't the case earlier. That should be easy to fix though by
passing data->comp[low].temp_c * 1000 and data->comp[high].temp_c * 1000
as parameters to fixp_linear_interpolate().

Also, the if/else in get_temp_mc() seems now unnecessary since
fixp_linear_interpolate() effectively does the same checks.

Guenter

>> ---
>>   drivers/hwmon/ntc_thermistor.c | 11 +++++++----
>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
>> index 8587189c7f15..61bd0e074ec9 100644
>> --- a/drivers/hwmon/ntc_thermistor.c
>> +++ b/drivers/hwmon/ntc_thermistor.c
>> @@ -14,6 +14,7 @@
>>   #include <linux/err.h>
>>   #include <linux/of.h>
>>   #include <linux/of_device.h>
>> +#include <linux/fixp-arith.h>
>>   
>>   #include <linux/platform_data/ntc_thermistor.h>
>>   
>> @@ -557,10 +558,12 @@ static int get_temp_mc(struct ntc_data *data, unsigned int ohm)
>>   		/* Unable to use linear approximation */
>>   		temp = data->comp[low].temp_c * 1000;
>>   	} else {
>> -		temp = data->comp[low].temp_c * 1000 +
>> -			((data->comp[high].temp_c - data->comp[low].temp_c) *
>> -			 1000 * ((int)ohm - (int)data->comp[low].ohm)) /
>> -			((int)data->comp[high].ohm - (int)data->comp[low].ohm);
>> +		temp = fixp_linear_interpolate(data->comp[low].ohm,
>> +					       data->comp[low].temp_c,
>> +					       data->comp[high].ohm,
>> +					       data->comp[high].temp_c,
>> +					       ohm);
>> +		temp *= 1000;
>>   	}
>>   	return temp;
>>   }
> 


      reply	other threads:[~2021-07-04 17:50 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-03 18:01 Linus Walleij
2021-07-04 16:14 ` Jonathan Cameron
2021-07-04 17:50   ` Guenter Roeck [this message]

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