I am still getting errors even with radeon.agpmode=4 is set
rican-linux@Debian-G5:~$ dmesg |grep -e radeom -e drm -e radeon.agpmode
[ 0.000000] Kernel command line: root=UUID=aeca9a67-31d7-4c4b-a0f8-4db328b33305 radeon.agpmode=4
[ 9.734366] [drm] Initialized drm 1.1.0 20060810
[ 10.934519] [drm] radeon kernel modesetting enabled.
[ 10.943743] fb: switching to radeondrmfb from OFfb ATY,Simone
[ 10.954622] fb: switching to radeondrmfb from OFfb ATY,Simone
[ 10.956515] [drm] initializing kernel modesetting (RV350 0x1002:0x4150 0x1002:0x4150 0x00).
[ 10.956552] [drm] register mmio base: 0xA0000000
[ 10.956558] [drm] register mmio size: 65536
[ 11.046327] [drm] Not an x86 BIOS ROM, not using.
[ 11.046358] [drm] Using device-tree clock info
[ 11.046369] [drm] AGP mode requested: 4
[ 11.046528] [drm] Generation 2 PCI interface, using max accessible memory
[ 11.046582] [drm] Detected VRAM RAM=256M, BAR=256M
[ 11.046587] [drm] RAM width 128bits DDR
[ 11.046858] [drm] radeon: 64M of VRAM memory ready
[ 11.046866] [drm] radeon: 256M of GTT memory ready.
[ 11.046935] [drm] radeon: 1 quad pipes, 1 Z pipes initialized.
[ 11.061306] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[ 11.061312] [drm] Driver supports precise vblank timestamp query.
[ 11.061363] [drm] radeon: irq initialized.
[ 11.061405] [drm] Loading R300 Microcode
[ 11.110467] [drm] radeon: ring at 0x0000000000010000
[ 11.257952] [drm:.r100_ring_test [radeon]] *ERROR* radeon: ring test failed (scratch(0x15E4)=0xCAFEDEAD)
[ 11.258060] [drm:.r100_cp_init [radeon]] *ERROR* radeon: cp isn't working (-22).
[ 11.404548] [drm:.r100_cp_fini [radeon]] *ERROR* Wait for CP idle timeout, shutting down CP.
[ 11.551199] [drm] radeon: cp finalized
[ 12.547388] [drm] radeon: cp finalized
[ 12.553854] [drm] radeon: ttm finalized
[ 12.553861] [drm] Forcing AGP to PCI mode
[ 12.643384] [drm] Not an x86 BIOS ROM, not using.
[ 12.643406] [drm] Using device-tree clock info
[ 12.643414] [drm] Generation 2 PCI interface, using max accessible memory
[ 12.643446] [drm] Detected VRAM RAM=256M, BAR=256M
[ 12.643452] [drm] RAM width 128bits DDR
[ 12.643712] [drm] radeon: 64M of VRAM memory ready
[ 12.643719] [drm] radeon: 512M of GTT memory ready.
[ 12.643761] [drm] GART: num cpu pages 8192, num gpu pages 131072
[ 12.645324] [drm] radeon: 1 quad pipes, 1 Z pipes initialized.
[ 12.645343] [drm] PCI GART of 512M enabled (table at 0x0000000062F80000).
[ 12.645462] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[ 12.645468] [drm] Driver supports precise vblank timestamp query.
[ 12.645512] [drm] radeon: irq initialized.
[ 12.645689] [drm] radeon: ring at 0x0000000090010000
[ 12.793131] [drm:.r100_ring_test [radeon]] *ERROR* radeon: ring test failed (scratch(0x15E4)=0xCAFEDEAD)
[ 12.793235] [drm:.r100_cp_init [radeon]] *ERROR* radeon: cp isn't working (-22).
[ 12.939961] [drm:.r100_cp_fini [radeon]] *ERROR* Wait for CP idle timeout, shutting down CP.
[ 12.940193] [drm] radeon: cp finalized
[ 12.944581] [drm] Connector Table: 12 (mac g5 9600)
[ 12.944609] [drm] No valid Ext TMDS info found in BIOS
[ 12.944621] [drm] No TV DAC info found in BIOS
[ 12.944798] [drm] No TMDS info found in BIOS
[ 12.944974] [drm] Radeon Display Connectors
[ 12.944981] [drm] Connector 0:
[ 12.944985] [drm] DVI-I-1
[ 12.944990] [drm] HPD1
[ 12.944995] [drm] DDC: 0x64 0x64 0x64 0x64 0x64 0x64 0x64 0x64
[ 12.945000] [drm] Encoders:
[ 12.945005] [drm] DFP2: INTERNAL_DVO1
[ 12.945010] [drm] CRT2: INTERNAL_DAC2
[ 12.945015] [drm] Connector 1:
[ 12.945019] [drm] DVI-I-2
[ 12.945023] [drm] HPD2
[ 12.945028] [drm] DDC: 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60
[ 12.945033] [drm] Encoders:
[ 12.945037] [drm] DFP1: INTERNAL_TMDS1
[ 12.945042] [drm] CRT1: INTERNAL_DAC1
[ 12.945046] [drm] Connector 2:
[ 12.945051] [drm] SVIDEO-1
[ 12.945055] [drm] Encoders:
[ 12.945059] [drm] TV1: INTERNAL_DAC2
[ 13.028211] [drm] fb mappable at 0xB0040000
[ 13.028229] [drm] vram apper at 0xB0000000
[ 13.028234] [drm] size 8323072
[ 13.028238] [drm] fb depth is 24
[ 13.028243] [drm] pitch is 7680
[ 13.122329] radeon 0000:f0:10.0: fb0: radeondrmfb frame buffer device
[ 13.175092] [drm] Initialized radeon 2.43.0 20080528 for 0000:f0:10.0 on minor 0
rican-linux@Debian-G5:~$