Re: [hatari-devel] SCU/VME register access?

[Eero Tamminen <oak@xxxxxxxxxxxxxx>]

Hi Christian & Nicolas,

On 14.6.2024 18.51, Christian Zietz wrote:
> With some help from Eero (thank you!) I now have a kernel that I can
> load on my real TT.
>
> I can confirm:
>
> 1.) The unmodified kernel is indeed broken: It hangs in an infinite loop
> "unexpected interrupt from 112" due to the active but unhandled VBL IRQ.
> This should be reported to m68k linux.
>
> 2.) When I patch the kernel (with Ghidra and a hex editor) to write 0x00
> to the SCU sys_mask register, it boots, only panicking in the end (as
> expected) because I did not provide a root FS in my test.

Thanks for testing!

I was thinking of following "Atari SCU support broken" mail to m68k 
Linux mailing list:
-------------------------------------------------
During the development of Atari TT / MegaSTe SCU support for Hatari 
emulator, it was noticed that m68k Linux does not properly initialize 
SCU registers / IRQ handlers on boot.

Currently Linux freezes at boot and spews "unexpected interrupt" 
messages constantly on TT.

Nicolas Pomarède and Christian Zietz investigated:
* Issue to be due to VBL IRQ handler removal [1]
* boot to succeed if either 0x00 is written to the SCU sys_mask, or 
handler is provided for VEC_INT4 [2] when SCU is used

Christian has verified the issue and its workaround on real Atari TT.

[1] 
https://github.com/torvalds/linux/commit/1fa0b29f3a43f9dd8080d24cd16790b15ef963f5
[2] 
https://git.tuxfamily.org/hatari/hatari.git/tree/tools/linux/0004-WA-avoid-unexpected-SCU-interrupts.patch
-------------------------------------------------

Does that sounds OK?


	- Eero