pwm_bl on i.MX6Q broken on 5.4-RC1+

pwm_bl on i.MX6Q broken on 5.4-RC1+

[Adam Ford]

I have an i.MX6Q with an LCD and PWM controlled backlight.  On 5.4-RC1
through the current master (16 Oct 2019), the backlight does not come
on by default.  I can get it come on by manually setting the
brightness, but any video activity seems to blank the screen again
until I change the brightness again.

I bisected the patch to 01ccf903edd6 ("pwm: Let pwm_get_state() return
the last implemented state")

If I revert this patch on 5.4-rc3, the issue goes away.

I was hoping someone might have a suggestion on how to fix it to play
nicely with the iMX6Q PWM or revert if no solution can be found.

thank you,

adam

Re: pwm_bl on i.MX6Q broken on 5.4-RC1+

[Ondřej Jirman]

On Wed, Oct 16, 2019 at 02:55:54PM -0500, Adam Ford wrote:
> I have an i.MX6Q with an LCD and PWM controlled backlight.  On 5.4-RC1
> through the current master (16 Oct 2019), the backlight does not come
> on by default.  I can get it come on by manually setting the
> brightness, but any video activity seems to blank the screen again
> until I change the brightness again.

You might want to check PWM driver that's used on your SoC. I had a similar
breakage on Allwinner SoCs, and it was caused by a broken get_state callback.
The problem was there for a long time and it was only exposed by the commit you
bisected to.

regards,
	o.

> I bisected the patch to 01ccf903edd6 ("pwm: Let pwm_get_state() return
> the last implemented state")
> 
> If I revert this patch on 5.4-rc3, the issue goes away.
> 
> I was hoping someone might have a suggestion on how to fix it to play
> nicely with the iMX6Q PWM or revert if no solution can be found.
> 
> thank you,
> 
> adam

Re: pwm_bl on i.MX6Q broken on 5.4-RC1+

[Uwe Kleine-König]

Hello Ondřej,

On Sun, Oct 20, 2019 at 08:24:28PM +0200, Ondřej Jirman wrote:
> On Wed, Oct 16, 2019 at 02:55:54PM -0500, Adam Ford wrote:
> > I have an i.MX6Q with an LCD and PWM controlled backlight.  On 5.4-RC1
> > through the current master (16 Oct 2019), the backlight does not come
> > on by default.  I can get it come on by manually setting the
> > brightness, but any video activity seems to blank the screen again
> > until I change the brightness again.
> 
> You might want to check PWM driver that's used on your SoC. I had a similar
> breakage on Allwinner SoCs, and it was caused by a broken get_state callback.
> The problem was there for a long time and it was only exposed by the commit you
> bisected to.

I added Adam to the recipents of the Thread "backlight: pwm_bl:
configure pwm only once per backlight toggle" here on this list. In this
thread the problem is analysed and it's a bit different than for sun4i.
I assume we don't need to follow up on this report given the discussion
is already ongoing elsewhere.

Best regards
Uwe

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