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
