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Date: Tue, 16 Jun 2009 20:05:29 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Register uhci_reset() callback.
Message-ID: <20090616190529.GO11893@shareable.org>
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Gleb Natapov <gleb@redhat.com>, Paul Brook <paul@codesourcery.com>

Jamie Lokier wrote:
> Blue Swirl wrote:
> It's analogous to a distributed atomic transaction problem.
> 
> So you need two phases:
> 
>     - Restoring: All devices states are restored, one by one including
>       output levels of interrupt lines and GPIOs, but nothing actually
>       _happens_ when those levels are set.
> 
>     - Running: All devices start running at the same instant from
>       their restored state.  They don't need to reexamine input
>       levels, because their internal states are already consistent
>       with the input levels.
> 
> In the restoring phase, you might still use the normal functions to
> set output levels, but they would be prevented from being passed as
> changes to other devices.
> 
> Anything else might be made to work with particular PICs etc., but
> two-phase restore is what you need to work with any wiring (including
> cycles) of arbitrary devices with arbitrary states.

I should say, the above applies to both restoring saved state, and
whole system resets.

That's why real hardware has a nice long reset pulse, during which
every input change is ignored until the reset pulse is removed.

-- Jamie