linux-kernel - Re: [PATCH v1] hwmon: max31790: revise the scale to write pwm

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Message-ID: <d82d5585-ec6c-4611-ad33-9c2d00745176@roeck-us.net>
Date: Fri, 12 Apr 2024 06:03:21 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Delphine CC Chiu <Delphine_CC_Chiu@...ynn.com>
Cc: patrick@...cx.xyz, Jean Delvare <jdelvare@...e.com>,
	linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] hwmon: max31790: revise the scale to write pwm

On Fri, Apr 12, 2024 at 11:25:58AM +0800, Delphine CC Chiu wrote:
> Since the value for PWMOUT Target Duty Cycle register is a 9 bit
> left-justified value that ranges from 0 to 511 and is contained in 2
> bytes.
> 
> There is an issue that the LSB of the 9 bit would always be zero if it
> just left shift 8 bit for the value that write to PWMOUT Target Duty
> Cycle register.
> 
> Therefore, revise the scale of the value that was writen to pwm input
> from 255 to 511 and modify the value to left-justified value.
> 

The only difference is that it writes 511 instead of 510. All other
values are the same. I am not sure if that is really worth the
trouble. It would have made a little more sense to me if you had
used DIV_ROUND_CLOSEST(), but you didn't do that. As it is, I really
don't understand the point. If it is really important to write 511
instead of 510, the commit description should explain that and not
talk about the last bit always being zero (which it still is after
this patch except, again, when writing 511 instead of 510).

Thanks,
Guenter

> Signed-off-by: Delphine CC Chiu <Delphine_CC_Chiu@...ynn.com>
> ---
>  drivers/hwmon/max31790.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/max31790.c b/drivers/hwmon/max31790.c
> index 3dc95196b229..bd201191da1c 100644
> --- a/drivers/hwmon/max31790.c
> +++ b/drivers/hwmon/max31790.c
> @@ -49,6 +49,9 @@
>  
>  #define NR_CHANNEL			6
>  
> +#define PWM_INPUT_SCALE	255
> +#define MAX31790_REG_PWMOUT_SCALE	511
> +
>  /*
>   * Client data (each client gets its own)
>   */
> @@ -343,10 +346,12 @@ static int max31790_write_pwm(struct device *dev, u32 attr, int channel,
>  			err = -EINVAL;
>  			break;
>  		}
> +
> +		val = val * MAX31790_REG_PWMOUT_SCALE / PWM_INPUT_SCALE;
>  		data->valid = false;
>  		err = i2c_smbus_write_word_swapped(client,
>  						   MAX31790_REG_PWMOUT(channel),
> -						   val << 8);
> +						   val << 7);
>  		break;
>  	case hwmon_pwm_enable:
>  		fan_config = data->fan_config[channel];
> -- 
> 2.25.1
>