[r300g] screen is garbled with drivers ati/modesetting + gallium (PCIe, Big Endian ppc64, mesa-19.3.0)

X does work properly with driver "fbdev" but the screen is garbled with driver "ati" or "modesetting" (see attached screenshots). Also colours are wrong widh driver "modesetting".

Machine is a PowerMac G5 + ATI Radeon X1600 Pro.

System:    Kernel: 5.4.3-gentoo-PowerMacG5 ppc64 bits: 64 Console: tty 1 
           Distro: Gentoo Base System release 2.6 
Machine:   Type: PowerPC Device System: PowerMac11 2 details: PowerMac11 2 rev: 1.1 (pvr 0044 0101) 
           serial: R6V 
CPU:       Dual Core: PPC970MP altivec supported type: MCP speed: 1150 MHz min/max: 1150/2300 MHz 
Graphics:  Device-1: NVIDIA NV43 [GeForce 6600] driver: N/A 
           Device-2: AMD RV530 [Radeon X1600 PRO] driver: radeon v: kernel 
           Display: server: X.org 1.20.5 driver: modesetting tty: 104x53 

ati_Xorg.0.log modesetting_Xorg.0.log fbdev_Xorg.0.log dmesg_543_g5.txt