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Re: [Qemu-devel] [Android-virt] [PATCH 03/12] hw/arm_boot.c: Make SMP bo


From: andrzej zaborowski
Subject: Re: [Qemu-devel] [Android-virt] [PATCH 03/12] hw/arm_boot.c: Make SMP boards specify address to poll in bootup loop
Date: Tue, 17 Jan 2012 00:31:54 +0100

On 16 January 2012 09:31, Peter Maydell <address@hidden> wrote:
> On 16 January 2012 01:56, Alexander Graf <address@hidden> wrote:
>>
>> On 13.01.2012, at 21:52, Peter Maydell wrote:
>>
>>> From: Evgeny Voevodin <address@hidden>
>>>
>>> The secondary CPU bootloader in arm_boot.c holds secondary CPUs in a
>>> pen until the primary CPU releases them. Make boards specify the
>>> address to be polled to determine whether to leave the pen (it was
>>> previously hardcoded to 0x10000030, which is a Versatile Express/
>>> Realview specific system register address).
>>
>> Is smp_boot implementing the same logic as hw/ppce500_spin.c? It
>> looks like the normal u-boot way of waiting for a magic address
>> to be written with boot info. What I don't understand is the WFI.
>> How can you wait for an interrupt if the trigger is a memory
>> write? Or are you actually getting IPIs?
>
> Basically this code is implementing the boot-rom half of
> what is technically a platform-specific contract[*] between
> the boot rom and the Linux kernel. The kernel end has to
> (a) write the secondary CPU's entry point into the register
> being polled and (b) send the CPU a softirq (an IPI, in
> other words). The WFI basically allows h/w to avoid running
> 3 cores at 100% when we're booting a uniprocessor OS.
> So the arm_boot end has to (a) enable the CPU's interrupt
> controller, (b) wait for interrupt, (c) get the entry point
> and jump to it.
>
> [*] realview, vexpress, highbank and exynos4 all do something
> that's sufficiently similar that we can handle them all by
> parameterising the secondary boot code a bit. omap is kinda
> different but then we don't support -kernel with omap anyway.

In the last sentence you mean the SMP case, right?

Cheers



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