[v2,0/3] pwm: stm32: Three fixes

Message

Uwe Kleine-König June 21, 2024, 2:37 p.m. UTC

Hello,

this series contains two fixes for the .apply() callback and a trivial
change to fix an error message in probe.

(Implicit) v1 can be found at
https://lore.kernel.org/linux-pwm/[e-mail]
Changes since then:

 - Improve working of the commit log of patch #2 (as noticed by myself
   after submission of v1)
 - Drop Cc: stable on patch #2, as the faulty commit isn't in a release
   yet.
 - patch #3 is new (but was sent out already separately)
 - Fix .apply in patch #2 for some more corner cases.
 - Added Trevor's Reviewed-by for patch #1. (I didn't apply it to patch
   #2 as this changed since then.)

The issue in patch #2 was found with CONFIG_PWM_DEBUG enabled. The
problematic setting was period = 941244 ns with input clk rate =
208877930 Hz. For this setting these hardware register values were
calculated:

	PSC = 3
	ARR = 0xbffe

resulting in a real period of 941238.741689943 ns. However a request for
941239 ns resulted in

	PSC = 2
	ARR = 0xfffd

which corresponds to 941229.1667195285 Hz. My error in reasoning was
that I thought I'd need

        period_ns * clkrate
   ------------------------------ ≤ max_arr
   NSEC_PER_SEC * (prescaler + 1)

but in fact the necessary equation is

        period_ns * clkrate
   ------------------------------ < max_arr + 1
   NSEC_PER_SEC * (prescaler + 1)

The value of the LHS fraction for the above mentioned request
(period = 941244 ns, clk rate = 208877930) with PSC = 2 is

	941244 * 208877930 / (1000000000 * 3) = 65535.03278164 > 0xffff = max_arr

But as it's still rounded down to 65535 it's a valid configuration.

Best regards
Uwe

Uwe Kleine-König (3):
  pwm: stm32: Refuse too small period requests
  pwm: stm32: Fix calculation of prescaler
  pwm: stm32: Fix error message to not describe the previous error path

 drivers/pwm/pwm-stm32.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)


base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0