net: dsa: felix: enable cut-through forwarding between ports by default

The VSC9959 switch embedded within NXP LS1028A (and that version of
Ocelot switches only) supports cut-through forwarding - meaning it can
start the process of looking up the destination ports for a packet, and
forward towards those ports, before the entire packet has been received
(as opposed to the store-and-forward mode).

The up side is having lower forwarding latency for large packets. The
down side is that frames with FCS errors are forwarded instead of being
dropped. However, erroneous frames do not result in incorrect updates of
the FDB or incorrect policer updates, since these processes are deferred
inside the switch to the end of frame. Since the switch starts the
cut-through forwarding process after all packet headers (including IP,
if any) have been processed, packets with large headers and small
payload do not see the benefit of lower forwarding latency.

There are two cases that need special attention.

The first is when a packet is multicast (or flooded) to multiple
destinations, one of which doesn't have cut-through forwarding enabled.
The switch deals with this automatically by disabling cut-through
forwarding for the frame towards all destination ports.

The second is when a packet is forwarded from a port of lower link speed
towards a port of higher link speed. This is not handled by the hardware
and needs software intervention.

Since we practically need to update the cut-through forwarding domain
from paths that aren't serialized by the rtnl_mutex (phylink
mac_link_down/mac_link_up ops), this means we need to serialize physical
link events with user space updates of bonding/bridging domains.

Enabling cut-through forwarding is done per {egress port, traffic class}.
I don't see any reason why this would be a configurable option as long
as it works without issues, and there doesn't appear to be any user
space configuration tool to toggle this on/off, so this patch enables
cut-through forwarding on all eligible ports and traffic classes.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211125125808.2383984-2-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index 42ac1952b39a..36f9c2e0e063 100644
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
@@ -2125,6 +2125,80 @@ static void vsc9959_psfp_init(struct ocelot *ocelot)
 	INIT_LIST_HEAD(&psfp->sgi_list);
 }
 
+/* When using cut-through forwarding and the egress port runs at a higher data
+ * rate than the ingress port, the packet currently under transmission would
+ * suffer an underrun since it would be transmitted faster than it is received.
+ * The Felix switch implementation of cut-through forwarding does not check in
+ * hardware whether this condition is satisfied or not, so we must restrict the
+ * list of ports that have cut-through forwarding enabled on egress to only be
+ * the ports operating at the lowest link speed within their respective
+ * forwarding domain.
+ */
+static void vsc9959_cut_through_fwd(struct ocelot *ocelot)
+{
+	struct felix *felix = ocelot_to_felix(ocelot);
+	struct dsa_switch *ds = felix->ds;
+	int port, other_port;
+
+	lockdep_assert_held(&ocelot->fwd_domain_lock);
+
+	for (port = 0; port < ocelot->num_phys_ports; port++) {
+		struct ocelot_port *ocelot_port = ocelot->ports[port];
+		int min_speed = ocelot_port->speed;
+		unsigned long mask = 0;
+		u32 tmp, val = 0;
+
+		/* Disable cut-through on ports that are down */
+		if (ocelot_port->speed <= 0)
+			goto set;
+
+		if (dsa_is_cpu_port(ds, port)) {
+			/* Ocelot switches forward from the NPI port towards
+			 * any port, regardless of it being in the NPI port's
+			 * forwarding domain or not.
+			 */
+			mask = dsa_user_ports(ds);
+		} else {
+			mask = ocelot_get_bridge_fwd_mask(ocelot, port);
+			mask &= ~BIT(port);
+			if (ocelot->npi >= 0)
+				mask |= BIT(ocelot->npi);
+			else
+				mask |= ocelot_get_dsa_8021q_cpu_mask(ocelot);
+		}
+
+		/* Calculate the minimum link speed, among the ports that are
+		 * up, of this source port's forwarding domain.
+		 */
+		for_each_set_bit(other_port, &mask, ocelot->num_phys_ports) {
+			struct ocelot_port *other_ocelot_port;
+
+			other_ocelot_port = ocelot->ports[other_port];
+			if (other_ocelot_port->speed <= 0)
+				continue;
+
+			if (min_speed > other_ocelot_port->speed)
+				min_speed = other_ocelot_port->speed;
+		}
+
+		/* Enable cut-through forwarding for all traffic classes. */
+		if (ocelot_port->speed == min_speed)
+			val = GENMASK(7, 0);
+
+set:
+		tmp = ocelot_read_rix(ocelot, ANA_CUT_THRU_CFG, port);
+		if (tmp == val)
+			continue;
+
+		dev_dbg(ocelot->dev,
+			"port %d fwd mask 0x%lx speed %d min_speed %d, %s cut-through forwarding\n",
+			port, mask, ocelot_port->speed, min_speed,
+			val ? "enabling" : "disabling");
+
+		ocelot_write_rix(ocelot, val, ANA_CUT_THRU_CFG, port);
+	}
+}
+
 static const struct ocelot_ops vsc9959_ops = {
 	.reset			= vsc9959_reset,
 	.wm_enc			= vsc9959_wm_enc,
@@ -2136,6 +2210,7 @@ static const struct ocelot_ops vsc9959_ops = {
 	.psfp_filter_add	= vsc9959_psfp_filter_add,
 	.psfp_filter_del	= vsc9959_psfp_filter_del,
 	.psfp_stats_get		= vsc9959_psfp_stats_get,
+	.cut_through_fwd	= vsc9959_cut_through_fwd,
 };
 
 static const struct felix_info felix_info_vsc9959 = {