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

From: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
To: Dimitri Fedrau <dima.fedrau@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Andrew Hepp <andrew.hepp@ahepp.dev>,
	linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iio: temperature: mcp9600: Fix temperature reading for negative values
Date: Sat, 27 Apr 2024 05:49:18 -0300	[thread overview]
Message-ID: <Ziy8DsMCeAGK79E7@debian-BULLSEYE-live-builder-AMD64> (raw)
In-Reply-To: <20240424185913.1177127-1-dima.fedrau@gmail.com>

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@gmail.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
> 
> 