Re: [PATCH for-next V2 0/9] Add completion timestamping support

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Wed, Jun 03, 2015 at 10:48:28PM +0300, Or Gerlitz wrote:

> >  - Should the frequency and mask be general, or driver private? If the
> >    cycles->ns conversion is a function they should be driver private.
> >    Even if they are general at libibverbs, they don't *have* to be in
> >    the kernel's general query response.
> 
> If they are general in libibverbs, what's the point not to put them in
> the kernel's general query response?

If there is a timestamp_to_ns API then they would not be general in
libiverbs either.

> >  - Should frequency even be frequency? Most clocks are expressed
> >    accurately as a period in picoseconds. Frequency is more often
> >    imprecise. (eg ethernet is 3200 ps or 312.5MHz)
> >    However FDR/EDR is fractional for both (4693.33333333 ps vs
> >    213.0681818181818 MHz)
> >    Precision is very important for time conversions, so a
> >    multiply-divide scheme would be ideal.
> 
> From Christoph's response I got the impression that our proposal of
> exposing frequency and mask combined with raw time stamps excellently
> fits typical user needs, so I thought we're good. Doug made a comment
> that things look OK to him and the rest of the work would be when we
> come to review the user-space patches.

This response ignores my point about precision.

MHz is fine *for mlx hardware* but someone elses hardware that uses,
say 312.5 MHz (ie the ethernet symbol clock) is NOT OK because MHz
looses too much precision.

Jason
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