linux-kernel - Re: [PATCH] regulator: qcom_spmi: Fix math of spmi_regulator_set_voltage_time

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Date:   Thu, 20 Jun 2019 15:55:29 +0200
From:   Niklas Cassel <niklas.cassel@...aro.org>
To:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Cc:     bjorn.andersson@...aro.org, lgirdwood@...il.com,
        broonie@...nel.org, jorge.ramirez-ortiz@...aro.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] regulator: qcom_spmi: Fix math of
 spmi_regulator_set_voltage_time_sel

On Wed, Jun 19, 2019 at 11:56:36AM -0700, Jeffrey Hugo wrote:
> spmi_regulator_set_voltage_time_sel() calculates the amount of delay
> needed as the result of setting a new voltage.  Essentially this is the
> absolute difference of the old and new voltages, divided by the slew rate.
> 
> The implementation of spmi_regulator_set_voltage_time_sel() is wrong.
> 
> It attempts to calculate the difference in voltages by using the
> difference in selectors and multiplying by the voltage step between
> selectors.  This ignores the possibility that the old and new selectors
> might be from different ranges, which have different step values.  Also,
> the difference between the selectors may encapsulate N ranges inbetween,
> so a summation of each selector change from old to new would be needed.
> 
> Lets avoid all of that complexity, and just get the actual voltage
> represented by both the old and new selector, and use those to directly
> compute the voltage delta.  This is more straight forward, and has the
> side benifit of avoiding issues with regulator implementations that don't
> have hardware register support to get the current configured range.
> 
> Fixes: e92a4047419c ("regulator: Add QCOM SPMI regulator driver")
> Reported-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> Reported-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@...aro.org>
> Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@...il.com>
> ---
>  drivers/regulator/qcom_spmi-regulator.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/regulator/qcom_spmi-regulator.c b/drivers/regulator/qcom_spmi-regulator.c
> index 13f83be50076..877df33e0246 100644
> --- a/drivers/regulator/qcom_spmi-regulator.c
> +++ b/drivers/regulator/qcom_spmi-regulator.c
> @@ -813,14 +813,10 @@ static int spmi_regulator_set_voltage_time_sel(struct regulator_dev *rdev,
>  		unsigned int old_selector, unsigned int new_selector)
>  {
>  	struct spmi_regulator *vreg = rdev_get_drvdata(rdev);
> -	const struct spmi_voltage_range *range;
>  	int diff_uV;
>  
> -	range = spmi_regulator_find_range(vreg);
> -	if (!range)
> -		return -EINVAL;
> -
> -	diff_uV = abs(new_selector - old_selector) * range->step_uV;
> +	diff_uV = abs(spmi_regulator_common_list_voltage(rdev, new_selector) -
> +		      spmi_regulator_common_list_voltage(rdev, old_selector));
>  
>  	return DIV_ROUND_UP(diff_uV, vreg->slew_rate);
>  }
> -- 
> 2.17.1
> 

Tested-by: Niklas Cassel <niklas.cassel@...aro.org>