https://bugs.freedesktop.org/show_bug.cgi?id=84500
--- Comment #7 from Michel Dänzer michel@daenzer.net ---
[25322.031213] 0x0000000b
This value is written to the R_028A3C_VGT_GROUP_VECT_1_FMT_CNTL register. However, the driver only ever writes 0 to that register, in si_init_config().
[25322.031214] 0x00000000 <--- [25322.031215] 0x00000295 [25322.031215] 0x00000080 [25322.031216] 0x00000040 [25322.031217] 0x00000002
The values after the arrow look like the following series of register writes to R_028A54_VGT_GS_PER_ES and the two following registers.
So, it looks like the value for the R_028A3C_VGT_GROUP_VECT_1_FMT_CNTL register and the following PKT3_SET_CONTEXT_REG header were scribbled over with the value 0x0000000b00000000. Looks like memory corruption to me.
Running firefox in valgrind or with something like the GCC / clang address sanitizers might give a clue, but might be painful.