linux-kernel - Re: [PATCH] iio: temperature: mcp9600: Fix temperature reading for negative values

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Message-ID: <Ziy8DsMCeAGK79E7@debian-BULLSEYE-live-builder-AMD64>
Date: Sat, 27 Apr 2024 05:49:18 -0300
From: Marcelo Schmitt <marcelo.schmitt1@...il.com>
To: Dimitri Fedrau <dima.fedrau@...il.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Andrew Hepp <andrew.hepp@...pp.dev>, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: temperature: mcp9600: Fix temperature reading for
 negative values

Hi Dimitri,

Interesting patch this one.
I think this does apply, although, the cold junction register has for sign bits
so I think we could also have a mask to clear those out.
Some code suggestions inline.

Regards,
Marcelo

On 04/24, Dimitri Fedrau wrote:
> Temperature is stored as 16bit value in two's complement format. Current
> implementation ignores the sign bit. Make it aware of the sign bit by
> using sign_extend32.
> 
> Fixes: 3f6b9598b6df ("iio: temperature: Add MCP9600 thermocouple EMF converter")
> Signed-off-by: Dimitri Fedrau <dima.fedrau@...il.com>
> ---
>  drivers/iio/temperature/mcp9600.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
> index 46845804292b..7a3eef5d5e75 100644
> --- a/drivers/iio/temperature/mcp9600.c
> +++ b/drivers/iio/temperature/mcp9600.c

#define MCP9600_COLD_JUNCTION_SIGN_MSK GENMASK(15,12)
..

> @@ -52,7 +52,8 @@ static int mcp9600_read(struct mcp9600_data *data,
>  
>  	if (ret < 0)
>  		return ret;
> -	*val = ret;
> +
> +	*val = sign_extend32(ret, 15);
	if (chan->address == MCP9600_COLD_JUNCTION)
		*val &= ~MCP9600_COLD_JUNCTION_SIGN_MSK;

>  
>  	return 0;
>  }
> -- 
> 2.39.2
> 
>