[Bug 108703] Screen sometimes jumps to full brightness on resume from suspend

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108703

--- Comment #7 from Jani Nikula <jani.nikula at intel.com> ---
I really can't think of any reason why this behaviour would have changed for
you because of a kernel change. For 10 years now, since the introduction of
i915 KMS support, we've had code in place to set the backlight to max if it's
at 0 on panel enable. (In retrospect, that was not such a good policy decision,
but I fear it's pretty much carved in stone now. Hindsight 20/20.)

I suspect something changed in your userspace so that it now sets brightness to
0 rather than some non-zero low value. That something would typically truncate
the 4438 levels provided by intel_backlight to a handful of discrete levels.

The kernel change to nuke the policy would technically be as simple as

diff --git a/drivers/gpu/drm/i915/intel_panel.c
b/drivers/gpu/drm/i915/intel_panel.c
index b6df63aa11e3..beb4801702d3 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1104,8 +1104,8 @@ void intel_panel_enable_backlight(const struct
intel_crtc_state *crtc_state,

        WARN_ON(panel->backlight.max == 0);

-       if (panel->backlight.level <= panel->backlight.min) {
-               panel->backlight.level = panel->backlight.max;
+       if (panel->backlight.level < panel->backlight.min) {
+               panel->backlight.level = panel->backlight.min;
                if (panel->backlight.device)
                        panel->backlight.device->props.brightness =
                                scale_hw_to_user(connector,

but if there's any userspace around that relies on the behaviour, we'd have to
revert back.

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