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Re: [PATCH 0/5] Pegasos2 fixes and audio output support

From: BALATON Zoltan
Subject: Re: [PATCH 0/5] Pegasos2 fixes and audio output support
Date: Wed, 22 Feb 2023 22:12:01 +0100 (CET) 
On Wed, 22 Feb 2023, Bernhard Beschow wrote:

Am 22. Februar 2023 19:25:16 UTC schrieb BALATON Zoltan <balaton@eik.bme.hu>:

On Wed, 22 Feb 2023, Bernhard Beschow wrote:

On Wed, Feb 22, 2023 at 4:38 PM Bernhard Beschow <shentey@gmail.com> wrote:

On Tue, Feb 21, 2023 at 7:44 PM BALATON Zoltan <balaton@eik.bme.hu> wrote:

This series fixes PCI interrupts on the ppc/pegasos2 machine and adds
partial implementation of the via-ac97 sound part enough to get audio
output. I'd like this to be merged for QEMU 8.0.

Regards,
BALATON Zoltan

BALATON Zoltan (5):
  hw/isa/vt82c686: Implement interrupt routing in via_isa_set_irq
  hw/isa/vt82c686: Implement PIRQ pins
  hw/ppc/pegasos2: Fix PCI interrupt routing
  hw/audio/ac97: Split off some definitions to a header
  hw/audio/via-ac97: Basic implementation of audio playback

 hw/audio/ac97.c            |  43 +---
 hw/audio/ac97.h            |  65 ++++++
 hw/audio/trace-events      |   6 +
 hw/audio/via-ac97.c        | 436 ++++++++++++++++++++++++++++++++++++-
 hw/ide/via.c               |   2 +-
 hw/isa/vt82c686.c          |  61 +++++-
 hw/pci-host/mv64361.c      |   4 -
 hw/ppc/pegasos2.c          |  26 ++-
 hw/usb/vt82c686-uhci-pci.c |   5 +-
 include/hw/isa/vt82c686.h  |  39 +++-
 10 files changed, 626 insertions(+), 61 deletions(-)
 create mode 100644 hw/audio/ac97.h

--
2.30.7



Wow, the MorphOS people paid attention to sound design. Thanks for
presenting it to us, Zoltan!

I've had a closer look at your series and I think it can be simplified:
Patch 2 can be implemented quite straight-forward like I proposed in a
private mail: https://github.com/shentok/qemu/commit/via-priq-routing.
Then, in order to make patch 3 "hw/ppc/pegasos2: Fix PCI interrupt routing"
working, one can expose the PCI interrupts with a single line like you do
in patch 2. With this, patch 1 "hw/isa/vt82c686: Implement interrupt
routing in via_isa_set_irq" isn't needed any longer and can be omitted.

In via-ac97, rather than using via_isa_set_irq(), pci_set_irq() can be
used instead. pci_set_irq() internally takes care of all the ISA interrupt
level tracking patch 1 attempted to address.



Here is a proof of concept branch to demonstrate that the simplification
actually works: https://github.com/shentok/qemu/commits/pegasos2 (Tested
with MorphOS with and without pegasos2.rom).


Does this only work because both the via-ac97 and the PCI interrupts are mapped 
to the same ISA IRQ and you've only tested sound? The guest could configure 
each device to use a different IRQ, also mapping them so they share one ISA 
interrupt. What happens if multiple devices are mapped to IRQ 9 (which is the 
case on pegasos2 where PCI cards, ac97 and USB all share this IRQ) and more 
than one such device wants to raise an interrupt at the same time? If you ack 
the ac97 interrupt but a PCI network card or the USB part still wants to get 
the CPUs attention the ISA IRQ should remain raised until all devices are 
serviced.


pci_bus_get_irq_level(), used in via_isa_set_pci_irq(), should handle
exactly that case very well.


I don't see a way to track the status of all devices in a single qemu_irq which 
can only be up or down so we need something to store the state of each source.


pci_set_irq() causes pci_bus_change_irq_level() to be called.
pci_bus_change_irq_level() tracks the sum of all irq levels of all
devices attached to a particular pin in irq_count. Have a look at
pci_bus_change_irq_level() and you will understand better.
I'm aware of that, we're using that in sam460ex which connects all PCI interrupt lines to a single IRQ and Peter explored and explained it in a comment there when that was discovered. First we had a patch with or-irq but due to this behaviot that's not needed for PCI interrupts. But the VT8132 could change what ISA IRQ you route the sub functions to. It happens that on pegasos2 by default all of those are routed to IRQ9 except IDE but what if a guest changes ac97 to use a different interrupt? Then it's not a PCI interrupt any more so you can't use pci_set_irq in via=ac97. There are only 4 PCI INT lines but the VIA components can be routed to 13 or 14 ISA IRQs. How do you keep track of that with only the PCI bus interrupts? I don't get your approach. 


My patch adds a state register to each ISA IRQ line for all possible sources 
which could probably be stored once but then for each change of ISA IRQ status 
all the mapped devices should be checked and combined so it's easier to store 
them for each IRQ. Does your approach still work if you play sound, and copy 
something from network to a USB device at the same time? (I'm not sure mine 
does not have remaining bugs but I don't think this can be simplified that way 
but if you can prove it would work I don't mind taking an alternative version 
but I'm not convinced yet.)


Well, I can't prove that my approach works but unfortunately I can
prove that both our approaches cause a freeze :/ Try:
1. Start `qemu-system-ppc -M pegasos2 -bios pegasos2.rom -rtc
base=localtime -device ati-vga,guest_hwcursor=true,romfile="" -cdrom
morphos-3.17.iso -device usb-mouse -device usb-kbd`
2. Move the mouse while sound is playing
-> Observe the VM to freeze

So there must be an issue somewhere else...
I'll have a look later but my patch attempts to handle the USB controller interrupts. There may be another bug somewhere in USB emulation though, we have similar problem with mac99 with older MacOS guests. Considering that USB devices probably did not work at all before this patch it's at least still an imptovement. :-) 

Regards,
BALATON Zoltan
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