Re: [PATCH 1/2] pwm: tegra: Improve required rate calculation

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello,

On Wed, Oct 26, 2022 at 11:13:04AM +0100, Jon Hunter wrote:
> For the case where dev_pm_opp_set_rate() is called to set the PWM clock
> rate, the requested rate is calculated as ...
> 
>  required_clk_rate = (NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;
> 
> The above calculation may lead to rounding errors because the
> NSEC_PER_SEC is divided by 'period_ns' before applying the
> PWM_DUTY_WIDTH multiplication factor. For example, if the period is
> 45334ns, the above calculation yields a rate of 5646848Hz instead of
> 5646976Hz. Fix this by applying the multiplication factor before
> dividing and using the DIV_ROUND_UP macro which yields the expected
> result of 5646976Hz.
> 
> Fixes: 1d7796bdb63a ("pwm: tegra: Support dynamic clock frequency configuration")
> Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
> ---
>  drivers/pwm/pwm-tegra.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
> index dad9978c9186..8a33c500f93b 100644
> --- a/drivers/pwm/pwm-tegra.c
> +++ b/drivers/pwm/pwm-tegra.c
> @@ -145,8 +145,8 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  		 * source clock rate as required_clk_rate, PWM controller will
>  		 * be able to configure the requested period.
>  		 */
> -		required_clk_rate =
> -			(NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;
> +		required_clk_rate = DIV_ROUND_UP_ULL((NSEC_PER_SEC << PWM_DUTY_WIDTH),
> +						     period_ns);

This also has the nice side effect that required_clk_rate doesn't become
zero any more for big period_ns values.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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