linux-kernel - [PATCH net] net: dsa: felix: re-enable TAS guard band mode

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Date:   Mon, 10 May 2021 13:07:08 +0200
From:   Michael Walle <michael@...le.cc>
To:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        UNGLinuxDriver@...rochip.com, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>,
        Michael Walle <michael@...le.cc>
Subject: [PATCH net] net: dsa: felix: re-enable TAS guard band mode

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/

Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
Signed-off-by: Michael Walle <michael@...le.cc>
---
 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