linux-kernel - [PATCH v3 2/4] pwm: jz4740: Enhance precision in calculation of duty cycle

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Message-Id: <20200527115225.10069-2-paul@crapouillou.net>
Date:   Wed, 27 May 2020 13:52:23 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     od@...c.me, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paul Cercueil <paul@...pouillou.net>,
        stable@...r.kernel.org
Subject: [PATCH v3 2/4] pwm: jz4740: Enhance precision in calculation of duty cycle

Calculating the hardware value for the duty from the hardware value of
the period resulted in a precision loss versus calculating it from the
clock rate directly.

(Also remove a cast that doesn't really need to be here)

Fixes: f6b8a5700057 ("pwm: Add Ingenic JZ4740 support")
Cc: <stable@...r.kernel.org>
Suggested-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Signed-off-by: Paul Cercueil <paul@...pouillou.net>
---

Notes:
    v2: New patch. I don't consider this a fix but an enhancement, since the old
    	behaviour was in place since the driver was born in ~2010, so no Fixes tag.
    v3: Add Fixes tag and Uwe's Reviewed-by

 drivers/pwm/pwm-jz4740.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
index 3cd5c054ad9a..4fe9d99ac9a9 100644
--- a/drivers/pwm/pwm-jz4740.c
+++ b/drivers/pwm/pwm-jz4740.c
@@ -158,11 +158,11 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	/* Calculate period value */
 	tmp = (unsigned long long)rate * state->period;
 	do_div(tmp, NSEC_PER_SEC);
-	period = (unsigned long)tmp;
+	period = tmp;
 
 	/* Calculate duty value */
-	tmp = (unsigned long long)period * state->duty_cycle;
-	do_div(tmp, state->period);
+	tmp = (unsigned long long)rate * state->duty_cycle;
+	do_div(tmp, NSEC_PER_SEC);
 	duty = period - tmp;
 
 	if (duty >= period)
-- 
2.26.2