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Re: [PATCH v5 5/7] hw/isa/vt82c686: Work around missing level sensitive

From: David Woodhouse
Subject: Re: [PATCH v5 5/7] hw/isa/vt82c686: Work around missing level sensitive irq in i8259 model
Date: Wed, 01 Mar 2023 14:05:31 +0000
User-agent: Evolution 3.44.4-0ubuntu1

On Wed, 2023-03-01 at 14:18 +0100, BALATON Zoltan wrote:
> > Are you sure the PIC ELCR is actually set for the lines you're having
> > trouble with? Is that something the Pegasos SmartFirmware would have
> > done, and MorphOS is expecting to inherit but isn't actually setting up
> > for itself?
> No, it works with other guests like Linux and AmigaOS that use PIC as set 
> up by the firmware but MorphOS tries to use it in level sensitive mode and 
> likely has an IRQ handler which expects this to work. This is where I've 
> debugged it and came to this workaround:
> https://lists.nongnu.org/archive/html/qemu-ppc/2023-02/msg00403.html
> When booting MorphOS with -d unimp I see these logs:
> i8259: level sensitive irq not supported
> i8259: level sensitive irq not supported
> which is I guess when it tries to set it for both PICs. (If you want to 
> try this MorphOS iso is downloadable and instructions how to boot it is 
> here: http://zero.eik.bme.hu/~balaton/qemu/amiga/#morphos

Wow. Even looking at the PIIX3 datasheet from 1996, That 'Edge/Level
Bank Select (LTIM)' bit was documented as 'This bit is disabled. Its
function is replaced by the Edge/Level Triggerede Control (ELCR)

We've been able to set the edge/level on a per-pin basis ever since the
ELCR was introduced with the IBM PS/2, I think.

It isn't a *correct* fix without a little bit more typing, but does
this make it work?

diff --git a/hw/intc/i8259.c b/hw/intc/i8259.c
index 17910f3bcb..36ebcff025 100644
--- a/hw/intc/i8259.c
+++ b/hw/intc/i8259.c
@@ -246,6 +246,7 @@ static void pic_ioport_write(void *opaque, hwaddr addr64,
             if (val & 0x08) {
                               "i8259: level sensitive irq not supported\n");
+                s->elcr = 0xff;
         } else if (val & 0x08) {
             if (val & 0x04) {

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