Re: [PATCH 01/16] math128: Introduce various 128bit primitives

[Juri Lelli]

Hi,
first of all thanks to everybody for all this comments!

On 10/26/2012 05:56 AM, Peter Zijlstra wrote:
> On Fri, 2012-10-26 at 12:44 +0200, Peter Zijlstra wrote:
> > > We can still have the user space interface handing in the information
> > > in nsec resolution, but it's reasonable to scale it down to something
> > > useful. Just shift the incoming information right by 10, so you're in
> > > the 1us resolution for all the internal math and all your limitation
> > > problems are gone. A shift by ten for converting back and forth to
> > > nsecs is not a real performance issue.
> >
> > I'm fine with that.. all I wanted was to not have the undefined overflow
> > we initially had.
>
> Note that we still need the constraint checking with this, although with
> both values shifted right 10 bits the range is now much bigger and
> shouldn't be a practical limit anymore.

I'll try to recap what seems to me you agreed and what will be the
changes for the next iteration.

- remove first two patches (u128 math) [and keep them in a safe place
  just in case following constraints will annoy future generation users
  :P]

- scale down (right by 10) incoming parameters as to do internal
  math with ~1us resolution (and scale up outgoing params)

- insert new constraints on -dl entities parameters:

    o since we have - dl_period >= dl_deadline >= dl_runtime - the
      only constraint we have to add for the overflow problem should
      be dl_period * dl_runtime < U64_MAX

    o to rule out problems with <= 1000ns parameters just force the user
      to pass > 1000ns parameters (in the end its our real resolution)

- WARN_ONCE() in proper places

- properly document all this (comments and Documentation)

What you think?

Thanks a lot and Regards,

- Juri
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