Re: [PATCH 2/2 v4] hwmon: (ntc_thermistor): try reading processed

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Mar 08, 2021 at 11:02:19AM +0100, Linus Walleij wrote:
> Before trying the custom method of reading the sensor
> as raw and then converting, we want to use
> iio_read_channel_processed_scale() which first tries to
> see if the ADC can provide a processed value directly,
> else reads raw and applies scaling inside of IIO
> using the scale attributes of the ADC. We need to
> multiply the scaled value with 1000 to get to
> microvolts from millivolts which is what processed
> IIO channels returns.
> 
> Keep the code that assumes 12bit ADC around as a
> fallback.
> 
> This gives correct readings on the AB8500 thermistor
> inputs used in the Ux500 HREFP520 platform for reading
> battery and board temperature.
> 
> Cc: Peter Rosin <peda@axentia.se>
> Cc: Chris Lesiak <chris.lesiak@licor.com>
> Cc: Jonathan Cameron <jic23@kernel.org>
> Cc: linux-iio@vger.kernel.org
> Link: https://lore.kernel.org/linux-iio/20201224011607.1059534-1-linus.walleij@linaro.org/
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Acked-by: Guenter Roeck <linux@roeck-us.net>

Up to Jonathan to decide if he wants to take both patches.

Thanks,
Guenter

> ---
> ChangeLog v3->v4:
> - Split out the new iio_read_channel_processed_scale()
>   API addition to a separate patch.
> - My suggestion is to apply both patches to the hwmon
>   tree.
> ChangeLog v2->v3:
> - After discussion about v2 we concludes that
>   iio_read_channel_processed() could loose precision
>   so we introduce a new API to read processed and
>   scale.
> - Include a link to the v2 discussion for reference.
> - For ease of applying to the hwmon tree, keep it all
>   in one patch.
> - This needs Jonathans ACK to be merged through hwmon.
> ChangeLog v1->v2:
> - Fix the patch to multiply the processed value by
>   1000 to get to microvolts from millivolts.
> - Fix up the confusion in the commit message.
> - Drop pointless comments about the code, we keep the
>   original code path around if processed reads don't
>   work, nothing bad with that.
> ---
>  drivers/hwmon/ntc_thermistor.c | 27 ++++++++++++++++++---------
>  1 file changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
> index 3aad62a0e661..8587189c7f15 100644
> --- a/drivers/hwmon/ntc_thermistor.c
> +++ b/drivers/hwmon/ntc_thermistor.c
> @@ -326,18 +326,27 @@ struct ntc_data {
>  static int ntc_adc_iio_read(struct ntc_thermistor_platform_data *pdata)
>  {
>  	struct iio_channel *channel = pdata->chan;
> -	int raw, uv, ret;
> +	int uv, ret;
>  
> -	ret = iio_read_channel_raw(channel, &raw);
> +	ret = iio_read_channel_processed_scale(channel, &uv, 1000);
>  	if (ret < 0) {
> -		pr_err("read channel() error: %d\n", ret);
> -		return ret;
> -	}
> +		int raw;
>  
> -	ret = iio_convert_raw_to_processed(channel, raw, &uv, 1000);
> -	if (ret < 0) {
> -		/* Assume 12 bit ADC with vref at pullup_uv */
> -		uv = (pdata->pullup_uv * (s64)raw) >> 12;
> +		/*
> +		 * This fallback uses a raw read and then
> +		 * assumes the ADC is 12 bits, scaling with
> +		 * a factor 1000 to get to microvolts.
> +		 */
> +		ret = iio_read_channel_raw(channel, &raw);
> +		if (ret < 0) {
> +			pr_err("read channel() error: %d\n", ret);
> +			return ret;
> +		}
> +		ret = iio_convert_raw_to_processed(channel, raw, &uv, 1000);
> +		if (ret < 0) {
> +			/* Assume 12 bit ADC with vref at pullup_uv */
> +			uv = (pdata->pullup_uv * (s64)raw) >> 12;
> +		}
>  	}
>  
>  	return uv;
> -- 
> 2.29.2
>