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

From: Jon Hunter <jonathanh@nvidia.com>
To: "Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: <linux-pwm@vger.kernel.org>, <linux-tegra@vger.kernel.org>,
	Jon Hunter <jonathanh@nvidia.com>
Subject: [PATCH 1/2] pwm: tegra: Improve required rate calculation
Date: Wed, 26 Oct 2022 11:13:04 +0100	[thread overview]
Message-ID: <20221026101305.30670-1-jonathanh@nvidia.com> (raw)

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);
 
 		err = dev_pm_opp_set_rate(pc->dev, required_clk_rate);
 		if (err < 0)
-- 
2.25.1

