Implement workaround for ERR051198
(https://www.nxp.com/docs/en/errata/IMX8MN_0N14Y.pdf)
PWM output may not function correctly if the FIFO is empty when a new SAR
value is programmed.
Description:
When the PWM FIFO is empty, a new value programmed to the PWM Sample
register (PWM_PWMSAR) will be directly applied even if the current timer
period has not expired. If the new SAMPLE value programmed in the
PWM_PWMSAR register is less than the previous value, and the PWM counter
register (PWM_PWMCNR) that contains the current COUNT value is greater
than the new programmed SAMPLE value, the current period will not flip
the level. This may result in an output pulse with a duty cycle of 100%.
Workaround:
Program the current SAMPLE value in the PWM_PWMSAR register before
updating the new duty cycle to the SAMPLE value in the PWM_PWMSAR
register. This will ensure that the new SAMPLE value is modified during
a non-empty FIFO, and can be successfully updated after the period
expires.
Write the old SAR value before updating the new duty cycle to SAR. This
avoids writing the new value into an empty FIFO.
This only resolves the issue when the PWM period is longer than 2us
(or <500kHz) because write register is not quick enough when PWM period is
very short.
Reproduce steps:
cd /sys/class/pwm/pwmchip1/pwm0
echo 2000000000 > period # It is easy to observe by using long period
echo 1000000000 > duty_cycle
echo 1 > enable
echo 8000 > duty_cycle # One full high pulse will be seen by scope
Fixes: 166091b189 ("[ARM] MXC: add pwm driver for i.MX SoCs")
Reviewed-by: Jun Li <jun.li@nxp.com>
Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20241008194123.1943141-1-Frank.Li@nxp.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>
a25351e4c7