[PATCH net] net: dsa: felix: re-enable TAS guard band mode
From: Michael Walle <hidden>
Date: 2021-05-10 12:16:34
Also in:
lkml
Subsystem:
networking drivers, networking [dsa], ocelot ethernet switch driver, the rest · Maintainers:
Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Andrew Lunn, Vladimir Oltean, Vladimir Oltean, Linus Torvalds
Commit 316bcffe4479 ("net: dsa: felix: disable always guard band bit for
TAS config") disabled the guard band and broke 802.3Qbv compliance.
There are two issues here:
(1) Without the guard band the end of the scheduling window could be
overrun by a frame in transit.
(2) Frames that don't fit into a configured window will still be sent.
The reason for both issues is that the switch will schedule the _start_
of a frame transmission inside the predefined window without taking the
length of the frame into account. Thus, we'll need the guard band which
will close the gate early, so that a complete frame can still be sent.
Revert the commit and add a note.
For a lengthy discussion see [1].
[1] https://lore.kernel.org/netdev/c7618025da6723418c56a54fe4683bd7@walle.cc/ (local)
Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
Signed-off-by: Michael Walle <redacted>
---
drivers/net/dsa/ocelot/felix_vsc9959.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index 2473bebe48e6..f966a253d1c7 100644
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c@@ -1227,12 +1227,17 @@ static int vsc9959_qos_port_tas_set(struct ocelot *ocelot, int port, if (taprio->num_entries > VSC9959_TAS_GCL_ENTRY_MAX) return -ERANGE; - /* Set port num and disable ALWAYS_GUARD_BAND_SCH_Q, which means set - * guard band to be implemented for nonschedule queues to schedule - * queues transition. + /* Enable guard band. The switch will schedule frames without taking + * their length into account. Thus we'll always need to enable the + * guard band which reserves the time of a maximum sized frame at the + * end of the time window. + * + * Although the ALWAYS_GUARD_BAND_SCH_Q bit is global for all ports, we + * need to set PORT_NUM, because subsequent writes to PARAM_CFG_REG_n + * operate on the port number. */ - ocelot_rmw(ocelot, - QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port), + ocelot_rmw(ocelot, QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM(port) | + QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q, QSYS_TAS_PARAM_CFG_CTRL_PORT_NUM_M | QSYS_TAS_PARAM_CFG_CTRL_ALWAYS_GUARD_BAND_SCH_Q, QSYS_TAS_PARAM_CFG_CTRL);
--
2.20.1