i915 | Bug in virtual PCH detection

[Andrey Toloknev]

Hello!

Thanks for your reply, Ville.

I looked at the code again and understood you are definitely right about
breaking other combinations of CPU+PCH with using IS_GEN9_BC in my patch.

Using libvirt (kvm) I can passthrough ISA/LPC bridge to VM, but the problem
is connected with method intel_detect_pch(). It searches only for the first
ISA Bridge.
And the virtual ISA/LPC Bridge PCI in libvirt is hardcoded to address
00:01.0 - it's always first.
So, method intel_detect_pch() correctly detects that the first bridge is
virtual and then calls method intel_virt_detect_pch(), which just checks
the iGPU platform and doesn't take into account the possible combination of
Comet Lake CPU and Tiger Lake PCH.

Of course, It would be nice if we can have a universal modparam to specify
PCH id by hand in future.
But as a fast fix of that small bug I think one more bool modparam may be
enough.
I wrote the second version of patch which adds that bool modparam -
force_tgp_vpch. It's false by default.
When this param is true, we also check that the CPU is Comet Lake and then
set PCH type as Tiger Lake in the method intel_virt_detect_pch().

The second version of patch is in attachment.


вт, 3 сент. 2024 г. в 18:38, Ville Syrjälä <ville.syrjala at linux.intel.com>:

> On Sun, Sep 01, 2024 at 02:56:07PM +0500, Andrey Toloknev wrote:
> > Hello!
> >
> > I have 2 machines with Comet Lake CPUs on Tiger Lake PCH (500 series of
> > Intel chipsets).
> > For that configuration there was a patch for adding support for Tiger
> Lake
> > PCH with CometLake CPU in 2021 -
> > https://patchwork.freedesktop.org/patch/412664/
> > This patch made possible correct detection of such chipset and cpu
> > configuration for i915 kernel module. Without it there was no output to
> any
> > display (HDMI/DP/DVI, even VGA).
> >
> > But this patch doesn't touch intel_virt_detect_pch method, when you
> > passthrough iGPU to a virtual machine.
> > So, virtual PCH incorrectly detects as Cannon Lake and you have no output
> > to a physical display with i915 driver:
> >
> > [    2.933139] i915 0000:00:02.0: [drm:intel_virt_detect_pch [i915]]
> > Assuming PCH ID a300
> > [    2.933308] i915 0000:00:02.0: [drm:intel_pch_type [i915]] Found
> Cannon
> > Lake PCH (CNP)
> >
> >
> > The bug is on line 173 in drivers/gpu/drm/i915/soc/intel_pch.c in method
> > intel_virt_detect_pch:
> >
> > else if (IS_TIGERLAKE(dev_priv) || IS_ROCKETLAKE(dev_priv))
> >
> > id = INTEL_PCH_TGP_DEVICE_ID_TYPE;
> >
> > It must be:
> >
> > else if (IS_TIGERLAKE(dev_priv) || IS_ROCKETLAKE(dev_priv) ||
> > IS_GEN9_BC(dev_priv))
> >
> > id = INTEL_PCH_TGP_DEVICE_ID_TYPE;
> >
> >
> > After that small change you get correct detection of PCH and have output
> to
> > a physical display in VM with passthrough iGPU:
> >
> > [   16.139809] i915 0000:00:02.0: [drm:intel_virt_detect_pch [i915]]
> > Assuming PCH ID a080
> > [   16.261151] i915 0000:00:02.0: [drm:intel_pch_type [i915]] Found Tiger
> > Lake LP PCH
> >
> >
> > All kernel versions in any distro since 2021 are affected by this small
> bug.
> > The patch for i915 module of the actual kernel version is in attachment.
>
> You fix one CPU+PCH combo, but break the other. I don't think there is
> any way to handle this mess in intel_virt_detect_pch(). The best thing
> would be if the virtual machine would advertise the correct ISA/LPC
> bridge, then the heiristic is not even invoked. If that's not possible
> for some reason then I suppose we'd need a modparam/etc. so the user
> can specify the PCH ID by hand.
>
> --
> Ville Syrjälä
> Intel
>


-- 
Best regards, Andrey Toloknev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/intel-gfx/attachments/20240904/cf012012/attachment-0001.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: tgl_vpch_fix_v2.patch
Type: application/octet-stream
Size: 2092 bytes
Desc: not available
URL: <https://lists.freedesktop.org/archives/intel-gfx/attachments/20240904/cf012012/attachment-0001.obj>