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man: tc-taprio.8: document the full offload feature 

Since this feature's introduction in commit 9c66d1564676 ("taprio: Add
support for hardware offloading") from kernel v5.4, it never got
documented in the man pages. Due to this reason, we see customer reports
of seemingly contradictory information: the community manpages claim
there is no support for full offload, nonetheless many silicon vendors
have already implemented it.

This patch documents the full offload feature (enabled by specifying
"flags 2" to the taprio qdisc) and gives one more example that tries to
illustrate some of the finer points related to the usage.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
This commit is contained in:
Vladimir Oltean 2021-01-22 03:22:11 +02:00 committed by Stephen Hemminger
parent 86d9660dc1
commit 4712a46174
1 changed files with 46 additions and 5 deletions
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51
man/man8/tc-taprio.8
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@ -92,7 +92,11 @@ in the schedule;
clockid
.br
Specifies the clock to be used by qdisc's internal timer for measuring
time and scheduling events.
time and scheduling events. This argument must be omitted when using the
full-offload feature (flags 0x2), since in that case, the clockid is
implicitly /dev/ptpN (where N is given by
.B ethtool -T eth0 | grep 'PTP Hardware Clock'
), and therefore not necessarily synchronized with the system's CLOCK_TAI.
.TP
sched-entry
@ -115,13 +119,27 @@ before moving to the next entry.
.TP
flags
.br
Specifies different modes for taprio. Currently, only txtime-assist is
supported which can be enabled by setting it to 0x1. In this mode, taprio will
set the transmit timestamp depending on the interval in which the packet needs
to be transmitted. It will then utililize the
This is a bit mask which specifies different modes for taprio.
.RS
.TP
.I 0x1
Enables the txtime-assist feature. In this mode, taprio will set the transmit
timestamp depending on the interval in which the packet needs to be
transmitted. It will then utililize the
.BR etf(8)
qdisc to sort and transmit the packets at the right time. The second example
can be used as a reference to configure this mode.
.TP
.I 0x2
Enables the full-offload feature. In this mode, taprio will pass the gate
control list to the NIC which will execute it cyclically in hardware.
When using full-offload, there is no need to specify the
.B clockid
argument.
The txtime-assist and full-offload features are mutually exclusive, i.e.
setting flags to 0x3 is invalid.
.RE
.TP
txtime-delay
@ -178,5 +196,28 @@ for more information about configuring the ETF qdisc.
offload delta 200000 clockid CLOCK_TAI
.EE
The following is a schedule in full offload mode. The
.B base-time
is 200 ns and the
.B cycle-time
is implicitly calculated as the sum of all
.B sched-entry
durations (i.e. 20 us + 20 us + 60 us = 100 us). Although the base-time is in
the past, the hardware will start executing the schedule at a PTP time equal to
the smallest integer multiple of 100 us, plus 200 ns, that is larger than the
NIC's current PTP time.
.EX
# tc qdisc add dev eth0 parent root taprio \\
num_tc 8 \\
map 0 1 2 3 4 5 6 7 \\
queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \\
base-time 200 \\
sched-entry S 80 20000 \\
sched-entry S a0 20000 \\
sched-entry S df 60000 \\
flags 0x2
.EE
.SH AUTHORS
Vinicius Costa Gomes <vinicius.gomes@intel.com>