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SubjectRe: Startup IPI (was: Re: test13-pre3)
On Tue, 19 Dec 2000, Petr Vandrovec wrote:

> I did... So it uses 'xchg %eax,APIC_ICR' instead of 'movl %eax,APIC_ICR',
> yes (as verified in generated code...)? No change, still dies, as expected
> (do not forget that before it dies, it can do ~0x1300 write-read cycles

I've forgotten indeed...

> from videomemory (AGP4x), so secondary CPU just does some thinking before

This might be the time needed to deliver the IPI. Remember that the
inter-APIC bus is serial and not that fast.

> it kills machine; only problem is that 0x1300 wr-rd cycles to VGA apperture
> take 3.48ms, and this does not correspond with needed 200us udelay.

Hmm, how do you calculate the time? Assuming AGP4x runs at 133MHz and a
read or write cycle lasts for a single clock tick (I don't know exact AGP
specs -- please correct me if I'm wrong), I find 0x1300 cycles to finish
in about 73usecs. The loop execution overhead may double the result and
it will still fit within 300usecs.

> Maybe chipset decides to do something when second CPU cannot obtain
> bus access in 100000 pci cycles?).

I guess a certain initial cycle from the AP confuses the chipset somehow.

> Do you (or anyone else) have code which can dump MTRR registers of each
> of CPU before mtrr driver takes over them? At least first CPU does not have
> any problem...

A brief look at arch/i386/kernel/mtrr.c reveals the bootstrap CPU's
settings do not get changed. As a result they may always be fetched from
the /proc filesystem. For APs you probably need to tweak sources.

> I even placed 'wbinvd' and 'wbinvd; cpuid' before sending startup IPI,
> but it does not matter. Secondary CPU just does not finish even first
> instruction when first CPU reads from videoram again and again.

Well, the CPU obeys the writeback and the invalidation, but does the

> Without VIA datasheet I cannot try to disable some PCI features to find
> which one is culprit, so I'm sorry.

But you may complain to the manufacturer and/or change hardware. I'm
still uncertain the delay should stay in...


+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+ e-mail:, PGP key available +

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