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Subject: odd behaviour with graphics, vt(4), VGA, and probing the text region
 leading to NMIs at boot
From: Adrian Chadd <adrian@freebsd.org>
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Hi,

I found -head didn't boot on a T400 I have here with external radeon
graphics (ie, not using the intel graphics.)

I was getting a memory parity error NMI upon boot, shortly after
probing option ROMs.

It turns out that it's the ISA device probe/attach path trying to
attach the VGA adapter, and doing testing of the memory regions at
0xb800:0000 (mono text) and 0xb000:0000 (colour text) - yes, I'm using
the 8086 real mode notation because I'm talking about ISA graphics
cards.

If I boot -HEAD with vt(4) in graphics mode, then it's using 640x480
graphics and that's mapped at 0xa000:0000 for 64k. There apparently
isn't anything at 0xb800:0000 or 0xb000:0000 and touching those
regions leads to an NMI.

Now, the tricksy bit is that it's not accessing those addresses
directly - it's going via the physical map, that's mapped it at
0xc00b8000 and 0xc00b0000 - yes, those are 32 bit (virtual, kernel
mode) addresses. I don't know if this is part of the problem.

If I boot -HEAD with vt(4) in text mode (hw.vga.textmode=1) then it boots fine.

So - has anyone else seen/debugged this? It doesn't happen on
everything - just this one T400 I have. But I can't help but wonder
what else is going to get finnicky about ISA probing like this.