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Date:   Thu, 06 May 2021 20:28:51 +0200
From:   Michael Walle <michael@walle.cc>
To:     Vladimir Oltean <olteanv@gmail.com>
Cc:     Vladimir Oltean <vladimir.oltean@nxp.com>,
        Xiaoliang Yang <xiaoliang.yang_1@nxp.com>,
        UNGLinuxDriver@microchip.com, alexandre.belloni@bootlin.com,
        allan.nielsen@microchip.com,
        Claudiu Manoil <claudiu.manoil@nxp.com>, davem@davemloft.net,
        idosch@mellanox.com, joergen.andreasen@microchip.com,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        Po Liu <po.liu@nxp.com>, vinicius.gomes@intel.com
Subject: Re: [net-next] net: dsa: felix: disable always guard band bit for TAS
 config
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Am 2021-05-06 17:07, schrieb Vladimir Oltean:
> On Thu, May 06, 2021 at 04:41:51PM +0200, Michael Walle wrote:
>> Am 2021-05-06 15:50, schrieb Vladimir Oltean:
>> > On Thu, May 06, 2021 at 03:25:07PM +0200, Michael Walle wrote:
>> > > Am 2021-05-04 23:33, schrieb Vladimir Oltean:
>> > > > [ trimmed the CC list, as this is most likely spam for most people ]
>> > > >
>> > > > On Tue, May 04, 2021 at 10:23:11PM +0200, Michael Walle wrote:
>> > > > > Am 2021-05-04 21:17, schrieb 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.
>> > > > >
>> > > > > We are talking about the same thing here. Why is that a poor tool?
>> > > >
>> > > > It is a poor tool because it revolves around the idea of "scheduled
>> > > > queues" and "non-scheduled queues".
>> > > >
>> > > > Consider the following tc-taprio schedule:
>> > > >
>> > > > 	sched-entry S 81 2000 # TC 7 and 0 open, all others closed
>> > > > 	sched-entry S 82 2000 # TC 7 and 1 open, all others closed
>> > > > 	sched-entry S 84 2000 # TC 7 and 2 open, all others closed
>> > > > 	sched-entry S 88 2000 # TC 7 and 3 open, all others closed
>> > > > 	sched-entry S 90 2000 # TC 7 and 4 open, all others closed
>> > > > 	sched-entry S a0 2000 # TC 7 and 5 open, all others closed
>> > > > 	sched-entry S c0 2000 # TC 7 and 6 open, all others closed
>> > > >
>> > > > Otherwise said, traffic class 7 should be able to send any time it
>> > > > wishes.
>> > >
>> > > What is the use case behind that? TC7 (with the highest priority)
>> > > may always take precedence of the other TCs, thus what is the point
>> > > of having a dedicated window for the others.
>> >
>> > Worst case latency is obviously better for an intermittent stream (not
>> > more than one packet in flight at a time) in TC7 than it is for any
>> > stream in TC6-TC0. But intermittent streams in TC6-TC0 also have their
>> > own worst case guarantees (assuming that 2000 ns is enough to fit one
>> > TC 7 frame and one frame from the TC6-TC0 range).
>> 
>> Oh and I missed that, TC0-TC6 probably won't work because that gate is
>> too narrow (12.5us guard band) unless of course you set MAXSDU to a
>> smaller value. Which would IMHO be the correct thing to do here.
> 
> I'm not sure that this is exactly true. I know that I tested once, and
> the switch is happy to send a packet as long as the time window is 33 
> ns
> or larger (Idon't . Can't remember if the ALWAYS_GUARD_BAND_SCH_Q bit 
> was set or
> not, and I'm traveling right now so I don't have an LS1028A board handy
> to re-test.

I've just double checked with a 5.12 kernel (that is with
ALWAYS_GUARD_BAND_SCH_Q bit still set). TC7 works perfectly fine at
line rate. Frames in TC0-TC6 aren't forwarded.

Clearing that bit manually (ie. devmem 0x1fc20f698 32 0x0; btw the
schedule is on swp1, so this will also prove that bit is global ;))
will make TC0-6 work. But it will also happily forward 1518b frames.

Behavior as expected. Please feel free to confirm, though.

-michael