linux-kernel - [PATCH 5.10 481/717] pwm: imx27: Fix overflow for bigger periods

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Message-Id: <20201228125044.013444789@linuxfoundation.org>
Date:   Mon, 28 Dec 2020 13:47:59 +0100
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 <uwe@...ine-koenig.org>,
        Johannes Pointner <johannes.pointner@...automation.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.10 481/717] pwm: imx27: Fix overflow for bigger periods

From: Uwe Kleine-König <uwe@...ine-koenig.org>

[ Upstream commit 1ce65396e6b2386b4fd54f87beff0647a772e1cd ]

The second parameter of do_div is an u32 and NSEC_PER_SEC * prescale
overflows this for bigger periods. Assuming the usual pwm input clk rate
of 66 MHz this happens starting at requested period > 606060 ns.

Splitting the division into two operations doesn't loose any precision.
It doesn't need to be feared that c / NSEC_PER_SEC doesn't fit into the
unsigned long variable "duty_cycles" because in this case the assignment
above to period_cycles would already have been overflowing as
period >= duty_cycle and then the calculation is moot anyhow.

Fixes: aef1a3799b5c ("pwm: imx27: Fix rounding behavior")
Signed-off-by: Uwe Kleine-König <uwe@...ine-koenig.org>
Tested-by: Johannes Pointner <johannes.pointner@...automation.com>
Signed-off-by: Thierry Reding <thierry.reding@...il.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/pwm/pwm-imx27.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index c50d453552bd4..86bcafd23e4f6 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -235,8 +235,9 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	period_cycles /= prescale;
 	c = clkrate * state->duty_cycle;
-	do_div(c, NSEC_PER_SEC * prescale);
+	do_div(c, NSEC_PER_SEC);
 	duty_cycles = c;
+	duty_cycles /= prescale;
 
 	/*
 	 * according to imx pwm RM, the real period value should be PERIOD
-- 
2.27.0