i915 backlight failure on resume with 2.6.27

[James Bottomley]

On Tue, 2008-12-02 at 13:08 +0000, Sergio Monteiro Basto wrote: 
> On Mon, 2008-12-01 at 11:42 -0600, James Bottomley wrote:
> > You probably remember the system, it's my fujitsu P7120 lifebook with
> > the funny backlight wiring.
> > 
> > Previously, suspend/resume was made to work by saving the PCI state
> > including the legacy backlight register setting (and worked just fine
> > when invoked with a hal quirk).
> > 
> > On FC9, with the 2.6.26 fedora kernels, hal no longer does anything and
> > relies on the i915 kernel driver.  This was perfectly fine, except that
> > the backlight was now being restored to full brightness on resume rather
> > than the setting on resume.  The other annoyance was that VT consoles
> > were now lost on resume (switching to them produces a black screen,
> > although switching back to vt7 where X is running is fine).
> > 
> > As of the 2.6.27 fedora kernels, the backlight is now off on resume,
> > which is even more annoying.  It can be turned on by doing a VT switch,
> > although all the console VTs are still blank, so there looks to be some
> > bug in the i915 driver that crept in between 2.6.26 and 2.6.27.
> 
> Hum as quick answer , kernel fedora 2.6.27 have change _DOS thing.
> So may be that is the problem.
> The fedora patch is talked about on lkml
> http://lkml.org/lkml/2008/11/10/233

Actually, they haven't.  I'm running  2.6.27.5-41.fc9.i686 and if I look
in the kernel SRPM for that kernel (by building it) the relevant area of
drivers/acpi/video.c is

/* acpi_video interface */

static int acpi_video_bus_start_devices(struct acpi_video_bus *video)
{
        return acpi_video_bus_DOS(video, 0, 0);
}

static int acpi_video_bus_stop_devices(struct acpi_video_bus *video)
{
        return acpi_video_bus_DOS(video, 0, 1);
}

So nothing's changed in that area since 2.6.26.

It could still be a disagreement between ACPI, BIOS and the driver over
who turns on the backlight, but the loss of the text console strongly
suggests that something is slightly wrong with the VGA modes.

> About this _DOS , I reported this problem a long time ago, which now
> is on http://bugzilla.kernel.org/show_bug.cgi?id=6001

James