Quick update: if this issue was ever fixed, the patch hasn't made it
into the mainline kernel as of 5.13.0.  I'm still getting the system
lock-up when X11 starts, and have to hit the reset switch to recover.

For whatever it might be worth, the mainline 5.10.0 kernel continues
to work properly alongside all the user space changes in "sid" that
have happened since late last year.

Respectfully,
--Bob

On Fri, Jun 04, 2021 at 12:37:14AM -0500, Bob Tracy wrote:

On Fri, Jun 04, 2021 at 12:18:58AM -0500, Bob Tracy wrote:

quoted

On Thu, Jun 03, 2021 at 03:15:05PM +0200, Maciej W. Rozycki wrote:

quoted

 I have lost track about this issue, so please fill me in as to whether 
the offending commit causing the regression has been bisected or not.

It has.  Michael Cree reported the following back on April 5th:

And the first bad commit is:

0fe3cf3a53b5c1205ec7d321be1185b075dff205 is the first bad commit
commit 0fe3cf3a53b5c1205ec7d321be1185b075dff205
Author: Christian König [off-list ref]
Date:   Sat Oct 24 13:12:23 2020 +0200

    drm/radeon: switch to new allocator v2
    
    It should be able to handle all cases here.
    
    v2: fix debugfs as well
    
    Signed-off-by: Christian König [off-list ref]
    Reviewed-by: Dave Airlie [off-list ref]
    Reviewed-by: Madhav Chauhan [off-list ref]
    Tested-by: Huang Rui [off-list ref]
    Link: https://patchwork.freedesktop.org/patch/397088/?series=83051&rev=1

:040000 040000 4e643ef861b921392bc67be21a42298c91c7ff7a b36453567c3176a3cd50fa0b23886b0fd642560d M	drivers

There were a few follow-up messages in this thread that left me with the
impression there *may* have been a patch submitted, although Christian
complained at the time he was having problems locating Alpha hardware to
test with.

The current (5.12.0 kernel) problem symptoms show some "improvement".
I at least got to the point that the login screen displayed, but it
had a bit of pixelation/distortion in a few areas indicative of "bad
things about to happen".  Then I got the expected system lock-up,
just as I originally reported: had to hit the reset switch to recover.