Re: [PATCH][2.6][5/5]Support for HPET based timer

[Vojtech Pavlik <vojtech@suse.cz>]

On Tue, Aug 19, 2003 at 06:28:40PM -0700, Pallipadi, Venkatesh wrote:
> 
> I experimented with HPET in native APIC routing mode. But, there 
> are couple of issues in that space:
> 
> 1) During boot up kernel expects to receive timer interrupt much
> before the IO-APIC initialization is done. If HPET uses native mode,
> it cannot generate timer interrupts till IOAPICs are initialized. So,
> we need to have some sort of Workarounds in generic kernel to avoid
> dependency on timer interrupt during the early boot.

Yes, that is a problem. We could do some switchover from PIT to HPET (or
from legacy to IOAPIC IRQs) after APICs are initialized, though I don't
like the idea very much. Best would be to remove the dependence on the
timer interrupt ticking so early. Or moving APIC detection earlier.

> 2) More important question is, do we really want to share timer
> interrupt with other PCI devices? This potentially can add some delay
> in the timer interrupt processing, and thus we may end up getting
> inaccurate time (and inaccurate timer interrupts) in the kernel.

Well, the APIC should have quite a number of free pins, which means that
the HPET shouldn't need to share an interrupt. Regarding lost and late
delivered timer interrupts - that happens nevertheless with drivers
disabling interrupts for a long time. The kernel timekeeping code can
cope with that.

> Thanks, -Venkatesh
> > -----Original Message----- From: Vojtech Pavlik
> > [mailto:vojtech@suse.cz] Sent: Tuesday, August 19, 2003 3:41 PM To:
> > Pallipadi, Venkatesh Cc: linux-kernel@vger.kernel.org;
> > torvalds@osdl.org; Nakajima, Jun; Mallick, Asit K Subject: Re:
> > [PATCH][2.6][5/5]Support for HPET based timer
> > 
> > 
> > On Tue, Aug 19, 2003 at 12:20:22PM -0700, Pallipadi, Venkatesh
> > wrote:
> > 
> > > 5/5 - hpet5.patch - This can be a standalone patch. Without this
> > > patch we loose interrupt generation capability of RTC (/dev/rtc),
> > > due to HPET. With this patch we basically try to emulate RTC
> > > interrupt functions in software using HPET counter 1.
> > > 
> > 
> > This is very wrong IMO. We shouldn't try to emulate the RTC
> > interrupt for the kernel, instead the HPET should use native APIC
> > interrupt routing. This way the RTC will keep working and the
> > 'legacy mode' of HPET doesn't need to be used. I must admit I was a
> > bit lazy when I was implementing the x86_64 variant and the native
> > IRQ for HPET is still on my to-do list.
> > 
> > -- Vojtech Pavlik SuSE Labs, SuSE CR
> > 
> 

-- 
Vojtech Pavlik
SuSE Labs, SuSE CR