linux-kernel - [PATCH 5.4 312/314] pwm: jz4740: Enhance precision in calculation of duty cycle

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Date:   Tue, 23 Jun 2020 21:58:27 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Paul Cercueil <paul@...pouillou.net>,
        Thierry Reding <thierry.reding@...il.com>
Subject: [PATCH 5.4 312/314] pwm: jz4740: Enhance precision in calculation of duty cycle

From: Paul Cercueil <paul@...pouillou.net>

commit 9017dc4fbd59c09463019ce494cfe36d654495a8 upstream.

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>
Signed-off-by: Thierry Reding <thierry.reding@...il.com>
[ukl: backport to v5.4.y and adapt commit log accordingly]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/pwm/pwm-jz4740.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/pwm/pwm-jz4740.c
+++ b/drivers/pwm/pwm-jz4740.c
@@ -108,8 +108,8 @@ static int jz4740_pwm_apply(struct pwm_c
 	if (prescaler == 6)
 		return -EINVAL;
 
-	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)