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 16:14:11 -0800
From:	Keith Packard <keithp@...thp.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
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

Chris Wilson <chris@...is-wilson.co.uk> writes:

> Ok, so as no vgaarb_clients have yet been registered and so the call to
> grab the IO resource does not actually disable VGA IO routing to the
> nvidia card.

Yikes! This explains a lot.

> If you care to update the changelog to explain the problem is that
> vgaarb is ineffective before all clients are registered, then I think
> this is a good temporary hack. It should be possible for vgaarb to mark
> resources as locked if the device is interpretting IO access and has no
> method for disabling the IO grab (and then a vga_tryget() check here).

Sounds like vgaarb should not assume that all devices using VGA are
registered drivers. That also sounds like a significantly harder fix.

If SR01 is required before disabling VGA, then perhaps we should not
disable VGA at all in this case though. That sounds safer, although if
VGA was enabled, it will suck a bit more power?

-- 
keith.packard@...el.com

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