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

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

On Tue, Jun 02, 2015 at 10:35:24AM -0400, Doug Ledford wrote:

> So, just so everyone is clear on this point: the current user space
> implementation of this feature creates an unversioned, newly named
> ibv_wc_ex struct that is ibv_wc with a 64bit timestamp tacked on at the
> end (not 64bit aligned either).  If we ever wanted to have a different
> extension to our ibv_wc struct, there is no good way to do that.

No, if they followed (I didn't check yes) the extension scheme then the
poll call is

 struct ibv_wc_ex wcs[num_wcs]
 ibv_poll_wc_ex(&wcs,num_wcs,sizeof(wcs[0]));

And the drivers decide what to do based on the 3rd argument, which is
essentially the ABI version.

> Jason, when the XRC and flow steering extensions were added to
> libibverbs, you complained loudly that they were not added in the agreed
> upon format and cited a previous on list discussion.  Do you have a link
> to that discussion?

Not off hand, but that was different, that was a misuse of comp_mask,
IIRC.

Or, the question in my mind based on looking at the UAPI patches is
what things should be driver private and what should be general.

Broadly my thoughts:
 - 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.
 - 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.
   This is suggesting to me these details really are not
   general.
 - There should be much better definition on what all this stuff is,
   units for frequency? When is the timestamp applied?
 - Should an app even be exposed to mask? This is very difficult
   to use correctly in the general case. Only cases where an app is
   restarted more often than a wrap period are trivial to use properly.

Jason
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