I am installing a Debian Jessie on a Mac Mini PowerPC G4 system.

After a couple of seconds of using the system, it generally freeze, looking at dmesg from another ssh connection I can see:

[   21.314765] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready
[   21.459715] b43-phy0: Loading firmware version 666.2 (2011-02-23 01:15:07)
[   21.531076] IPv6: ADDRCONF(NETDEV_UP): wlan0: link is not ready
[   23.719199] gem 0002:20:0f.0 eth0: Link is up at 100 Mbps, full-duplex
[   23.719318] gem 0002:20:0f.0 eth0: Pause is disabled
[   23.719357] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[  180.631048] radeon 0000:00:10.0: ring 0 stalled for more than 16096msec
[  180.631062] radeon 0000:00:10.0: GPU lockup (waiting for
0x000000000000031b last fence id 0x0000000000000301 on ring 0)
[  180.768184] radeon: wait for empty RBBM fifo failed ! Bad things
might happen.
[  180.905284] Failed to wait GUI idle while programming pipes. Bad
things might happen.
[  180.908156] radeon 0000:00:10.0: Saved 827 dwords of commands on ring 0.
[  180.908176] radeon 0000:00:10.0: (r100_asic_reset:2551)
RBBM_STATUS=0x8002C139
[  181.405690] radeon 0000:00:10.0: (r100_asic_reset:2572)
RBBM_STATUS=0x8002C139
[  181.899237] radeon 0000:00:10.0: (r100_asic_reset:2580)
RBBM_STATUS=0x8002C139
[  181.899264] radeon 0000:00:10.0: GPU reset succeed
[  181.899269] radeon 0000:00:10.0: GPU reset succeeded, trying to resume
[  181.899281] radeon 0000:00:10.0: (r100_asic_reset:2551)
RBBM_STATUS=0x8002C139
[  182.396784] radeon 0000:00:10.0: (r100_asic_reset:2572)
RBBM_STATUS=0x8002C139
[  182.890328] radeon 0000:00:10.0: (r100_asic_reset:2580)
RBBM_STATUS=0x8002C139
[  182.890351] radeon 0000:00:10.0: GPU reset succeed
[  182.890376] radeon 0000:00:10.0: WB disabled
[  182.890384] radeon 0000:00:10.0: fence driver on ring 0 use gpu
addr 0x0000000000000000 and cpu addr 0xe2819000
[  183.027494] radeon: wait for empty RBBM fifo failed ! Bad things
might happen.
[  183.164599] Failed to wait GUI idle while programming pipes. Bad
things might happen.
[  183.164643] [drm] radeon: ring at 0x0000000000001000
[  183.171448] [drm] ring test succeeded in 0 usecs
[  193.171046] radeon 0000:00:10.0: ring 0 stalled for more than 10000msec
[  193.171061] radeon 0000:00:10.0: GPU lockup (waiting for
0x000000000000031c last fence id 0x0000000000000301 on ring 0)
[  193.171068] [drm:r100_ib_test] *ERROR* radeon: fence wait failed (-35).
[  193.171079] [drm:radeon_ib_ring_tests] *ERROR* radeon: failed
testing IB on GFX ring (-35).
[  193.171084] radeon 0000:00:10.0: ib ring test failed (-35).
[  193.308210] radeon: wait for empty RBBM fifo failed ! Bad things
might happen.
[  193.445310] Failed to wait GUI idle while programming pipes. Bad
things might happen.
[  193.446307] radeon 0000:00:10.0: (r100_asic_reset:2551)
RBBM_STATUS=0x8002C136
[  193.943817] radeon 0000:00:10.0: (r100_asic_reset:2572)
RBBM_STATUS=0x8002C136
[  194.437363] radeon 0000:00:10.0: (r100_asic_reset:2580)
RBBM_STATUS=0x8002C136
[  194.437390] radeon 0000:00:10.0: GPU reset succeed
[  194.437395] radeon 0000:00:10.0: GPU reset succeeded, trying to resume
[  194.437407] radeon 0000:00:10.0: (r100_asic_reset:2551)
RBBM_STATUS=0x8002C136
[  194.934911] radeon 0000:00:10.0: (r100_asic_reset:2572)
RBBM_STATUS=0x8002C136
[  195.428455] radeon 0000:00:10.0: (r100_asic_reset:2580)
RBBM_STATUS=0x8002C136
[  195.428476] radeon 0000:00:10.0: GPU reset succeed
[  195.428501] radeon 0000:00:10.0: WB disabled
[  195.428509] radeon 0000:00:10.0: fence driver on ring 0 use gpu
addr 0x0000000000000000 and cpu addr 0xe2819000
[  195.565605] radeon: wait for empty RBBM fifo failed ! Bad things
might happen.
[  195.702705] Failed to wait GUI idle while programming pipes. Bad
things might happen.
[  195.702749] [drm] radeon: ring at 0x0000000000001000
[  195.710956] [drm] ring test succeeded in 0 usecs
[  205.707048] radeon 0000:00:10.0: ring 0 stalled for more than 10000msec
[  205.707063] radeon 0000:00:10.0: GPU lockup (waiting for
0x000000000000031d last fence id 0x0000000000000301 on ring 0)
[  205.707070] [drm:r100_ib_test] *ERROR* radeon: fence wait failed (-35).
[  205.707081] [drm:radeon_ib_ring_tests] *ERROR* radeon: failed
testing IB on GFX ring (-35).
[  205.707086] radeon 0000:00:10.0: ib ring test failed (-35).
[  205.758647] radeon 0000:00:10.0: GPU reset failed

Using

# apt-cache policy xserver-xorg-video-r128
xserver-xorg-video-r128:
  Installé : 6.9.2-1+b2
  Candidat : 6.9.2-1+b2
 Table de version :
 *** 6.9.2-1+b2 0
        500 http://ftp.fr.debian.org/debian/ jessie/main powerpc Packages
        100 /var/lib/dpkg/status