Patch "pwm: stm32: Improve precision of calculation in .apply()" has been added to the 6.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    pwm: stm32: Improve precision of calculation in .apply()

to the 6.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     pwm-stm32-improve-precision-of-calculation-in-.apply.patch
and it can be found in the queue-6.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e910bcd4d018e3f4b0f49efd8e10b11703b5ef58
Author: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Date:   Sun Mar 17 22:52:14 2024 +0100

    pwm: stm32: Improve precision of calculation in .apply()
    
    [ 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>

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);