On Tuesday, September 10, 2013 04:53:40 PM Jani Nikula wrote: > On Mon, 09 Sep 2013, "Rafael J. Wysocki" <rjw@xxxxxxx> wrote: > > On Monday, September 09, 2013 05:21:18 PM Daniel Vetter wrote: > >> On Mon, Sep 09, 2013 at 02:16:12PM +0200, Rafael J. Wysocki wrote: > >> > Hi, > >> > > >> > On Monday, September 09, 2013 11:32:10 AM Daniel Vetter wrote: > >> > > Hi Aaaron, > >> > > > >> > > Have we grown any clue meanwhile about which Intel boxes need this and for > >> > > which we still need to keep the acpi backlight around? > >> > > >> > First of all, there is a bunch of boxes where ACPI backlight works incorrectly > >> > because of the Win8 compatibility issue. [In short, if we say we are compatible > >> > with Win8, the backlight AML goes into a special code path that is broken on > >> > those machines. Presumably Win8 uses native backlight control on them and that > >> > AML code path is never executed there.] The collection of machines with this > >> > problem appears to be kind of random (various models from various vendors), but > >> > I *think* they are machines that originally shipped with Win7 with a Win8 > >> > "upgrade" option (which in practice requires the BIOS to be updated anyway). > >> > > >> > Because of that, last time we tried to switch all of the systems using i915 > >> > and telling the BIOS that they are compatible with Win8 over to native backlight > >> > control, but that didn't work for two reasons. The first reason is that some > >> > user space doesn't know how to use intel_backlight and needs to be taught about > >> > that (so some systems are just not ready for that switch). The second and more > >> > fundamental reason is that the native backlight control simply doesn't work on > >> > some machines and we don't seem to have any idea why and how to debug this > >> > particular problem (mostly because we don't have enough information and we > >> > don't know what to ask for). > >> > > >> > > I've grown _very_ reluctant to just adding tons of quirks to our driver for > >> > > the backlight. > >> > > > >> > > Almost all the quirks we have added recently (or that have been proposed > >> > > to be added) are for the backlight. Imo that indicates we get something > >> > > fundamentally wrong ... > >> > > >> > This thing isn't really a quirk. It rather is an option for the users of > >> > the systems where ACPI backlight doesn't work to switch over to the native > >> > backlight control using a command line switch. This way they can at least > >> > *see* if the native backlight control works for them and (hopefully) report > >> > problems if that's not the case. This gives us a chance to get more > >> > information about what the problem is and possibly to make some progress > >> > without making changes for everyone, reverting those changes when they don't > >> > work etc. > >> > > >> > An alternative for them is to pass acpi_osi="!Windows 2012" which will probably > >> > make the ACPI backlight work for them again, but this rather is a step back, > >> > because we can't possibly learn anything new from that. > >> > >> If Win8 is as broken as we are I'm ok with the module option. It just > >> sounded to me like right now we don't know of a way to make all machines > >> somewhat happy, combined with the other pile of random backlight issues > >> the assumption that we do something (maybe something a bit racy) that > >> windows doesn't do isn't too far-fetched. So I'm not wary of the machines > >> where the aml is busted for acpi_os=win8, but for the others where this > >> broke stuff. > >> > >> Or do I miss something here? > > > > The ACPI video driver doesn't do anything new now. It only does things that > > did work before we started to tell BIOSes we're compatible with Win8. The > > reason why they don't work on some machines now is not related to whether or > > not Win8 is broken, but to what is there in the ACPI tables on those machines. > > That *is* pure garbage, but Win8 users don't see that (presumably, because > > Win8 does't execute the AML in question). We don't see that either with > > acpi_osi="!Windows 2012" (because then we don't execute that AML either). > > > > Whether or not Win8 is broken doesn't matter here. What matters is that we > > have to deal with broken AML somehow. > > > > One way may be to tell everyone affected by this to pass > > acpi_osi="!Windows 2012" in the kernel command line or possibly create a > > blacklist of those machines in the kernel (which Felipe has been pushing for > > recently) and wash our hands clean of this, but that leaves some exceptionally > > bad taste in my mouth. > > > > The alternative is to try to use native backlight in i915, which we did, but > > that didn't work on some machines. I don't think we will know why it didn't > > work there before we collect some more information and that's not possible > > without user testing. So, the idea is to make that testing possible without > > hacking the kernel and that's why we're introducing the new command line > > switch. And we're going to ask users to try it and report back. > > The thing that slightly bugs me with the proposed patches is that > they're adding a module parameter to i915 to tell ACPI video driver > whether to quirk the backlight or not. Before you know, we *will* have > requests to add quirks to i915 to tell ACPI video driver this. > > I think the parameter "Does the ACPI backlight interface work or not" > belongs to the ACPI video driver. > > Feel free to file this in your bikeshedding folder, but I think i915 > should only tell ACPI "I have a native backlight interface". It kind of does that already through the intel_opregion_init() thing, so it would be trivial to test intel_opregion_present() in acpi_video_register(). So yes, we can add a command line option, say 'use_native_backlight' to the ACPI video driver that will work like this: "if the Intel opregion is present and video.use_native_backlight is set, skip registering ACPI backlight". Is that what you want? 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