[Intel-gfx] [PATCH] drm/intel: Only smash VGA SR01 register if intel is default VGA device

[Chris Wilson]

On Tue, Dec 17, 2013 at 09:12:10AM -0800, Keith Packard wrote:
> We want to disable the (unused) VGA plane on the intel hardware, which
> should be a simple matter of writing the vga control register. However,
> in 2009 (commit 24f119c769bacac5729297b682fec7811a983cc6), that simple
> code was changed to also smash the SR01 VGA register to fix "random
> crash and lockups".
> 
> When running efifb on an nVidia card, executing this store to SR01
> ends up causing the screen to go black.
> 
> I'm not sure if this store is still required; this patch limits it to
> when the intel card is primary.

The bspec still says we must assert SR01 bit5 prior to disabling the VGA
plane.

Perhaps the test should be whether (vga_reg & VGA_DISP_DISABLE) == 0 and
do nothing if the plane is already off.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre