[PATCH v2 11/11] eventdev: RFC clarify docs on event object fields

[Bruce Richardson]

On Fri, Feb 02, 2024 at 10:38:10AM +0100, Mattias Rönnblom wrote:
> On 2024-02-01 17:59, Bruce Richardson wrote:
> > On Wed, Jan 24, 2024 at 12:34:50PM +0100, Mattias Rönnblom wrote:
> > > On 2024-01-19 18:43, Bruce Richardson wrote:
> > > > Clarify the meaning of the NEW, FORWARD and RELEASE event types.
> > > > For the fields in "rte_event" struct, enhance the comments on each to
> > > > clarify the field's use, and whether it is preserved between enqueue and
> > > > dequeue, and it's role, if any, in scheduling.
> > > > 
> > > > Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
> > > > ---
> > > > 
<snip>
> > > Is the normalized or unnormalized value that is preserved?
> > > 
> > Very good point. It's the normalized & then denormalised version that is
> > guaranteed to be preserved, I suspect. SW eventdevs probably preserve
> > as-is, but HW eventdevs may lose precision. Rather than making this
> > "implementation defined" or "not preserved" which would be annoying for
> > apps, I think, I'm going to document this as "preserved, but with possible
> > loss of precision".
> > 
> 
> This makes me again think it may be worth noting that Eventdev -> API
> priority normalization is (event.priority * PMD_LEVELS) / EVENTDEV_LEVELS
> (rounded down) - assuming that's how it's supposed to be done - or something
> to that effect.
> 
Following my comment on the thread on the other patch about looking at
numbers of bits of priority being valid, I did a quick check of the evdev PMDs
by using grep for "max_event_priority_levels" in each driver. According to
that (and resolving some #defines), I see:

0 - dpaa, dpaa2
1 - cnxk, dsw, octeontx, opdl
4 - sw
8 - dlb2, skeleton

So it looks like switching to a bit-scheme is workable, where we measure
supported event levels in powers-of-two only. [And we can cut down priority
fields if we like].

Question for confirmation. For cases where the eventdev does not support
per-event prioritization, I suppose we should say that the priority field
is not preserved, as well as being ignored?

/Bruce