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Re: [Qemu-devel] [PATCH 11/22] i8259: Update IRQ state after reset

From: Blue Swirl
Subject: Re: [Qemu-devel] [PATCH 11/22] i8259: Update IRQ state after reset
Date: Thu, 29 Sep 2011 19:35:32 +0000

On Wed, Sep 28, 2011 at 9:38 PM, Peter Maydell <address@hidden> wrote:
> On 28 September 2011 19:42, Blue Swirl <address@hidden> wrote:
>> We also assume that the reset states of devices and their outputs are
>> coherent i.e. the output of one device during reset would not change
>> the state of another device from its reset state.
> This seems a very dubious assumption to make. Consider the
> realview board: the 'card present' line from the pl181
> MMC card controller is wired up to (1) a GPIO input on the
> 'sysctl' system register module [which causes it to appear
> as a status bit in a register] and (2) via an inverter to
> an input on a pl061 GPIO module. On coming out of reset
> something has to be done to make the reset value of the
> output from the pl181 be reflected in the internal status
> of the sysctl and pl061 devices.
> (On real hardware this happens when the pl061/sysctl
> devices come out of reset and sample their input pins.
> If we wanted to model it that way we'd need internal state
> on all gpio lines and a two phase "enter reset"+"leave reset"
> or something.)

Maybe the rule should be that during reset, all inputs should be in
high impedance mode. We'd need a two-phase reset for QEMU for that
too, or to disconnect all signals before reset and then reconnect them
after that.

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