This is an update on the ongoing issue with a backlight in Fedora. I first experienced it in Fedora 15, although I suppose it is a much longer story.
Basically, backlight works, then update, stops working, then update works again, sometimes it needs a little tuning, sometimes not. Simply it is a problem Fedora (or upstream kernel) developers struggle for a very long time and cannot fix it. However, in Windows it works for years without a single issue.
I have a friend, who is a fantastic Linux expert and now I understand why he always says "Linux, on server absolutely yes, on workstation - no way."
And he is the update for Fedora 21 on Acer Aspire 5742.
acpi_backlight=vendor option does not work, even though it pretty much, nearly always was a good fix.
video.use_native_backlight=1 does not work at all.
pcie_aspm=force acpi_osi=Linux acpi_backlight=vendor partially works, it changes the brightness of the screen while using Fn keys, however the slider in Gnome settings and extension does not have effect at all.
Let's see what the next update will bring. ;-)