Red Hat Bugzilla – Bug 647406
max_brightness value is incorrect for some chipsets
Last modified: 2012-08-16 13:39:57 EDT
Description of problem:
On a system with both nouveau and ACPI backlight controls, g-p-m prefers the nouveau ones. This is a problem for chipsets which aren't yet supported properly by nouveau (see bug 625171 for example).
Always, with certain chipsets
Steps to Reproduce:
1. Open g-p-m preferences
2. Manipulate brightness slider
1. Boot machine on battery power
2. Wait for g-p-m to dim brightness according to power-saving preferences
Brightness is set relative to nouveau's max_brightness value, which is incorrect for some chipsets (in my case, much too low). Consequently, I am plunged into darkness even at the highest setting.
Brightness is set relative to the ACPI max_brightness value, using the ACPI controls
Graphics card in question has an NV50 0x0af chipset
(In reply to comment #0)
> Brightness is set relative to nouveau's max_brightness value, which is
> incorrect for some chipsets (in my case, much too low). Consequently, I am
> plunged into darkness even at the highest setting.
Surely this is a nouveau bug then?
(In reply to comment #1)
> Surely this is a nouveau bug then?
Yes and no. ACPI is presumably a more reliable set of controls when using reverse-engineered drivers such as the ones that ship with Fedora by default, so surely g-p-m should prefer using these controls on ACPI systems which provide them? Or is there a reason why it doesn't?
(In reply to comment #2)
> Yes and no. ACPI is presumably a more reliable set of controls when using
> reverse-engineered drivers such as the ones that ship with Fedora by default,
> so surely g-p-m should prefer using these controls on ACPI systems which
> provide them? Or is there a reason why it doesn't?
No, g-p-m prefers the XOrg backlight control over the platform or ACPI system controls. Whether the hardware is reverse engineered or not makes no difference. On other systems using the xorg controls is more accurate than using the limited number of steps in the acpi driver.
I'll reassign this to the xorg component.
What about cases where xrandr does not support the backlight property? This is what is happening in my case, and g-p-m seems to fall back to gnome-power-backlight-helper which prefers the nouveau control over the ACPI one.
It's hard to deduce any consistent policy by looking at the priority order of backlight interfaces in gpm-backlight-helper.c, except that platform controls are preferred over ACPI for (only?) NVIDIA chipsets - a faulty policy in my opinion, as ACPI leaves the system in a much more consistent state than manipulating hardware registers directly. It seems that this interface should be prioritised down, or even removed completely.
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