Regression: Atari SCU support

Regression: Atari SCU support

[Eero Tamminen]

Hi,

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 
(preferred), or handler is provided for VEC_INT4 [2] when SCU is used

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


	- Eero

[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

Re: Regression: Atari SCU support

[Michael Schmitz]

Hi Eero,

while the workaround will shut up the unexpected interrupt messages, it 
still does not prevent that interrupt from happening.

falcon_hblhandler() will not set the IPL sufficiently high for that, and 
I do not know whether permanently setting IPL 4 will quiesce other 
system interrupts.

Setting tt_scu.sys_mask zero is indeed the preferred option.

If you or Christian want to submit a patch to that effect, I'll gladly 
add my review tag.

Cheers,

     Michael


On 20/06/24 02:43, Eero Tamminen wrote:
> Hi,
>
> 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 
> (preferred), or handler is provided for VEC_INT4 [2] when SCU is used
>
> Christian has verified the issue and its workaround on real Atari TT.
>
>
>     - Eero
>
> [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