Commit 297c4de6 authored by Michael Walle's avatar Michael Walle Committed by David S. Miller
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net: dsa: felix: re-enable TAS guard band mode 

Commit 316bcffe ("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/



Fixes: 316bcffe ("net: dsa: felix: disable always guard band bit for TAS config")
Signed-off-by: default avatarMichael Walle <michael@walle.cc>
Reviewed-by: default avatarVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 3058e01d

drivers/net/dsa/ocelot/felix_vsc9959.c

+10 −5
Original line number Diff line number Diff line
@@ -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);