Hello, On Wed, February 23, 2011 02:09, Linus Torvalds wrote: > On Tue, Feb 22, 2011 at 2:31 PM, Tino Keitel <tino.keitel at tikei.de> wrote: >> >> I just tried 2.6.38-rc6 on my ThinkPad X61s without any other DRM >> related patches, and my backlight issue is gone. > > I applied Indan's fix in -rc6 (commit 951f3512dba5), since it had > several testers and seemed to simplify the code nicely too.
Sadly, as so often in life, it's not correct. At this point I'm not sure if it's better to revert that patch and add a correct one, or to just fix it up. The end result is the same I suppose. I've also found more documentation, namely: ACPI_IGD_OpRegion_Spec.pdf, which has the ASLE stuff in intel_opregion.c, and VOL_1_graphics_core.pdf, which mentions LBPC (I was looking at 3 before). Apparently the undocumented stuff the old code did was correct. What I don't understand is how BIOS makers could know about those bits. The good side is that that big warning in my patch description is invalid, something else was going on: The BIOS used LBPC to set and restore brightness, while the driver only used BLC_PWM_CTL after my patch. All credits to Intel for making something simple as backlight control as stupid and complex as possible: - It has two registers to control brightness, sometimes one is used, sometimes the other, sometimes both, and it's unknown what the BIOS uses, and it's undefined what registers are restored by the BIOS after reboot/resume. - When using ACPI and ASLE, the kernel requests a brightness change via a standard ACPI method, which in turns lets the BIOS generate an ASLE interrupt, which is handled by the driver. The brightness to set is between 0 and 255, and the driver is supposed to store the current brightness in another register. That register stores the brightness in percentages, which is used by the BIOS to restore brightness. How it does that is undefined, so it can use either register. So the BIOS obviously knows how to change the brightness, and it's still seemed like a good idea to bother the driver with it. The ASLE interface is a mess. All in all, after my patch, systems using ASLE and a BIOS storing the brightness in LBPC stopped working. The reason it works without ASLE is because the brightness is never changed by the driver, the backlight is only enabled or disabled. I'd love to clean up the whole backlight mess, but it's too late in the RC cycle to do that. So please revert my patch and apply Takashi Iwai's, which fixes the most immediate bug without changing anything else. This should go in stable too.
