linux-kernel - Re: [PATCH] ina3221: correct the update

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Date:   Tue, 8 Nov 2022 14:24:13 +0100
From:   Thierry Reding <treding@...dia.com>
To:     Ninad Malwade <nmalwade@...dia.com>
Cc:     jonathanh@...dia.com, linux@...ck-us.net, jdelvare@...e.com,
        nicolinc@...dia.com, rkasirajan@...dia.com,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] ina3221: correct the update_interval value

On Tue, Nov 08, 2022 at 01:00:29PM +0800, Ninad Malwade wrote:
> As per the INA3221 datasheet the samples value should not be
> considered while calculating the update_interval value.
> Section 8.4.2.2 from datasheet says - "The conversion-time
> settings, along with the programmable-averaging mode, enable
> the INA3221 to optimize available timing requirements in a given
> application. For example, if a system requires data to be read
> every 2 ms with all three channels monitored, configure the INA3221
> with the conversion times for the shunt- and bus-voltage
> measurements set to 332 μs"
> 
> As per above only conversion time and number of channels are
> required to set the update_interval value. Correcting the same in
> the driver.
> 
> Signed-off-by: Ninad Malwade <nmalwade@...dia.com>
> ---
>  Documentation/hwmon/ina3221.rst | 3 +--
>  drivers/hwmon/ina3221.c         | 4 +---
>  2 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/hwmon/ina3221.rst b/Documentation/hwmon/ina3221.rst
> index 8c12c54d2c24..a4f107d1e489 100644
> --- a/Documentation/hwmon/ina3221.rst
> +++ b/Documentation/hwmon/ina3221.rst
> @@ -61,10 +61,9 @@ samples                 Number of samples using in the averaging mode.
>  
>  update_interval         Data conversion time in millisecond, following:
>  
> -                          update_interval = C x S x (BC + SC)
> +                          update_interval = C x (BC + SC)
>  
>                            * C:	number of enabled channels
> -                          * S:	number of samples
>                            * BC:	bus-voltage conversion time in millisecond
>                            * SC:	shunt-voltage conversion time in millisecond
>  
> diff --git a/drivers/hwmon/ina3221.c b/drivers/hwmon/ina3221.c
> index 2a57f4b60c29..e3aa57e3b039 100644
> --- a/drivers/hwmon/ina3221.c
> +++ b/drivers/hwmon/ina3221.c
> @@ -183,11 +183,9 @@ static const int ina3221_avg_samples[] = {
>  static inline u32 ina3221_interval_ms_to_conv_time(u16 config, int interval)
>  {
>  	u32 channels = hweight16(config & INA3221_CONFIG_CHs_EN_MASK);
> -	u32 samples_idx = INA3221_CONFIG_AVG(config);
> -	u32 samples = ina3221_avg_samples[samples_idx];
>  
>  	/* Bisect the result to Bus and Shunt conversion times */
> -	return DIV_ROUND_CLOSEST(interval * 1000 / 2, channels * samples);
> +	return DIV_ROUND_CLOSEST(interval / 2, channels);

Why do you drop the * 1000 multiplication factor? That seems to be there
to account for the input "interval" being specified in ms whereas the
values in ina3221_conv_time[] are in us. Isn't this new code going to
give us values that are in the wrong unit?

Thierry

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