Created attachment 29350 [details]
Links2 (the text/low-graphic browser) with framebuffer support on triggers the attached lockdep trace when exiting.
Both KMS and links2 work correctly afterwards.
KMS is enabled and working, drm-next commit cecc6b63a5de547a345c491bb4c18c01a15984a4
The card is RV670, Mobility Radeon HD3850.