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[Xen-devel] Re: A question about PIT platform timer

  • To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir@xxxxxxxxxxxxx>
  • Date: Mon, 11 Dec 2006 09:36:18 +0000
  • Delivery-date: Mon, 11 Dec 2006 01:36:23 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AccdA44RYK8xaKWxSleokF0BH7qTYQABEEJ1
  • Thread-topic: A question about PIT platform timer

On 11/12/06 09:05, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> Just note that pit_read_counter is implemented on top of CH2 of PIT,
> which is known as mode 0 to calibrate TSC earlier. However I see no
> place to re-initialize it again and it's used as the source of platform
> timer count (when using_pit==1) immediately. It seems like weird
> usage since mode0 channel will fall into a loop starting counting down
> from 0xFFFF forever if nobody re-latches it again, which indicates a
> 18.2065 Hz source while platform_timer_scale is set as CLOCK_TICK_RATE.
> Seems ch0 is what's really wanted... Is it a typo, or any special usage
> on ch2?

We use it as a free-running globally-accessible counter of known frequency.
We don't take interrupts from it (in fact we can't even if we wanted to as
it's not wired up to an IRQ).

 -- Keir

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