radeonkms broken on PowerMac G5 (powerpc64)

[realchonk]

Describe the bug
I have a PowerMac G5 (powerpc64) and an old Radeon card in it,
which I tried using with the radeonkms driver (scfb worked fine, but slow).

FreeBSD version
FreeBSD powerkek 14.1-RELEASE FreeBSD 14.1-RELEASE releng/14.1-n267679-10e31f0946d8 GENERIC powerpc 1401000 1401000

PCI Info

pciconf -lv

Paste the output of pciconf -lv
vgapci0@pci0:0:16:0: class=0x030000 rev=0x00 hdr=0x00 vendor=0x1002 device=0x4152 subvendor=0x1002 subdevice=0x4152
vendor = 'Advanced Micro Devices, Inc. [AMD/ATI]'
device = 'RV360 [Radeon 9600/X1050 Series]'
class = display
subclass = VGA
hostb0@pci0:0:11:0: class=0x060000 rev=0x00 hdr=0x00 vendor=0x106b device=0x0058 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'U3L AGP Bridge'
class = bridge
subclass = HOST-PCI
pcib2@pci1:0:1:0: class=0x060400 rev=0x00 hdr=0x01 vendor=0x106b device=0x0053 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta PCI Bridge'
class = bridge
subclass = PCI-PCI
pcib3@pci1:0:2:0: class=0x060400 rev=0x00 hdr=0x01 vendor=0x106b device=0x0054 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta PCI Bridge'
class = bridge
subclass = PCI-PCI
pcib4@pci1:0:3:0: class=0x060400 rev=0x00 hdr=0x01 vendor=0x106b device=0x0055 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta PCI Bridge'
class = bridge
subclass = PCI-PCI
gem0@pci1:3:15:0: class=0x020000 rev=0x00 hdr=0x00 vendor=0x106b device=0x0051 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta (Sun GEM)'
class = network
subclass = ethernet
macio0@pci1:1:7:0: class=0xff0000 rev=0x00 hdr=0x00 vendor=0x106b device=0x004f subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta Mac I/O'
none0@pci1:1:1:0: class=0x028000 rev=0x03 hdr=0x00 vendor=0x14e4 device=0x4320 subvendor=0x106b subdevice=0x004e
vendor = 'Broadcom Inc. and subsidiaries'
device = 'BCM4306 802.11b/g Wireless LAN Controller'
class = network
ohci0@pci1:1:11:0: class=0x0c0310 rev=0x43 hdr=0x00 vendor=0x1033 device=0x0035 subvendor=0x1033 subdevice=0x0035
vendor = 'NEC Corporation'
device = 'OHCI USB Controller'
class = serial bus
subclass = USB
ohci1@pci1:1:11:1: class=0x0c0310 rev=0x43 hdr=0x00 vendor=0x1033 device=0x0035 subvendor=0x1033 subdevice=0x0035
vendor = 'NEC Corporation'
device = 'OHCI USB Controller'
class = serial bus
subclass = USB
ehci0@pci1:1:11:2: class=0x0c0320 rev=0x04 hdr=0x00 vendor=0x1033 device=0x00e0 subvendor=0x1033 subdevice=0x00e0
vendor = 'NEC Corporation'
device = 'uPD72010x USB 2.0 Controller'
class = serial bus
subclass = USB
atapci0@pci1:2:12:0: class=0x01018f rev=0x00 hdr=0x00 vendor=0x1166 device=0x0240 subvendor=0x1166 subdevice=0x0240
vendor = 'Broadcom'
device = 'K2 SATA'
class = mass storage
subclass = ATA
ata0@pci1:2:13:0: class=0xff0000 rev=0x00 hdr=0x00 vendor=0x106b device=0x0050 subvendor=0x0000 subdevice=0x0000
vendor = 'Apple Inc.'
device = 'Shasta IDE'
fwohci0@pci1:2:14:0: class=0x0c0010 rev=0x00 hdr=0x00 vendor=0x106b device=0x0052 subvendor=0x106b subdevice=0x5811
vendor = 'Apple Inc.'
device = 'Shasta Firewire'
class = serial bus
subclass = FireWire

DRM KMOD version
Paste the output of pkg query -x '%n %v' 'drm.*kmod'
drm-510-kmod 5.10.163_8
This was the only drm-kmod version available through pkg for this machine.

To Reproduce

# pkg install drm-510-kmod gpu-firmware-radeon-kmod-r300-20220511
# kldload radeonkms

now you should see yellow vertical stripes on a black background.

Screenshots

The screenshot is when I run kldload radeonkms && startx.
On X the colored rectangle is the cursor, which I can freely move around.
It's not shown, when I only run kldload radeonkms.

Additional context
This machine is big-endian, which might be a possible reason for this.