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

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Date:	Tue, 17 Dec 2013 17:27:40 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Keith Packard <keithp@...thp.com>
Cc:	intel-gfx@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm/intel: Only smash VGA SR01 register if
 intel is default VGA device

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
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