GNOME session fails to start with some AMD/ATI radeon GPU

we’ve got multiple reports of users unable to get to the desktop after the greeter, using computers with AMD/ATI radeon GPU.

most of them get the “oh no! Something has gone wrong. A problem has occurred and the system can’t recover. Please contact a system administrator” screen

other get to the desktop for few seconds, then they are sent back to the greeter. log in again result in a black screen or the console and a freeze.

one get to the desktop but with a very low resolution

I have tails-debugging-info from failsafe sessions (none of the users could get one from a ‘real’ session) that show this error multiple times:

[drm] VGACON disable radeon kernel modesetting.
[drm:radeon_init] *ERROR* No UMS support in radeon module!

this happened with these GPU :

Advanced Micro Devices, Inc. [AMD/ATI] Mars XTX [Radeon HD 8790M]
Advanced Micro Devices, Inc. [AMD/ATI] Mars XTX [Radeon HD 8790M] (rev ff)
Advanced Micro Devices, Inc. [AMD/ATI] Cedar [Radeon HD 5000/6000 7350/8350 Series]
Advanced Micro Devices, Inc. [AMD/ATI] Broadway PRO [Mobility Radeon HD 5850]
Advanced Micro Devices, Inc. [AMD/ATI] RV730/M96 [Mobility Radeon HD 4650/5165]

(the last one is the one able to access the desktop)

Related issues

• Related to #11225 (closed)
• Related to #11392 (closed)
• Related to #12482 (closed)

Original created by @goupille on 11095 (Redmine)