Ati radeon driver generating screen garbage when with agpmode=4

[Gianni Cessel]

Greetings,

I moved to Lubuntu 18.04 (i386) a few months ago and since then I 
experienced a problem with my old laptop HP zt3300.

Whenever I enable to AGP rendering in the radeon driver with agpmode=4, 
then the image displayed on the monitor is corrupted with some garbage 
(See attached image)

Running over the garbage with the pointer clears temporally the garbage, 
and then reappears

Of course, if I use agpmode=-1 the problem and the garbage disappear, 
but the laptop is also much slower since all graphical processing is 
done by the (old) CPU.

I tried every possible radeon driver parameter, or combinations thereof 
but without any lack to circumvent this problem.

I don't thing it is a matter of monitor because I also used and new 
external monitor and it still happens.

Do you know if there is any way to avoid this artifact on the video 
output and maintain the AGP enabled?

See below some of the technical characteristics of the laptop in case it 
is of any help.

Thank you very much

Gianni

gc at bcn-lap:~$   inxi -G
Graphics:  Card: Advanced Micro Devices [AMD/ATI] RV250/M9 GL [Mobility 
FireGL 9000/Radeon 9000]
            Display Server: x11 (X.Org 1.19.6 ) drivers: ati,radeon 
(unloaded: modesetting,fbdev,vesa)
            Resolution: 1920x1080 at 60.00hz
            OpenGL: renderer: llvmpipe (LLVM 10.0.0, 128 bits) version: 
3.3 Mesa 20.0.8


gc at bcn-lap:~$ lspci -vvv

...

00:01.0 PCI bridge: Intel Corporation 82855PM Processor to AGP 
Controller (rev 03) (prog-if 00 [Normal decode])
     Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- 
Stepping- SERR- FastB2B- DisINTx-
     Status: Cap- 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- 
<TAbort- <MAbort- >SERR- <PERR- INTx-
     Latency: 128
     Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
     I/O behind bridge: 00003000-00003fff
     Memory behind bridge: 90400000-904fffff
     Prefetchable memory behind bridge: 98000000-9fffffff
     Secondary status: 66MHz+ FastB2B+ ParErr- DEVSEL=medium >TAbort- 
<TAbort- <MAbort+ <SERR- <PERR-
     BridgeCtl: Parity- SERR+ NoISA+ VGA+ MAbort- >Reset- FastB2B-
         PriDiscTmr- SecDiscTmr- DiscTmrStat- DiscTmrSERREn-
....

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. 
[AMD/ATI] RV250/M9 GL [Mobility FireGL 9000/Radeon 9000] (rev 01) 
(prog-if 00 [VGA controller])
     Subsystem: Compaq Computer Corporation RV250/M9 GL [Mobility FireGL 
9000/Radeon 9000]
     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: 128 (2000ns min), Cache Line Size: 32 bytes
     Interrupt: pin A routed to IRQ 10
     Region 0: Memory at 98000000 (32-bit, prefetchable) [size=128M]
     Region 1: I/O ports at 3000 [size=256]
     Region 2: Memory at 90400000 (32-bit, non-prefetchable) [size=64K]
     [virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
     Capabilities: [58] AGP version 2.0
         Status: RQ=48 Iso- ArqSz=0 Cal=0 SBA+ ITACoh- GART64- HTrans- 
64bit- FW+ AGP3- Rate=x1,x2,x4
         Command: RQ=1 ArqSz=0 Cal=0 SBA+ AGP- GART64- 64bit- FW- 
Rate=<none>
     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: radeonfb, radeon



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