linux-kernel - Re: [PATCH 2/2] x86/apic: check global clockevent in lapic timer setup

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Date:	Sat, 19 Dec 2009 01:44:31 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Pan, Jacob jun" <jacob.jun.pan@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 2/2] x86/apic: check global clockevent in lapic timer
	setup

On Fri, Dec 18, 2009 at 10:13:52AM -0800, Pan, Jacob jun wrote:
...
> >
> >No, we need to fix the whole lapic timer calibration logic first.
> >
> >There is no reason why we don't calibrate the lapic at the same time
> >as we calibrate the TSC.
> [[JPAN]] that seems to be much more efficient and we can have platform specific
> way of calibration too with the x86_init abstraction.

Good idea I think!

> >
> >Another question is why there is no way to read out the lapic clock
> >frequency from some config registers or wherever the chip designers
> >decided to hide that. There is no real reason why the lapic timer
> >calibration needs to be extremly precise.
> >
> [[JPAN]] x86 does have MSR_FSB_FREQ to read bus frequency then the DCR to figure
> out LAPIC timer freq. but i guess not all CPU models have that. so having
> the abstraction would be a plus for those do have reliable reporting of lapic
> freq.

IIRC old apics may use independent clock signal too, though I dont think that we
ever switch (espec novadays) to use it due to obsolescense of such chips :)

> 
> >> Honestly, i don't fully understand how the dummy lapic event device
> >> is related to the broadcast mechanism. can someone explain why we
> >> register the dummy lapic clockevent?
> >
> >The broadcast mechanism is there in the first place to work around the
> >APIC stops in deeper C-states idiocy.
> >
> >Then we need to support the disable lapic timer command line option
> >(even on SMP) so we make use of the existing broadcast mechanism and
> >register the dummy device to have a per cpu clock event device.
> >
> [[JPAN]] thanks for the explanation. so if we have per cpu timer that is
> always-on, and don't have a broadcast timer, then the dummy device would not
> be needed, correct?
> 
> 

Hmm... We may be using nmi detector, so I think we still need dummy clockevent
device to send broadcast "time" IPI, or per-cpu timer interrupt handler have
to call the local apic interrupt routine. At least that is how I imagine this
scheme :)

> >Thanks,
> >
> >	tglx
> 
	-- Cyrill
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