Re: [net-next] net: dsa: felix: disable always guard band bit for TAS config

[Vladimir Oltean]

On Tue, May 04, 2021 at 09:08:00PM +0200, Michael Walle wrote:
> > > > > As explained in another mail in this thread, all queues are marked as
> > > > > scheduled. So this is actually a no-op, correct? It doesn't matter if
> > > > > it set or not set for now. Dunno why we even care for this bit then.
> > > >
> > > > It matters because ALWAYS_GUARD_BAND_SCH_Q reduces the available
> > > > throughput when set.
> > > 
> > > Ahh, I see now. All queues are "scheduled" but the guard band only
> > > applies
> > > for "non-scheduled" -> "scheduled" transitions. So the guard band is
> > > never
> > > applied, right? Is that really what we want?
> > 
> > Xiaoliang explained that yes, this is what we want. If the end user
> > wants a guard band they can explicitly add a "sched-entry 00" in the
> > tc-taprio config.
> 
> You're disabling the guard band, then. I figured, but isn't that
> suprising for the user? Who else implements taprio? Do they do it in the
> same way? I mean this behavior is passed right to the userspace and have
> a direct impact on how it is configured. Of course a user can add it
> manually, but I'm not sure that is what we want here. At least it needs
> to be documented somewhere. Or maybe it should be a switchable option.
> 
> Consider the following:
> sched-entry S 01 25000
> sched-entry S fe 175000
> basetime 0
> 
> Doesn't guarantee, that queue 0 is available at the beginning of
> the cycle, in the worst case it takes up to
> <begin of cycle> + ~12.5us until the frame makes it through (given
> gigabit and 1518b frames).
> 
> Btw. there are also other implementations which don't need a guard
> band (because they are store-and-forward and cound the remaining
> bytes). So yes, using a guard band and scheduling is degrading the
> performance.

What is surprising for the user, and I mentioned this already in another
thread on this patch, is that the Felix switch overruns the time gate (a
packet taking 2 us to transmit will start transmission even if there is
only 1 us left of its time slot, delaying the packets from the next time
slot by 1 us). I guess that this is why the ALWAYS_GUARD_BAND_SCH_Q bit
exists, as a way to avoid these overruns, but it is a bit of a poor tool
for that job. Anyway, right now we disable it and live with the overruns.

FWIW, the ENETC does not overrun the time gate, the SJA1105 does. You
can't really tell just by looking at the driver code, just by testing.
It's a bit of a crapshoot.

> > Sorry, I don't understand what you mean to say here.
> 
> I doubt that ALWAYS_GUARD_BAND_SCH_Q is a per-port setting. But that is
> only a guess. One would have to check with the IP vendor.

Probably not, but I'm not sure that this is relevant one way or another,
as the driver unconditionally clears it regardless of port (or unconditionally
set it, before Xiaoliang's patch).

> > > > May I know what drew your attention to this patch? Is there something
> > > > wrong?
> 
> See private mail.

Responded. I'm really curious if this change makes any difference at all
to your use case.