RE: [PATCH 2/2] x86/apic: check global clockevent in lapic timersetup

From: Pan, Jacob jun
Date: Fri Dec 18 2009 - 13:14:45 EST

>-----Original Message-----
>From: Thomas Gleixner [mailto:tglx@xxxxxxxxxxxxx]
>Sent: Friday, December 18, 2009 8:36 AM
>To: Pan, Jacob jun
>Cc: H. Peter Anvin; Cyrill Gorcunov; linux-kernel@xxxxxxxxxxxxxxx;
>Subject: RE: [PATCH 2/2] x86/apic: check global clockevent in lapic timer setup
>On Thu, 17 Dec 2009, Pan, Jacob jun wrote:
>> >-----Original Message-----
>> >From: H. Peter Anvin [mailto:hpa@xxxxxxxxxxxxxxx]
>> >Sent: Thursday, December 17, 2009 2:34 PM
>> >To: Pan, Jacob jun
>> >Cc: Cyrill Gorcunov; linux-kernel@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx
>> >Subject: Re: [PATCH 2/2] x86/apic: check global clockevent in lapic timer
>> >
>> >On 12/17/2009 02:31 PM, Pan, Jacob jun wrote:
>> >>> Wouldn't be better to operate the same way as in case of "noapictimer"
>> >>> boot option. I guess the non-pc x86 midplatforms you're mentioning
>> >>> do not use SMP ever but just to be consistent in code.
>> >>>
>> >> [[JPAN]] We do use SMP with hyper threading in Moorestown.
>> >> In that case we have a per cpu platform timer without global clockevent.
>> >> so i think we don't want the dummy lapic event. we don't want to use the
>> >> broadcast mechanism as mentioned in the comments before disabling lapic
>> >> timer.
>> >>
>> >> For moorestown, I can use x86_init.timers.setup_percpu_clockev
>> >> abstraction function so that Moorestown platform does not need to call
>> >> setup_boot_APIC_clock() if per cpu platform timer is used. so many IFs :).
>> >>
>> >> But in the case of having constant and always on LAPIC timer, we still do
>> >> not want the dummy lapic clockevent and the broadcast. we will just have
>> >> per cpu always on local apic timers without global clockevent device.
>> >
>> >OK, I'm not entirely sure I follow this, and I'm not sure someone trying
>> >to understand the code in five years would, either. I don't really see
>> >the above translating into "we don't have a global clockevent, therefore
>> >we shouldn't initialize (but should still not disable) the local APIC
>> >timer."
>> >
>> > -hpa
>> [[JPAN]] There is some thing wrong in my logic.
>> If we have always running lapic timer, and per cpu platform timers, we would
>> still want to set up the lapic timer without global clockevent, just without
>> calibration. perhaps use a platform specific setup_percpu_clockev() function.
>> So i don't think we need this patch at the moment, maybe we only need to do a
>> sanity check for global clockevent in calibrate_APIC_clock().
>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.
>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

>> 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?

> tglx
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