Re: [PATCH v2 net 1/3] net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet

[Michael Walle]

Am 2022-09-06 02:11, schrieb Vladimir Oltean:
> On Tue, Sep 06, 2022 at 12:53:20AM +0200, Michael Walle wrote:
> > I haven't looked at the overall code, but the solution described
> > above sounds good.
> >
> > FWIW, I don't think such a schedule, where exactly one frame
> > can be sent, is very likely in the wild though. Imagine a piece
> > of software is generating one frame per cycle. It might happen
> > that during one (hardware) cycle there is no frame ready (because
> > it is software and it jitters), but then in the next cycle, there
> > are now two frames ready. In that case you'll always lag one frame
> > behind and you'll never recover from it.
> >
> > Either I'd make sure I can send at two frames in one cycle, or
> > my software would only send a frame every other cycle.
>
> A 10 us interval is a 10 us interval, it shouldn't matter if you slice
> it up as one 1250B frame, or two 500B frames, or four 200B frames, etc.
> Except with the Microchip hardware implementation, it does. In v1, we
> were slicing the 10 us interval in half for useful traffic and half for
> the guard band. So we could fit more small packets in 5 us. In v2, at
> your proposal, we are slicing it in 33 ns for the useful traffic, and
> 10 us - 33 ns for the guard band. This indeed allows for a single
> packet, be it big or small. It's how the hardware works; without any
> other input data point, a slicing point needs to be put somewhere.
> Somehow it's just as arbitrary in v2 as where it was in v1, just
> optimized for a different metric which you're now saying is less 
> practical.

I actually checked the code before writing and saw that one could
change the guard band by setting the MTU of the interface. I though,
"ah ok, then there is no issue". After sleeping, I noticed that you'd
restrict the size of all the frames on the interface. Doh ;)

-michael

> By the way, I was a fool in last year's discussion on guard bands for
> saying that there isn't any way for the user to control per-tc MTU.
> IEEE 802.1Qbv, later standardized as IEEE 802.1Q clause 8.6.8.4
> Enhancements for scheduled traffic, does contain a queueMaxSDUTable
> structure with queueMaxSDU elements. I guess I have no choice except to
> add this to the tc-taprio UAPI in a net-next patch, because as I've
> explained above, even though I've solved the port hanging issue, this
> hardware needs more fine tuning to obtain a differentiation between 
> many
> small packets vs few large packets per interval.