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pwm: stm32: Fix calculation of prescaler

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BugLink: https://bugs.launchpad.net/bugs/2076435

commit dab8f9f0fe3aada61c0eb013dcf7d3ff75a2c336 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>
This commit is contained in:
Uwe Kleine-König
2024-06-21 16:37:13 +02:00
committed by Stefan Bader
parent c7ff906634
commit 59651ef7f8
1 changed files with 14 additions and 10 deletions
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drivers/pwm/pwm-stm32.c
+14 -10
View File
@@ -1,5 +1,5 @@
// SPDX-License-Identifier: GPL-2.0
/*
// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (C) STMicroelectronics 2016
*
* Author: Gerald Baeza <gerald.baeza@st.com>
@@ -321,19 +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;
if (prescaler > MAX_TIM_PSC)
return -EINVAL;
(u64)NSEC_PER_SEC * ((u64)priv->max_arr + 1));
prd = mul_u64_u64_div_u64(period_ns, clk_get_rate(priv->clk),
(u64)NSEC_PER_SEC * (prescaler + 1));