Re: [Linuxptp-devel] [RFC 1/1] Add avnu_ap to enable AVnu Automotive Profile

[Cliff S. <csp...@go...>]

On Wed, Jul 18, 2018 at 10:36 AM Patel, Vedang <ved...@in...>
wrote:

> To take advantage of the quicker startup time the profile specifies.
>
> There will be more features added later on which will further optimize
> linuxptp to run more efficiently on the automotive network. I have
> described all of the future enhancements in the cover letter:


So the motivation behind these changes is quicker startup time, and faster
synchronization of slave clocks. That is a good goal, but best implemented
directly into an existing spec. Instead, this spec deeply modifies 802.1as
creating yet another non-conforming variant of IEEE 1588.

Removing Announce and BMCA seems to cause all sorts of additional
complexity, such as all this logic for what a boundary clock should do
based on the lack of a Sync message. If it just ran normal PTP it wouldn't
need all this complexity - it could just promote its local clock as
grandmaster.

This spec has a lot of special logic that you haven't yet implemented - see
"9.3.2 Non-Continuous Sync Values" for example. One of your planned
features is to slow down Sync messages after stabilization, but that
appears to require a special port-local signalling message to be sent and
received.

Normally I'd say that this spec could be implemented by creating a separate
state machine (similar to the slaveOnly state machine), but this is all
just too different. Personally, I think that if you really need to
implement this spec, it would be better to fork linuxptp, strip out all the
disabled and unnecessary features, and implement all special-case logic
directly, instead of conditionally. As far as I can see, this spec will
only become more divergent from IEEE 1588 and 802.1as over time with
planned security and path redundancy features.

If we look at table 6 in Section 5.5 of the spec[1], grand master is
> expected to start sending Sync/Follow-Up messages within 250ms. Without
> skipping BMCA, I don't see it happening.


Without any patches ptp4l can already become grandmaster quickly: setting
logAnnounceInterval to -10 gets through BMCA in 3 milliseconds on my
computer.