On Thu, Aug 27, 2020 at 10:55:28PM +0300, Andrey Lebedev wrote: > Hello, > > I think I'm experiencing problem with pwm-sun4i module. I'll describe > the symptoms first. > > I have a device, based on Allwinner A20 (Cubieboard 2) with LVDS display > that has a PWM-based backlight. The problem is: when linux shuts down, > the backlight stays on. I expect it to be turned off. This used to work > as expected on kernel 5.2-rc2, but after upgrade to 5.8 the backlight > does not turn off anymore (most of the times, see below). > > The backlight is configured in the device tree [1]. The brightness can > be changed by writing to "brightness" file on sysfs. So, linux can > control the PWM line. Backlight sysfs directory also has a "bl_power" > file, which can accept "0" to power on or "4" to power off the backlight > (according to [2]). > > Now, writing "4" to bl_power sometimes turns the backlight off and > sometimes not. I've found that the probability of backlight turning off > pretty much correlates with the current screen brightness: on 100% > brightness it will never turn off, on 50% brightness it will turn off on > about half of the times. When backlight does not turn off, it goes on > full brightness. It feels like the line, controlled by pwm stays in > whatever state it was the moment backlight was powered down - either > full 1 or 0. > > The pwm backlight device driver (pwm_bl) requests to set the duty cycle > to 0 and disable the pwm with the same request [3], but I suspect the > implementation driver (pwm-sun4i) does not actually set the duty cycle > to 0 before disabling the pulse width modulation. > > Is there anything that can be done to fix this?
There's some rather odd logic in sun4i_pwm_apply() that results in the PWM being disabled twice... once when it applies the initial config and again after waiting for a duty_cycle. I suspect disabling the initial disable would solve your issue... but it might provoke some new ones! Anyhow, try removing the else clause starting at line 299 and see what happens: https://elixir.bootlin.com/linux/v5.8/source/drivers/pwm/pwm-sun4i.c#L299 Daniel.
