[Bug 87171] 3.18: i855GM KMS regression

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87171

--- Comment #10 from Bruno <bonbons67 at internet.lu> ---
Done bisecting.

Offending commit is:
commit 69769f9a422bfc62e17399da3590c5e31ac37f24
Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
Date:   Fri Aug 15 01:22:08 2014 +0300

    drm/i915: Preserve VGACNTR bits from the BIOS

    My Fujistsu-Siemens Lifebook S6010 doesn't like to resume from
    S3 unless VGACNTR has been restore to the original value. The BIOS
    value in this case was 0x0124008E. Setting the "VGA disable" bit
    doesn't interfere with the S3 resume fortunately.

    Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
    Tested-by: Thomas Richter <richter at rus.uni-stuttgart.de>
    Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Reverting just this commit from 3.18 makes the display issue go away.


It seems i915_vgacntrl_reg() [VGACNTRL on i855GM] still has some effect on LVDS
configuration when VGA_DISP_DISABLE bit is set.

In i915_reg.h the following bits are defined:
# define VGA_DISP_DISABLE                       (1 << 31)
# define VGA_2X_MODE                            (1 << 30)
# define VGA_PIPE_B_SELECT                      (1 << 29)

Of those bits VGA_2X_MODE might very well be related to the end result I'm
seeing here. (my VGA mode is set to 2X mode if I'm not mistaken...)
I'm going to check VGACNTRL values as handled by this commit and check if
zeroing VGA_2X_MODE or VGA_PIPE_B_SELECT is sufficient to get proper output.

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