[Bug 108225] [LPT SPT KBP] Acer V3-772G screen brightness cannot be adjusted with i915.fastboot enabled

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108225

--- Comment #47 from Tolga Cakir <cevelnet at gmail.com> ---
(In reply to Maarten Lankhorst from comment #46)
> Backlight turning off during S4 hibernate is expected. The kernel performs
> all steps of suspend. A whole snapshot of the memory is made. It then
> performs a resume so all devices are accessible again, and then the memory
> snapshot is written to disk. After this the system really shuts down.
> 
> This is probably what you're seeing when hibernating.
Thanks for the explanation!

> In the first patch to fix backlight on resume, can you change in
> intel_panel.c:
> if (panel->backlight.enabled && conn_state->crtc) {
> 
> to:
> if (panel->backlight.level && conn_state->crtc) {
> 
> ?
I have applied your mentioned changes. It indeed fixes the S4 resume issue. I
have tested fresh boot, S3 resume, S4 resume and couldn't observe any sort of
issues. Backlight levels are restored correctly and backlight controls work
fine after all these actions. No flicker occur after resuming to console from
all tested states (S3, S4, S5).

A single flicker occurs when S4 resuming to GNOME, but it looks like it's a
GNOME-related issue. Doesn't happen after S3 resume and I couldn't spot any
further issues, other than the single flicker. Maybe Hans has some insight on
this, as (afaik) he has worked on multiple projects regarding flickerfree
experience.

Furthermore, I have checked, whether enable_fbc=1 is affected by fastboot in
any way. According to dmesg, fbc gets enabled, regardless of fastboot. I have
verified this by measuring power consumption: with enable_fbc=1, my laptop
draws ~6.5W; without it draws 10W. Setting fastboot=1 doesn't affect it in any
way: I still achieve 6.5W power draw by enabling_fbc=1 and fastboot=1 at the
same time.

I couldn't test the effects on enable_psr=1, as it's bugged on my laptop
(PSR1). It allowed my laptop to enter PC7 on 4.18 kernels, but I had flickering
issues, so disabled it. 4.19.4 flickers even more badly and no PC7 anymore.
Latest 4.20.x doesn't seem to flicker anymore, but no PC7. So, I don't know, if
it *really* works (dmesg says it's enabled). Therefore, I can't say anything
about that.

All in all, I think this can be viewed as fixed. Feel free to add my Tested-by
Tag.

Thanks for all your efforts! If you need further testing, please let me know.
Also feel free to CC me, if you ever need me to test anything in the future for
HSW laptops.

Cheers,
Tolga

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