Re: [Qemu-devel] [PATCH] Register usb-uhci reset function.

[Filip Navara <filip.navara@gmail.com>]

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On Tue, Jun 16, 2009 at 2:47 PM, Gleb Natapov <gleb@redhat.com> wrote:

> Update irq line on reset. Reseting irq line is required because
> racing irq from pci device will call piix3_set_irq(). piix3_set_irq()
> will remember current level in pci_irq_levels[]. The PIC line will be
> triggered if one of pci_irq_levels[] is set (depends on piix3 config).
> If for instance pci_irq_levels[0] and pci_irq_levels[1] are mapped to
> the same PIC irq and during reset pci_irq_levels[1] == 1, but device
> that drives pci_irq_levels[0] is initialized first the device driver
> will not be able to lower irq line.
>

I have been trying to stay away from the discussion for a long while, but I
can't keep it anymore. The patch is wrong. Since qemu_irq doesn't hold any
state, the information on reset has to be cleared on the places where the
state is maintained. Under no circumstances should any *_set_irq() function
should be called from reset handlers! Especially since the order of reset
handlers is not guaranteed. The reseting of the interrupt state in practice
means that interrupt status registers of individual devices should be
cleared, the PCI bus interrupt levels should be cleared - *in the PCI reset
handler* and so on. Eventually you will end up with reset handlers that
clear the state at every level, so there won't be any "hanging interrupts"
after reset.

Best regards,
Filip Navara

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