Re: device driver for the SGI system clock, mmtimer

[Christoph Lameter]

On Thu, 16 Sep 2004, Robert Picco wrote:

> >Is there something specific that drivers/char/hpet.c expects that
> >your hardware doesn't implement?
> Look at HPET revision history.  Specifically 0.98 01/20/2002
>     * Product name changed: from Multimedia Timer to HPET (High
> Precision Event Timer)

The HPET timer has a specific memory layout of registers that is mappable
to user space. The mmtimer driver only allows the mapping of a single 64
bit counter to use space. We have lots of applications at SGI
that rely on mmtimer since mmtimer provides a locally accessible
clock in an NUMA environment with hundreds of CPU. A hpet device would
have to show up in the global address space and require cross node
accesses in our NUMA environment that would make access to the timer
slow. All CPU would content for access to a certain global memory address.

The HPET hardware and the sgi mmtimer are totally different architectures
that are not easily reconcilable.

The software API to handle both is similar and we would like it to be as
compatible as possible.

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