[Xen-devel] Re: Question on virtual HPET's behavior

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

My reading of the (really bad) specification doc is that in periodic mode writes to the comparator register change the period only, unless VAL_SET_CNF is set in which case only the comparator counter value is set. The Linux HPET code agrees with this interpretation — it writes to the comparator register twice. Do you have an example of an OS that assumes different HPET behaviour?

 -- Keir

On 26/12/07 12:37, "Shan, Haitao" <haitao.shan@xxxxxxxxx> wrote:

> Here hpet.period is updated only when timer 0 is in periodic mode and SET_VAL is not set. But from HPET spec 1.0a, I can not find the reason. The spec provides a recommended sequence to reset main counter and comparator.
>
> If the software resets the main counter, the value in the comparator’s value register needs to reset as well. This can be done by setting the  n_VAL_SET_CNF bit. Again, to avoid race conditions, this should be done with the main counter halted. The following usage model is expected:
>
> 1) Software clears the GLOBAL_ENABLE_CNF bit to prevent any interrupts
>
> 2) Software Clears the main counter by writing a value of 00000000h to it.
>
> 3) Software sets the TIMER0_VAL_SET_CNF bit.
>
> 4) Software writes the new value in the TIMER0_COMPARATOR_VAL register
>
> 5) Software sets the GLOBAL_ENABLE_CNF bit to enable interrupts.
>
> What will the result is system software only writes the comparator once after set SET_VAL like the above sequence?
>
> Can you give me some hints? Thanks in advance!




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