Re: [PATCH] i915: Update VGA arbiter support for newer devices

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Dave Airlie <airlied@gmail.com>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	Dave Airlie <airlied@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] i915: Update VGA arbiter support for newer devices
Date: Fri, 16 Aug 2013 13:20:17 +0300	[thread overview]
Message-ID: <20130816102017.GK7159@intel.com> (raw)
In-Reply-To: <1376607255.13642.155.camel@ul30vt.home>

On Thu, Aug 15, 2013 at 04:54:15PM -0600, Alex Williamson wrote:
> On Fri, 2013-08-16 at 08:49 +1000, Dave Airlie wrote:
> > On Fri, Aug 16, 2013 at 8:43 AM, Alex Williamson
> > <alex.williamson@redhat.com> wrote:
> > > This is intended to add VGA arbiter support for Intel HD graphics on
> > > Core processors.  The old GMCH registers no longer exist, so even
> > > though it appears that i915 participates in VGA arbitration, it doesn't
> > > work.  On Intel HD graphics we already attempt to disable VGA regions
> > > of the device.  This makes registering as a VGA client unnecessary since
> > > we don't intend to operate differently depending on how many VGA devices
> > > are present.  We can disable VGA memory regions by clearing a memory
> > > enable bit in the VGA MSR.  That only leaves VGA IO, which we update
> > > the VGA arbiter to know that we don't participate in VGA memory
> > > arbitration.  We also add a hook on unload to re-enable memory and
> > > reinstate VGA memory arbitration.
> > 
> > I would think there is still a VGA disable bit on the Intel device
> > somewhere, we'd just need
> > Intel to look in the docs and find it. A bit that can nuke both i/o
> > and cmd regs.
> 
> The only bit available is in the GGC and is a keyed/locked register that
> not only disables VGA memory and I/O, but also modifies the class code
> of the device.  Early Core processors didn't lock this, but it's
> untouchable in newer ones AFAICT.  Thanks,

I've not found anything else in the docs. And also we _need_ VGA I/O
access to make i915_disable_vga() work. It's not 100% clear whether
we really need to poke at the sequencer register in modern hardware,
but the docs do still list it as a mandatory step. So even if we were
to have a global "disable VGA I/O and mem bit" we'd need to make sure
we already disabled VGA eg. after resume when the BIOS had a chance to
turn the VGA display back on. I think there were also some BIOSen that
turned VGA display back on when closing/opening the laptop lid. Not
sure what would even happen with those if totally disabled VGA I/O
access. I'm not sure they actually frob with the VGA regs though.
Could be they just turn on the VGA display bit in the VGA_CONTROL
register.

-- 
Ville Syrjälä
Intel OTC