linux-kernel - Re: [PATCH v2 3/3] hwmon: tmp421: fix rounding for negative values

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Message-ID: <20210924022141.GA3032469@roeck-us.net>
Date:   Thu, 23 Sep 2021 19:21:41 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Paul Fertser <fercerpav@...il.com>
Cc:     linux-hwmon@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] hwmon: tmp421: fix rounding for negative values

On Thu, Sep 23, 2021 at 12:48:01PM +0300, Paul Fertser wrote:
> Current code produces -24999 for 0b1110011100000000 input in standard
> format due to always rounding up rather than "away from zero". Use the
> common macro for division, unify and simplify the conversion code along
> the way.
> 
> Signed-off-by: Paul Fertser <fercerpav@...il.com>

This patch should probably have a Fixes: tag.

Guenter

> ---
> 
> Changes from v1:
>  - Trivial rebase
> 
>  drivers/hwmon/tmp421.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/hwmon/tmp421.c b/drivers/hwmon/tmp421.c
> index 9f0a4db695db..c77c9f3bbd21 100644
> --- a/drivers/hwmon/tmp421.c
> +++ b/drivers/hwmon/tmp421.c
> @@ -100,23 +100,17 @@ struct tmp421_data {
>  	s16 temp[4];
>  };
>  
> -static int temp_from_s16(s16 reg)
> +static int temp_from_raw(u16 reg, bool extended)
>  {
>  	/* Mask out status bits */
>  	int temp = reg & ~0xf;
>  
> -	return (temp * 1000 + 128) / 256;
> -}
> -
> -static int temp_from_u16(u16 reg)
> -{
> -	/* Mask out status bits */
> -	int temp = reg & ~0xf;
> -
> -	/* Add offset for extended temperature range. */
> -	temp -= 64 * 256;
> +	if (extended)
> +		temp = temp - 64 * 256;
> +	else
> +		temp = (s16)temp;
>  
> -	return (temp * 1000 + 128) / 256;
> +	return DIV_ROUND_CLOSEST(temp * 1000, 256);
>  }
>  
>  static int tmp421_update_device(struct tmp421_data *data)
> @@ -175,10 +169,8 @@ static int tmp421_read(struct device *dev, enum hwmon_sensor_types type,
>  
>  	switch (attr) {
>  	case hwmon_temp_input:
> -		if (tmp421->config & TMP421_CONFIG_RANGE)
> -			*val = temp_from_u16(tmp421->temp[channel]);
> -		else
> -			*val = temp_from_s16(tmp421->temp[channel]);
> +		*val = temp_from_raw(tmp421->temp[channel],
> +				     tmp421->config & TMP421_CONFIG_RANGE);
>  		return 0;
>  	case hwmon_temp_fault:
>  		/*