HPET drift question

HPET drift question

[Alex Williamson]

Hi Venki,

   I'm confused by the calculation of the drift value in the hpet
driver.  The specs defines the recommended minimum hardware
implementation is a frequency drift of 0.05% or 500ppm.  However, the
drift passed in when registering with the time interpolator is:

ti->drift = ti->frequency * HPET_DRIFT / 1000000;

Isn't that absolute number of ticks per second drift?  The time
interpolator defines the drift in parts per million.  Shouldn't this
simply be:

ti->drift = HPET_DRIFT;

The current code seems to greatly penalize any hpet timer with greater
than a 1MHz frequency.  Thanks,

	Alex

-- 
Alex Williamson                             HP Linux & Open Source Lab

RE: HPET drift question

[Pallipadi, Venkatesh]

Yes. Looks like "ti->drift = HPET_DRIFT;" is right here. However, I
would 
like to double check this with Bob.

Thanks,
Venki

>-----Original Message-----
>From: Alex Williamson [mailto:alex.williamson@hp.com] 
>Sent: Thursday, August 25, 2005 8:17 AM
>To: Pallipadi, Venkatesh
>Cc: robert.picco@hp.com; linux-kernel@vger.kernel.org
>Subject: HPET drift question
>
>Hi Venki,
>
>   I'm confused by the calculation of the drift value in the hpet
>driver.  The specs defines the recommended minimum hardware
>implementation is a frequency drift of 0.05% or 500ppm.  However, the
>drift passed in when registering with the time interpolator is:
>
>ti->drift = ti->frequency * HPET_DRIFT / 1000000;
>
>Isn't that absolute number of ticks per second drift?  The time
>interpolator defines the drift in parts per million.  Shouldn't this
>simply be:
>
>ti->drift = HPET_DRIFT;
>
>The current code seems to greatly penalize any hpet timer with greater
>than a 1MHz frequency.  Thanks,
>
>	Alex
>
>-- 
>Alex Williamson                             HP Linux & Open Source Lab
>
>

Re: HPET drift question

[Bob Picco]

Pallipadi, Venkatesh wrote:	[Fri Aug 26 2005, 08:53:35PM EDT]
> 
> Yes. Looks like "ti->drift = HPET_DRIFT;" is right here. However, I
> would 
> like to double check this with Bob.
> 
> Thanks,
> Venki
> 
> >-----Original Message-----
> >From: Alex Williamson [mailto:alex.williamson@hp.com] 
> >Sent: Thursday, August 25, 2005 8:17 AM
> >To: Pallipadi, Venkatesh
> >Cc: robert.picco@hp.com; linux-kernel@vger.kernel.org
> >Subject: HPET drift question
> >
> >Hi Venki,
> >
> >   I'm confused by the calculation of the drift value in the hpet
> >driver.  The specs defines the recommended minimum hardware
> >implementation is a frequency drift of 0.05% or 500ppm.  However, the
> >drift passed in when registering with the time interpolator is:
> >
> >ti->drift = ti->frequency * HPET_DRIFT / 1000000;
> >
> >Isn't that absolute number of ticks per second drift?  The time
> >interpolator defines the drift in parts per million.  Shouldn't this
> >simply be:
> >
> >ti->drift = HPET_DRIFT;
> >
> >The current code seems to greatly penalize any hpet timer with greater
> >than a 1MHz frequency.  Thanks,
> >
> >	Alex
> >
> >-- 
> >Alex Williamson                             HP Linux & Open Source Lab
> >
> >
Hi Venki:

Alex and I had an earlier IRC discussion where we agreed that HPET_DRIFT
should be the value.  We were just verifying with you.

thanks,

bob