Re: VGA arbiter support for Intel HD?

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Aug 14, 2013 at 11:14:48AM -0600, Alex Williamson wrote:
> On Wed, 2013-08-14 at 17:47 +0300, Ville Syrjälä wrote:
> > On Wed, Aug 14, 2013 at 07:23:57AM -0600, Alex Williamson wrote:
> > > Hi,
> > > 
> > > I'm trying to add support for device assignment of PCI VGA devices with
> > > VFIO and QEMU.  For normal, discrete discrete graphics the Linux VGA
> > > arbiter works fairly well, disabling VGA on one bridge and adding it to
> > > another (though I wish all the kernel VGA drivers made use of it).  The
> > > i915 driver only seems to support disabling VGA on really old GMCH
> > > devices (see intel_modeset_vga_set_state).  This means that if I boot
> > > with IGD as the primary graphics and attempt to assign a discrete
> > > graphics device, all the VGA range accesses are still routed to IGD, I
> > > end up getting some error messages from the IGD interrupt handler, and
> > > the discrete card never initializes.
> > > 
> > > I spent some time looking through the Sand Bridge, Ivy Bridge, and
> > > Haswell datasheets, and I'm a bit concerned whether the hardware even
> > > provides a reasonable way to disable VGA anymore.  Quoting 2.17 from the
> > > Haswell docs:
> > > 
> > >         Accesses to the VGA memory range are directed to IGD depend on
> > >         the configuration.  The configuration is specified by:
> > >               * Internal graphics controller in Device 2 is enabled
> > >                 (DEVEN.D2EN bit 4)
> > >               * Internal graphics VGA in Device 0 Function 0 is enabled
> > >                 through register GGC bit 1.
> > >               * IGD's memory accesses (PCICMD2 04 – 05h, MAE bit 1) in
> > >                 Device 2 configuration space are enabled.
> > >               * VGA compatibility memory accesses (VGA Miscellaneous
> > >                 Output register – MSR Register, bit 1) are enabled.
> > >               * Software sets the proper value for VGA Memory Map Mode
> > >                 register (VGA GR06 Register, bits 3-2). See the
> > >                 following table for translations.
> > > 
> > > (There's a similar list for VGA I/O range)  I've found that if I disable
> > > memory and I/O in the PCI command register for IGD then I do get VGA
> > > routing to the PEG device and the discrete VBIOS works.  This obviously
> > > isn't a good option for the VGA arbiter since it entirely disables IGD.
> > > 
> > > The GGC registers aren't meant for runtime switching and are actually
> > > locked.  Disabling IGD via the device 2 enable bit doesn't seem like and
> > > option.  I don't quite understand the VGA miscellaneous output register
> > > and VGA memory map mode, but the table provided for the latter makes me
> > > think they just augment the VGA ranges and don't disable them.
> > 
> > Bit 1 of MSR (0x3c2/0x3cc) should allow you to turn off VGA mem
> > access while leaving other memory space access working.
> > 
> > As for VGA I/O decode, IIRC there's no standard bit for that in VGA
> > or PCI config registers, and I can't see any other  bit for it in the
> > docs. But I guess you could just turn off I/O space completely
> > via the PCI_COMMAND register. We shouldn't need it for anything beyond
> > i915_disable_vga() and that has the necessary vgaarb calls already.
> 
> Thanks Ville.  The MSR seems to work for VGA memory.  Disabling I/O via
> PCI_COMMAND does works, but something is re-enabling it after
> intel_modeset_vga_set_state().  If I manually disable I/O with setpci
> then I do have VGA routing to PEG and can still interact with the KMS
> console on IGD.  It's unfortunate that the MSR bit for I/O only disables
> pieces of the range.  If we have no other options, I'll try to hunt down
> where I/O is being re-enabled and see how feasible it is to avoid.
> Thanks,

Hmm. Now that I look at vgaarb more it seems I misunderstood the way it
works. Based on the code it looks like it will permanently remove the
device from the arbiration if set_vga_decode indicates that it doesn't
decode legacy resources. And it calls set_vga_decode w/ decode=false
if there are more than two VGA cards in the system. That means
i915_disable_vga() is actually broken whenever there is another
VGA card in the system.

To make it work I think what we'd need to do is always return 
VGA_RSRC_LEGACY_IO from i915_vga_set_decode(), which will leave the
PCI_COMMAND I/O bit in the hands of vgaarb, and then poke at the
MSR register to disable the VGA mem decode permanently. But to touch
MSR we actually need VGA I/O, so I guess we'd have to do that right
after registering w/ vgaarb. Doing it from i915_vga_set_decode()
doesn't look possible since there's no guarantee that VGA I/O would
end up at the right device at the time that is called.

So maybe the following patch might work (although maybe vgaarb itself
should be extended a bit to properly support this use case).

From 90070e547f1582f8e73d9221d6a31502dea8246d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Wed, 14 Aug 2013 21:20:57 +0300
Subject: [PATCH] vga arb stuf

---
 drivers/gpu/drm/i915/i915_dma.c      | 13 +++++--------
 drivers/gpu/drm/i915/intel_display.c |  9 ---------
 2 files changed, 5 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 0adfe40..9c8f9725 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1225,14 +1225,7 @@ intel_teardown_mchbar(struct drm_device *dev)
 /* true = enable decode, false = disable decoder */
 static unsigned int i915_vga_set_decode(void *cookie, bool state)
 {
-	struct drm_device *dev = cookie;
-
-	intel_modeset_vga_set_state(dev, state);
-	if (state)
-		return VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM |
-		       VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM;
-	else
-		return VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM;
+	return VGA_RSRC_LEGACY_IO | VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM;
 }
 
 static void i915_switcheroo_set_state(struct pci_dev *pdev, enum vga_switcheroo_state state)
@@ -1291,6 +1284,10 @@ static int i915_load_modeset_init(struct drm_device *dev)
 	if (ret && ret != -ENODEV)
 		goto out;
 
+	vga_get_uninterruptible(dev->pdev, VGA_RSRC_LEGACY_IO);
+	outb(inb(VGA_MSR_READ) & ~VGA_MSR_MEM_EN, VGA_MSR_WRITE);
+	vga_put(dev->pdev, VGA_RSRC_LEGACY_IO);
+
 	intel_register_dsm_handler();
 
 	ret = vga_switcheroo_register_client(dev->pdev, &i915_switcheroo_ops);
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 1c8441b..c402308 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -10359,15 +10359,6 @@ void intel_connector_attach_encoder(struct intel_connector *connector,
  */
 int intel_modeset_vga_set_state(struct drm_device *dev, bool state)
 {
-	struct drm_i915_private *dev_priv = dev->dev_private;
-	u16 gmch_ctrl;
-
-	pci_read_config_word(dev_priv->bridge_dev, INTEL_GMCH_CTRL, &gmch_ctrl);
-	if (state)
-		gmch_ctrl &= ~INTEL_GMCH_VGA_DISABLE;
-	else
-		gmch_ctrl |= INTEL_GMCH_VGA_DISABLE;
-	pci_write_config_word(dev_priv->bridge_dev, INTEL_GMCH_CTRL, gmch_ctrl);
 	return 0;
 }
 
-- 
1.8.1.5

-- 
Ville Syrjälä
Intel OTC
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