Results of requested backlight testing for 6.1

[hansdegoede]

I have received quite a few test reports in response to my previous blog post. Many thanks to everyone who has run the tests and send me their results!

These tests show that as a result of the current 6.1 changes quite a few laptop models will end up with an empty "/sys/class/backlight", breaking users ability to control their laptop panel's brightness.

I have submitted a patch-set for 6.1 upstream to fix this.

More detailed summary/analysis of the received test reports:

• Known Windows laptop models affected by this:
  • Acer Aspire 1640
  • HP Compaq nc6120
  • IBM ThinkPad X40
  • System76 Starling Star1
• Known MacBook models affected by this:
  • Apple MacBook 2.1
  • Apple MacBook 4.1
  • Apple MacBook Pro 7.1
• 30 unaffected models
• The Dell Inspiron N4010 has acpi_video support and acpi_osi_is_win8() returns false, so acpi_video_get_backlight_type() returns acpi_video, but acpi_video fails to register a backlight device due to a _BCM eval error. This model already has a DMI quirk for this, so it is unaffected.
• The following laptop models use vendor backlight control method, while also having a native backlight entry under /sys/class/backlight:
  • Asus EeePC 901 (native backlight confirmed to work better then vendor)
  • Dell Latitude D610 (native backlight confirmed to also work)
  • Sony Vaio PCG-FRV3 (native backlight control does not work, BIOS from 2003!)

The fixes for 6.1 restore the behavior where userspace can see multiple entries under "/sys/class/backlight" for a single panel and the kernel leaves figuring out which one actually works up to userspace. This is undesirable and having more then 1 backlight device for a single panel also blocks the new backlight userspace API work which I have planned.

This first round of testing has shown that native works well even on systems so old that they don't have acpi_video backlight control support.

So I have prepared a patch series to try again with 6.2 by making native be preferred over vendor, which should avoid the problems seen with the 6.1 changes before the fixes.