slot-based crash! not SBusFPGA (Re: CG14 in 8-bit color)

[Romain Dolbeau <romain%dolbeau.org@localhost>]

Hello,

During the development of framebuffer emulation in my SBusFPGA, I had
this strange bug:

Le ven. 3 déc. 2021 à 20:29, Romain Dolbeau <romain%dolbeau.org@localhost> a écrit :
> Neither, it doesn't crash while accessing hardware in X11, that's part
> of why it's been a pain to diagnose. When using my own
> re-implementation of a cg6 or a cg3 (and the cg3 is a very passive and
> simple device), X11 works apparently fine (visually speaking), but
> after I exit X11 the kernel will crash sooner or later (this does not
> happen with a real cg6 of course, so it concerns literally just me! I
> don't have a real cg3 handy to triple-check). It happens even when my
> device is set to a lower resolution like 1024x768 or 1152x900. If I
> deliberately use swap while running X11, it apparently gets corrupted;
> if I use swap after leaving X11, a crash is guaranteed, and it's
> always in 'pv_syncflags4m' IIRC.

I finally tracked it down ... to the SBus slot, as far as I can tell !
(I wasn't moving the real TGX+ around, it was in slot 3 while the
SBusFPGA was in or out of slot 1).

After emulating a cg3/cg6 with a genuine PROM (so the SW stack was
strictly identical), my design would still crash.
I tried single-headed by removing the 'real' TGX+, but then also moved
the SBusFPGA to a different slot (from 1 to 2). No crash.
Back to 1, crash again.
Try again in 2, fine.
In 2 with my own PROM and other devices, fine.
Remove SBusFPGA, put the real TGX+ in slot 1 ... and crash again. With
no SBusFPGA in the machine.

No idea why this happens, but while the SBusFPGA with other devices
were fine in that slot, I can't use it for a TGX+, real or emulated.
I will have to try with a different SS20 to see if it's a design issue
(unlikely!) or if the machine is somehow damaged.
I did plug and unplug the SBusFPGA quite a lot, and it was the easiest
slot to work with, so it may have taken some damage. Yet USB and the
RAM disk from SBusFPGA seem OK in it.

It did get me to code a hardware-based initializer and a bw2 emulated
device, so it's not a complete waste of time (I'll probably need both
if I ever get around to getting a NuBusFPGA made).

Cordially,

-- 
Romain Dolbeau