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

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 11/12/06 14:35, "Keir Fraser" <keir@xxxxxxxxxxxxx> wrote:

> On 11/12/06 12:10, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
>> Yes, there's no interrupt from it. But what's the known frequency?
>> CALIBRATE_LATCH or CLOCK_TICK_RATE? platform_timer_scale
>> is set to the later, and so ch2 needs to work on same frequency or
>> else count value will result incorrect offset. However I didn't see such
>> setting on ch2. Does I miss something? :-)
> 
> All the PIT channels tick at CLOCK_TICK_RATE. Mode-0 is not auto-relatching,
> so when the counter reaches 0000 it carries on FFFF, FFFE, FFFD, ....
> 
> So we calculate platform system time as:
>  system_timestamp + (pit_timestamp - pit_timenow) * (delta_system/delta_pit)

Where, to be explicit, since system time is in nanoseconds we have:
 delta_system = 1000000000
 delta_pit    = CLOCK_TICK_RATE

 -- Keir


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