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pwm: imx27: Fix overflow for bigger periods 

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: aef1a3799b ("pwm: imx27: Fix rounding behavior")
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Tested-by: Johannes Pointner <johannes.pointner@br-automation.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
zero-sugar-mainline-defconfig
Uwe Kleine-König 2020-12-07 15:13:24 +01:00 committed by Thierry Reding
parent 2f81b51d0d
commit 1ce65396e6
1 changed files with 2 additions and 1 deletions
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3
drivers/pwm/pwm-imx27.c
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@ -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