[PATCH] drm/i915: fix inconsistent brightness after resume

[Jani Nikula]

On Sat, 10 Jan 2015, Jeremiah Mahler <jmmahler at gmail.com> wrote:
> Commit 6dda730e55f4 introduced a bug which resulted in inconsistent
> brightness levels on different machines. If a suspended was entered
> with the screen off some machines would resume with the screen
> at minimum brightness and others at maximum brightness.
>
> The following commands can be used to produce this behavior.
>
>   xset dpms force off
>   sleep 1
>   sudo systemctl suspend
>   (resume ...)
>
> The root cause of this problem is a comparison which checks to see if
> the backlight level is zero when the panel is enabled.  If it is zero,
> it is set to the maximum level.  Unfortunately, not all machines have a
> minimum level of zero. On those machines the level is left at the
> minimum instead of begin set to the maximum.

Good catch!

I think part of the problem is that the userspace sets brightness to
minimum before suspend, but apparently does not restore it after
resume. The dmesg would confirm this. But I guess it doesn't matter,
since we're pretty much stuck with having to do this anyway.

> Fix the bug by updating the comparison to check for the minimum
> backlight level instead of zero.
>
> Fixes: 6dda730e55f4 ("respect the VBT minimum backlight brightness")
> Signed-off-by: Jeremiah Mahler <jmmahler at gmail.com>
> ---
>  drivers/gpu/drm/i915/intel_panel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 4d63839..4ef4d66 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -962,7 +962,7 @@ void intel_panel_enable_backlight(struct intel_connector *connector)
>  
>  	WARN_ON(panel->backlight.max == 0);
>  
> -	if (panel->backlight.level == 0) {
> +	if (panel->backlight.level == panel->backlight.min) {

Perhaps <= instead of == would be safest?


BR,
Jani.

>  		panel->backlight.level = panel->backlight.max;
>  		if (panel->backlight.device)
>  			panel->backlight.device->props.brightness =
> -- 
> 2.1.4
>

-- 
Jani Nikula, Intel Open Source Technology Center