[Bug 2044131] Re: i915 regression introduced with 5.5 kernel

This bug is awaiting verification that the linux-xilinx-
zynqmp/5.15.0-1029.33 kernel in -proposed solves the problem. Please
test the kernel and update this bug with the results. If the problem is
solved, change the tag 'verification-needed-jammy-linux-xilinx-zynqmp'
to 'verification-done-jammy-linux-xilinx-zynqmp'. If the problem still
exists, change the tag 'verification-needed-jammy-linux-xilinx-zynqmp'
to 'verification-failed-jammy-linux-xilinx-zynqmp'.


If verification is not done by 5 working days from today, this fix will
be dropped from the source code, and this bug will be closed.


See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how
to enable and use -proposed. Thank you!


** Tags added: kernel-spammed-jammy-linux-xilinx-zynqmp-v2 verification-needed-jammy-linux-xilinx-zynqmp

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https://bugs.launchpad.net/bugs/2044131

Title:
i915 regression introduced with 5.5 kernel

Status in Linux:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in linux-hwe-5.15 package in Ubuntu:
Invalid
Status in linux source package in Focal:
Invalid
Status in linux-hwe-5.15 source package in Focal:
Fix Released
Status in linux source package in Jammy:
Fix Released
Status in linux-hwe-5.15 source package in Jammy:
Invalid
Status in linux source package in Mantic:
Fix Released
Status in linux-hwe-5.15 source package in Mantic:
Invalid
Status in linux source package in Noble:
Fix Released
Status in linux-hwe-5.15 source package in Noble:
Invalid

Bug description:
[ Impact ]
Commit 8f4b1068e7fc3df1a77ac8151767e56b208cc87f introduced some timing checks which have been proven invalid for at least some hardware setups. A user trying to run Focal with HWE 5.15 kernel is not able to get any video output.
After going through bisection between 5.4 and 5.5 this commit was identified. Reverting it on top of Focal HWE kernel fixes the issue.

The issue has been addressed upstream in DRM tree
(20c2dbff342aec13bf93c2f6c951da198916a455) and has been merge into
linux-next (e0ef2daa8ca8ce4dbc2fd0959e383b753a87fd7d).

dmesg and lspci from the affected configuration are attached to this
bug.

[ Test Plan ]
1. Boot the affected hardware with Ubuntu desktop running kernel v5.5 or higher.
2. Wait until boot finishes and see the blank screen.

Actual result: there is no video output visible.
Expected result: normal boot process should be visible (e.g. splash), then GUI should appear.

[ Where problem could occur ]
This bug was a result of assumptions in the checks that turned out to be not valid for some hardware. The checks were removed from global intel_mode_valid function and moved into connector specific .mode_valid() hooks, entirely skiping BXT/GLK DSI connectors.

This should keep the checks where appropriate and skip for hardware
that does not comply to them.

[ Other info ]

Original bug description:

There is a regression preventing a user to upgrade from 5.4 kernel to anything that's higher than 5.5. When using such kernel the image orientation is wrong (rotated by 90°C). Also the kernel log contains:
wrz 15 09:19:49 desktop kernel: i915 0000:00:02.0: [drm] Unknown revid 0x06
wrz 15 09:19:49 desktop kernel: rtw_8821ce 0000:01:00.0: Firmware version 24.8.0, H2C version 12
wrz 15 09:19:49 desktop kernel: Console: switching to colour dummy device 80x25
wrz 15 09:19:49 desktop kernel: i915 0000:00:02.0: vgaarb: deactivate vga console
wrz 15 09:19:49 desktop kernel: i915 0000:00:02.0: [drm] couldn't get memory information
wrz 15 09:19:49 desktop kernel: i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:o>
wrz 15 09:19:49 desktop kernel: i915 0000:00:02.0: [drm] Finished loading DMC firmware i915/glk_dmc_ver1_04.bin (v>
wrz 15 09:19:49 desktop kernel: ================================================================================
wrz 15 09:19:49 desktop kernel: UBSAN: array-index-out-of-bounds in /build/linux-hwe-5.15-DZkSuT/linux-hwe-5.15-5.>
wrz 15 09:19:49 desktop kernel: index 5 is out of range for type 'u32 [5]'

(full stack trace attached)

The video hardware in use is:
00:02.0 VGA compatible controller [0300]: Intel Corporation UHD Graphics 605 [8086:3185] (rev 06) (prog-if 00 [VGA controller])
(...)
Kernel driver in use: i915
Kernel modules: i915

The user wanted to upgrade from bionic to focal with HWE kernel (they
needed it due to some networking hardware they wanted to have
supported by the newer kernel).

The user was testing mainline stable kernels and noticed that the last
working kernel was the 5.4 series, while anything starting from 5.5
and above is BROKEN (symptoms as described in the first paragraph
above).

Together with the user we have run a bisection between v5.4 and v5.5 on the upstream stable kernel and we were able to identify the first broken commit to be:
8f4b1068e7fc3df1a77ac8151767e56b208cc87f drm/i915: Check some transcoder timing minimum limits

To confirm I have reverted this change on top of the HWE kernel for
focal and the user have confirmed that this resolved the issue (test
build available at ppa:dgadomski/kernel-sf368743).

I am not fully sure what the purpose of this patch was, but I assume
the regression was not intended and the hardware should be still
supported.

Attaching lshw and lspci output.

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