[Bug 254472] graphics/drm-fbsd13-kmod Xorg crash on resume

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254472

--- Comment #6 from Maurizio <maurizio1018@gmail.com> ---
I have found a workaround for this bug, an i915kms kernel driver patch.
I have tested  the patch on a Lenovo Thinkpad T450.

NOTICE: I don’t know if this  patch can damage your laptop, if it happens it is
not my responsibility.

The patch changes only one line of code in the source file
drm-kmod-drm_v5.4.144_6/drivers/gpu/drm/i915/i915_gem.c in the function
wait_for_timelines:
I have replaced the line:
timeout = i915_request_wait(rq, wait, timeout);
with:      
    timeout = i915_request_wait(rq, wait, MAX_SCHEDULE_TIMEOUT);

the rest of the patch lines are only printf messages to display the time spent
in the function. The i915_request_wait function never timeouts. I have warned
you with the above notice because my laptop can take more than 4 seconds to
resume.

Before applying the patch you must be able to build the
graphics/drm-fbsd13-kmod port from source.

Building the patched i915kms driver:
# cd /usr/ports/graphics/drm-fbsd13-kmod/
# make clean
# make config
select “Build with debugging support”
# mkdir files
Copy the patch file patch-drivers_gpu_drm_i915_i915__gem.c in the files
directory and verify it
# ls /usr/ports/graphics/drm-fbsd13-kmod/files
patch-drivers_gpu_drm_i915_i915__gem.c
# make build deinstall reinstall
should compile and install the patched code without errors. Reboot for running
the new driver.

How this patch works on my laptop in the suspend/resume cycle.
When suspend/resume works fast, I have this output at the console:

vgapci0: child drmn0 requested pci_enable_io

Start of i915_gem_wait_for_idle()
flags=12 (locked), timeout=200

Calling wait_for_timelines()
flags=12 (locked), timeout=2147483647 (forever)

End of i915_gem_wait_for_idle() in 0 tick(s)
rtsx0: Resume

The i915_gem_wait_for_idle function takes less than one ms (tick = ms).  

When the suspend/resume cycle is slow, I have an output similar to:
vgapci0: child drmn0 requested pci_enable_io

Start of i915_gem_wait_for_idle()
flags=12 (locked), timeout=200

Calling wait_for_timelines()
drmn0: Resetting rcs0 for hang on rcs0
flags=12 (locked), timeout=2147483647 (forever)

End of i915_gem_wait_for_idle() in 4138 tick(s)

Start of i915_gem_wait_for_idle()
flags=12 (locked), timeout=200

Calling wait_for_timelines()
flags=12 (locked), timeout=2147483647 (forever)

End of i915_gem_wait_for_idle() in 0 tick(s)
rtsx0: Resume

The message “drmn0: Resetting rcs0 for hang on rcs0” is written to the console
and the i915_gem_wait_for_idle function takes more than 4 seconds to finish.
The i915_gem_wait_for_idle is called again without errors and the laptop
resumes.

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