i915 | Bug in virtual PCH detection

[Andrey Toloknev]

>
> Hmm. I wonder how many systems we'd break if we make it look
> through all the bridges for a real match first, and only fall
> back to intel_virt_detect_pch() if nothing was found...


Yes, I definitely understand this, that's why I didn't touch this code at
all in the second patch.
I just add bool modparam force_tgp_vpch in i915_params and a bit modify
method intel_virt_detect_pch() in intel_pch.c



ср, 4 сент. 2024 г. в 17:52, Ville Syrjälä <ville.syrjala at linux.intel.com>:

> On Wed, Sep 04, 2024 at 05:25:06PM +0500, Andrey Toloknev wrote:
> > 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.
>
> Hmm. I wonder how many systems we'd break if we make it look
> through all the bridges for a real match first, and only fall
> back to intel_virt_detect_pch() if nothing was found...
>
> > 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
>
>
>
> --
> Ville Syrjälä
> Intel
>


-- 
С Уважением, Толокнев Андрей
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