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From: Richard Henderson <rth@twiddle.net>
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Date: Tue, 25 Apr 2017 12:11:00 +0200
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Subject: Re: [Qemu-devel] [PATCH RFC] target/openrisc: Support non-busy idle
 state using PMR SPR
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To: Stafford Horne <shorne@gmail.com>
Cc: QEMU Development <qemu-devel@nongnu.org>, Openrisc <openrisc@lists.librecores.org>

On 04/23/2017 11:54 PM, Stafford Horne wrote:
> The OpenRISC architecture has the Power Management Register (PMR)
> special purpose register to manage cpu power states.  The interesting
> modes are:
> 
>   * Doze Mode (DME) - Stop cpu except timer & pic - wake on interrupt
>   * Sleep Mode (SME) - Stop cpu and all units - wake on interrupt
>   * Suspend Model (SUME) - Stop cpu and all units - wake on reset
> 
> The linux kernel will set DME when idle.

And SUME would be, essentially, poweroff?  Perhaps at least for the purposes of 
QEMU; on real hardware one could press a button to assert reset and reboot.

> Also, I don't know if its due to this patch of an issue with the timer
> interrupts.  After applying this patch the timer interrupts do not trigger
> until a keypress is make.  i.e. something like this...
> 
>    $ sleep 5
>    <hangs forever until a key is pressed>
...
> +            cpu_restore_state(cs, GETPC() + 4);

This isn't correct.  You want

	cpu_restore_state(cs, GETPC());
	cs->env.pc += 4;

So what's happening is that you're re-executing the MTSPR and going back to 
sleep again.  Which probably explains the hang.


r~