[Intel-wired-lan] [RFC v2 net-next 1/2] rtnetlink: Add new RTM_GETSYNCESTATE message to get SyncE status

[Machnikowski, Maciej]

> -----Original Message-----
> From: Jakub Kicinski <kuba at kernel.org>
> Sent: Tuesday, August 31, 2021 4:18 PM
> To: Machnikowski, Maciej <maciej.machnikowski at intel.com>
> Subject: Re: [RFC v2 net-next 1/2] rtnetlink: Add new RTM_GETSYNCESTATE
> message to get SyncE status
> 
> On Tue, 31 Aug 2021 14:07:32 +0000 Machnikowski, Maciej wrote:
> > > > I agree that this also is useful for Time card, yet it's also useful here.
> > > > PTP subsystem should implement a similar logic to this one for
> > > > DPLL-driven timers which can lock its frequency to external sources.
> > >
> > > Why would we have two APIs for doing the same thing? IIUC Richard
> does
> > > not want this in the PTP ioctls which is fair, but we need to cater to
> devices
> > > which do not have netdevs.
> >
> > From technical point of view - it can be explained by the fact that the DPLL
> > driving the SyncE logic can be separate from the one driving PTP.  Also
> > SyncE is frequency-only oriented and doesn't care about phase and
> > Time of Day that PTP also needs. The GNSS lock on the PTP side will be
> > multi-layered, as the full lock would mean that our PTP clock is not only
> > syntonized, but also has its time and phase set correctly.
> 
> Just because GNSS lock addresses more parameters (potentially) doesn't
> mean the syntonization part shouldn't be addressed by the same API.

Fair enough.

> 
> > A PTP can reuse the "physical" part of this interface later on, but it also
> needs
> > to solve more SW-specific challenges, like reporting the PTP lock on a SW
> level.
> >
> > I agree that having such API for PTP subsystem will be very useful,
> > but let's address SyncE in netdev first and build the PTP netlink on top of
> what
> > we learn here. We can always move the structures defined here to the
> layer
> > above without affecting any APIs.
> 
> It's a reasonable SW design strategy to start simple. Unfortunately,
> it doesn't apply to stable uAPI design. You're adding a RTNL op, which
> will have to be supported for ever. If we add anything "later" it will
> be a strict addition, and will have to be backward compatible. Which
> I'm not sure how to do when the object we'd operate on would be
> completely different (clock vs netdev).

I agree - the point I'm trying to make here is that the existence of
the PTP-specific interface will not invalidate the need of having 
SyncE-specific one as well. Even if we report lock-states for the clock
we will still need to report lock-states for devices that don't use PTP
clocks, but support SyncE. (that's also a reason why RTNL is still required).

The RTNL interface will also address devices that only need the 
frequency syntonization (especially in Radio Access Networks).

> 
> As I said I can write the boilerplate code for you if you prefer, the
> code implementing the command and the driver interface will be almost
> identical.

I think it's a great idea to start that in parallel to this patch. Then move
the common structures to the generic layer and use them in both
SyncE-specific RTNL implementation and PTP-specific part that will
be added. This won't affect SyncE specific APIs. The "worst" that can
happen is that the driver will put the same info for PTP part and
SyncE part if that's the design someone follows.

Regards
Maciek

> 
> Is there a reason why RTNL is better?