[Bug 90075] [BYT LPSS backlight] Acer Aspire Switch 10 backlight control

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90075

--- Comment #17 from Miha Frangež <miha.frangez at gmail.com> ---
Hi, Shobhit.

I've applied your new patches to 4.13.0-rc2 with PWM_LPSS_PLATFORM=y and
DRM_I915=m. No PWM chips showed up in /sys/class/pwm. (more details:
https://frangez.tech/switch10/4.13-rc2-switch10-17/)

Then I set PWM_CRC=y and this gave me pwmchip0 in sysfs. It only has one
channel, which would make sense for a backlight chip.
After exporting that channel however, things stop making sense.

Reading /sys/class/pwm/pwmchip0/pwm0/enable shows the channel disabled. Since
the screen is at full brightness, this could either mean that it's not used, or
that the device has some kind of fallback (so the UEFI team didn't have to mess
with PWM in firmware).

Writing random values into duty_cycle and period and enabling it succeeded, but
didn't do anything (no effect on backlight).
(more details: https://frangez.tech/switch10/4.13-rc2-switch10-17-16/)

If there is a CRC PWM chip in there, it's either not used for backlight or the
PWM_CRC driver can't control it.

Let me know if you need any more info.

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