Enable KMS leads to totally frozen of the X-window, but the machine (HP Compaq nx6125) can still accessable via SSH.

# cat /proc/cmdline 
ro root=UUID=63f5071e-45a2-41b4-aec1-7578b59cb52d rhgb quiet LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=uk rd_plytheme=charge

# dmesg
...
[TTM] Failed moving buffer. Proposed placement 0x00060004
[TTM] Out of aperture space or DRM memory quota.
[drm:radeon_object_list_validate] *ERROR* radeon: failed to validate.
[drm:radeon_cs_ioctl] *ERROR* Failed to parse relocation !

# lspci -xxxvvv
...
01:05.0 VGA compatible controller: ATI Technologies Inc Radeon XPRESS 200M 5955 (PCIE) (prog-if 00 [VGA controller])
        Subsystem: Hewlett-Packard Company MX6125
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B+ DisINTx-
        Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
        Latency: 64 (2000ns min), Cache Line Size: 64 bytes
        Interrupt: pin A routed to IRQ 17
        Region 0: Memory at c0000000 (32-bit, prefetchable) [size=64M]
        Region 1: I/O ports at 2000 [size=256]
        Region 2: Memory at c4400000 (32-bit, non-prefetchable) [size=64K]
        [virtual] Expansion ROM at c4420000 [disabled] [size=128K]
        Capabilities: [50] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
        Kernel driver in use: radeon
        Kernel modules: radeon, radeonfb
00: 02 10 55 59 07 02 b0 02 00 00 00 03 10 40 00 00
10: 08 00 00 c0 01 20 00 00 00 00 40 c4 00 00 00 00
20: 00 00 00 00 00 00 00 00 00 00 00 00 3c 10 8b 30
30: 00 00 00 00 50 00 00 00 00 00 00 00 05 01 08 00
40: 00 00 00 00 00 00 00 00 00 00 00 00 3c 10 8b 30
50: 01 00 02 06 00 00 00 00 02 50 20 00 30 02 00 4f
60: 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00
70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
80: 05 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00