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| author |  Uwe Kleine-König <u.kleine-koenig@baylibre.com>
| 2024-06-21 16:37:13 +0200 |
|---|---|---|
| committer |  Uwe Kleine-König <ukleinek@kernel.org>
| 2024-06-22 16:13:19 +0200 |
| commit | dab8f9f0fe3aada61c0eb013dcf7d3ff75a2c336 (patch) | |
| tree | a70724c2aaafbc57849e0ab6a0b3f42bdb92f65a /drivers | |
| parent | pwm: stm32: Refuse too small period requests (diff) | |
| download | linux-dab8f9f0fe3aada61c0eb013dcf7d3ff75a2c336.tar.gz linux-dab8f9f0fe3aada61c0eb013dcf7d3ff75a2c336.tar.bz2 linux-dab8f9f0fe3aada61c0eb013dcf7d3ff75a2c336.zip | |
pwm: stm32: Fix calculation of prescaler
A small prescaler is beneficial, as this improves the resolution of the
duty_cycle configuration. However if the prescaler is too small, the
maximal possible period becomes considerably smaller than the requested
value.
One situation where this goes wrong is the following: With a parent
clock rate of 208877930 Hz and max_arr = 0xffff = 65535, a request for
period = 941243 ns currently results in PSC = 1. The value for ARR is
then calculated to
ARR = 941243 * 208877930 / (1000000000 * 2) - 1 = 98301
This value is bigger than 65535 however and so doesn't fit into the
respective register field. In this particular case the PWM was
configured for a period of 313733.4806027616 ns (with ARR = 98301 &
0xffff). Even if ARR was configured to its maximal value, only period =
627495.6861167669 ns would be achievable.
Fix the calculation accordingly and adapt the comment to match the new
algorithm.
With the calculation fixed the above case results in PSC = 2 and so an
actual period of 941229.1667195285 ns.
Fixes: 8002fbeef1e4 ("pwm: stm32: Calculate prescaler with a division instead of a loop")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/b4d96b79917617434a540df45f20cb5de4142f88.1718979150.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>
Diffstat (limited to 'drivers')
| -rw-r--r-- | drivers/pwm/pwm-stm32.c | 18 |
1 files changed, 12 insertions, 6 deletions
diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c index 3e7b2a8e34e7..97d3de24f312 100644 --- a/drivers/pwm/pwm-stm32.c +++ b/drivers/pwm/pwm-stm32.c @@ -321,17 +321,23 @@ static int stm32_pwm_config(struct stm32_pwm *priv, unsigned int ch, * First we need to find the minimal value for prescaler such that * * period_ns * clkrate - * ------------------------------ + * ------------------------------ < max_arr + 1 * NSEC_PER_SEC * (prescaler + 1) * - * isn't bigger than max_arr. + * This equation is equivalent to + * + * period_ns * clkrate + * ---------------------------- < prescaler + 1 + * NSEC_PER_SEC * (max_arr + 1) + * + * Using integer division and knowing that the right hand side is + * integer, this is further equivalent to + * + * (period_ns * clkrate) // (NSEC_PER_SEC * (max_arr + 1)) ≤ prescaler */ prescaler = mul_u64_u64_div_u64(period_ns, clk_get_rate(priv->clk), - (u64)NSEC_PER_SEC * priv->max_arr); - if (prescaler > 0) - prescaler -= 1; - + (u64)NSEC_PER_SEC * ((u64)priv->max_arr + 1)); if (prescaler > MAX_TIM_PSC) return -EINVAL; |