[Bug 111095] Backlight flicker after suspend on certain Intel CherryTrail tablets

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111095

--- Comment #20 from Hans de Goede <jwrdegoede at fedoraproject.org> ---
I have an Terra Pad 1061 which has this too, it also has another problem only
90-100% of the brightness range of the backlight sysfs interface is usable.

I can fix the brightness range problem by disabling the pwm backlight support
in the i915 driver with a quick hack. Then I can control the backlight smoothly
using the entire duty-cycle range using /sys/class/pwm.

I believe that the problem is that the i915 driver overrides the pwm frequency.
At least the LPSS pwm driver supports the pwm atomic APIs now, which means the
frequency set by the GOP gets read back from the PWM controller.  I plan to add
support for the PWM atomic API to drivers/pwm/pwm-crc.c and then convert the
i915 pwm backlight code to use the atomic API and preserve the frequency.

This does raise the question of what to do if the GOP has not initialized the
PWM. I can test this on BYT / CHT hardware and see what happens when the GOP
has not initialized the panel.

I think that the flickering russianneuromancer is seeing might be related.

###

Mostly unrelated: I found out this week that on BYT with a DSI panel when using
the SoC/LPSS for PWM, and booting with a HDMI cable inserted so that the GOP
will not initialize the panel, the panel will not light up. The problems is
that when using the Crystal Cove PMIC for PWM we manually set the panel enable
GPIO, we need similar code for the SoC/LPSS case. So far I have tested 3 BYT +
DSI + LPSS devices and all 3 need this. I have writing a patch-set for this on
my to do list.

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
You are the assignee for the bug.
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/intel-gfx-bugs/attachments/20191114/768a9b32/attachment.html>