On Thu, Oct 17, 2019 at 11:48:08AM +0200, Michal Vokáč wrote:

On 17. 10. 19 10:10, Uwe Kleine-König wrote:

quoted

A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let
pwm_get_state() return the last implemented state")) changed the
semantic of pwm_get_state() and disclosed an (as it seems) common
problem in lowlevel PWM drivers. By not relying on the period and duty
cycle being retrievable from a disabled PWM this type of problem is
worked around.

Apart from this issue only calling the pwm_get_state/pwm_apply_state
combo once is also more effective.

Signed-off-by: Uwe Kleine-König <redacted>
---
Hello,

There are now two reports about 01ccf903edd6 breaking a backlight. As
far as I understand the problem this is a combination of the backend pwm
driver yielding surprising results and the pwm-bl driver doing things
more complicated than necessary.

So I guess this patch works around these problems. Still it would be
interesting to find out the details in the imx driver that triggers the
problem. So Adam, can you please instrument the pwm-imx27 driver to
print *state at the beginning of pwm_imx27_apply() and the end of
pwm_imx27_get_state() and provide the results?

Note I only compile tested this change.

Hi Uwe,
I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+"
thread that I have a similar problem when you submitted this patch.

So here are my few cents:

My setup is as follows:
 - imx6dl-yapp4-draco with i.MX6Solo
 - backlight is controlled with inverted PWM signal
 - max brightness level = 32, default brightness level set to 32 in DT.

1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let
   pwm_get_state() return the last implemented state):

 - System boots to userspace and backlight is enabled all the time from
   power up.

   $ dmesg | grep state
   [    1.763381] get state end: -1811360608, enabled: 0

What is -1811360608? When I wrote "print *state" above, I thought about
something like:

	pr_info("%s: period: %u, duty: %u, polarity: %d, enabled: %d",
		__func__, state->period, state->duty_cycle, state->polarity, state->enabled);

A quick look into drivers/pwm/pwm-imx27.c shows that this is another
driver that yields duty_cycle = 0 when the hardware is off.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |