linux-kernel - Re: [PATCH] tick: add check for the existence of broadcast clock event device

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Date:	Mon, 8 Jun 2009 14:47:40 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"Pan, Jacob jun" <jacob.jun.pan@...el.com>
Subject: Re: [PATCH] tick: add check for the existence of broadcast clock
 event device

On Mon, 8 Jun 2009 14:33:14 +0800
Thomas Gleixner <tglx@...utronix.de> wrote:

> Feng,
> 
> On Mon, 8 Jun 2009, Feng Tang wrote:
> > Our apbt driver is pretty similar with HPET's, including its cpu
> > hotplug notifier. But our platform only has 2 available apbt to
> > use, otherwise we will configure it just like HPET, using one timer
> > as bc and others for per-cpu ones, then it won't hit this case
> > 
> > There are 2 situations, one is for the normal boot, apbt0 will be
> > inited first and registered to OS as cpu0's timer, then tsc/lapic
> > is calculated based on it, and apbt1 is registered later in a
> > fs_initcall() (just like hpet.c does) after basic kernel core is
> > up. so the sequence is: apbt0 --> lapic0 --> lapic1 --> apbt1
> 
> Hmm, I do not like that at all. That explicitely relies on CPU0 doing
> some work which will kick CPU1. That's fragile as hell. 
I understand the concern. apbt0 is inited in a very early boot phase when
the cpu1 is not up yet, and os don't even know wether there is a cpu1, that's
why we registered apbt1 in fs_initcall(). If we explicitly setup apbt1 when
OS brings up cpu1, it is a little brutal and not generic as only our platform
has apbt, and I guess cpu hotplug maintainer won't like it :p

As I said if we have one more apbt, we will make it totally follow the HPET way,
sad thing is we don't have it :(

Thanks,
Feng

> 
> Why do you want to make the CPU1 case special? You setup apbt0 before
> you setup local APIC on CPU0, so why can't you do the same for apbt1
> on CPU1 ? That will also remove the complete hotplug logic from your
> code.
> 
> Thanks,
> 
> 	tglx
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