6.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> [ Upstream commit e419617847b688799a91126eb6c94b936bfb35ff ] While mathematically it's ok to calculate the number of cyles for the duty cycle as: duty_cycles = period_cycles * duty_ns / period_ns this doesn't always give the right result when doing integer math. This is best demonstrated using an example: With the input clock running at 208877930 Hz a request for duty_cycle = 383 ns and period = 49996 ns results in period_cycles = clkrate * period_ns / NSEC_PER_SEC = 10443.06098828 Now calculating duty_cycles with the above formula gives: duty_cycles = 10443.06098828 * 383 / 49996 = 80.00024719 However with period_cycle truncated to an integer results in: duty_cycles = 10443 * 383 / 49996 = 79.99977998239859 So while a value of (a little more than) 80 would be the right result, only 79 is used here. The problem here is that 14443 is a rounded result that should better not be used to do further math. So to fix that use the exact formular similar to how period_cycles is calculated. Link: https://lore.kernel.org/r/7628ecd8a7538aa5a7397f0fc4199a077168e8a6.1710711976.git.u.kleine-koenig@xxxxxxxxxxxxxx Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> Stable-dep-of: c45fcf46ca23 ("pwm: stm32: Refuse too small period requests") Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- drivers/pwm/pwm-stm32.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c index 0c028d17c0752..d50194ad24b1c 100644 --- a/drivers/pwm/pwm-stm32.c +++ b/drivers/pwm/pwm-stm32.c @@ -351,8 +351,9 @@ static int stm32_pwm_config(struct stm32_pwm *priv, unsigned int ch, regmap_set_bits(priv->regmap, TIM_CR1, TIM_CR1_ARPE); /* Calculate the duty cycles */ - dty = prd * duty_ns; - do_div(dty, period_ns); + dty = (unsigned long long)clk_get_rate(priv->clk) * duty_ns; + do_div(dty, prescaler + 1); + do_div(dty, NSEC_PER_SEC); regmap_write(priv->regmap, TIM_CCR1 + 4 * ch, dty); -- 2.43.0
