[Bug 1908219] Re: [drm:qxl_enc_commit [qxl]] *ERROR* head number too large or missing monitors config:

This bug is awaiting verification that the 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-
bionic' to 'verification-done-bionic'. If the problem still exists,
change the tag 'verification-needed-bionic' to 'verification-failed-
bionic'.

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: verification-needed-bionic

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https://bugs.launchpad.net/bugs/1908219

Title:
[drm:qxl_enc_commit [qxl]] *ERROR* head number too large or missing
monitors config:

Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Bionic:
Fix Committed

Bug description:
[Impact]

* Ubuntu 18.04 used as a guest in KVM with Spice/QXL in use may lead to a DRM error displayed during xorg launch:
[drm:qxl_enc_commit [qxl]] *ERROR* head number too large or missing monitors config: (ptrval), 0

[Fix]

* 00e5d217fa19bcbec13135898e1b9ca2c1c3e89b qxl: hook monitors_config
updates into crtc, not encoder.

[Test Case]

* Ubuntu 18.04 desktop guest with 4.15-series kernel with Spice/QXL.
* I used Ubuntu 20.04 as the host, but I was reported that the issue is similar also on Centos 7.8 used as a host.

[Regression Potential]

* Fix is limited to the QXL driver, so any regressions will be related
to graphics (either potential drm errors or graphical artifacts).

[Other]

* This has been fixed in HWE kernels and in later Ubuntu releases. Only Bionic is affected.
* According to the description in drivers/gpu/drm/qxl/qxl_dev.h:
struct qxl_monitors_config {
(...)
uint16_t max_allowed; /* If it is 0 no fixed limit is given by the
driver */
(...)
};

In the message this value is 0 which should be a completely correct situation in that context. However, it is incorrectly compared against current qxl_output.
This has been fixed soon after Bionic release and in Bionic is marked with:
/* TODO: ugly, do better */

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