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acfcdb78d5
With this port schedule:
tc qdisc replace dev $send_if parent root handle 100 taprio \
num_tc 8 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
map 0 1 2 3 4 5 6 7 \
base-time 0 cycle-time 10000 \
sched-entry S 01 1250 \
sched-entry S 02 1250 \
sched-entry S 04 1250 \
sched-entry S 08 1250 \
sched-entry S 10 1250 \
sched-entry S 20 1250 \
sched-entry S 40 1250 \
sched-entry S 80 1250 \
flags 2
ptp4l would fail to take TX timestamps of Pdelay_Resp messages like this:
increasing tx_timestamp_timeout may correct this issue, but it is likely caused by a driver bug
ptp4l[4134.168]: port 2: send peer delay response failed
It turns out that the driver can't take their TX timestamps because it
can't transmit them in the first place. And there's nothing special
about the Pdelay_Resp packets - they're just regular 68 byte packets.
But with this taprio configuration, the switch would refuse to send even
the ETH_ZLEN minimum packet size.
This should have definitely not been the case. When applying the taprio
config, the driver prints:
mscc_felix 0000:00:00.5: port 0 tc 0 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 1 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 2 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 3 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 4 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 5 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 6 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 7 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 132 octets including FCS
and thus, everything under 132 bytes - ETH_FCS_LEN should have been sent
without problems. Yet it's not.
For the forwarding path, the configuration is fine, yet packets injected
from Linux get stuck with this schedule no matter what.
The first hint that the static guard bands are the cause of the problem
is that reverting Michael Walle's commit 297c4de6f780 ("net: dsa: felix:
re-enable TAS guard band mode") made things work. It must be that the
guard bands are calculated incorrectly.
I remembered that there is a magic constant in the driver, set to 33 ns
for no logical reason other than experimentation, which says "never let
the static guard bands get so large as to leave less than this amount of
remaining space in the time slot, because the queue system will refuse
to schedule packets otherwise, and they will get stuck". I had a hunch
that my previous experimentally-determined value was only good for
packets coming from the forwarding path, and that the CPU injection path
needed more.
I came to the new value of 35 ns through binary search, after seeing
that with 544 ns (the bit time required to send the Pdelay_Resp packet
at gigabit) it works. Again, this is purely experimental, there's no
logic and the manual doesn't say anything.
The new driver prints for this schedule look like this:
mscc_felix 0000:00:00.5: port 0 tc 0 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 1 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 2 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 3 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 4 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 5 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 6 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
mscc_felix 0000:00:00.5: port 0 tc 7 min gate length 1250 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 131 octets including FCS
So yes, the maximum MTU is now even smaller by 1 byte than before.
This is maybe counter-intuitive, but makes more sense with a diagram of
one time slot.
Before:
Gate open Gate close
| |
v 1250 ns total time slot duration v
<---------------------------------------------------->
<----><---------------------------------------------->
33 ns 1217 ns static guard band
useful
Gate open Gate close
| |
v 1250 ns total time slot duration v
<---------------------------------------------------->
<-----><--------------------------------------------->
35 ns 1215 ns static guard band
useful
The static guard band implemented by this switch hardware directly
determines the maximum allowable MTU for that traffic class. The larger
it is, the earlier the switch will stop scheduling frames for
transmission, because otherwise they might overrun the gate close time
(and avoiding that is the entire purpose of Michael's patch).
So, we now have guard bands smaller by 2 ns, thus, in this particular
case, we lose a byte of the maximum MTU.
Fixes: 11afdc6526de ("net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Link: https://patch.msgid.link/20241210132640.3426788-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.