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net: dsa: yt921x: Add DCB/QoS support

This commit adds DCB/QoS support to the yt921x DSA switch driver,
setting up global DSCP/PCP priority mappings and adding related DCB
methods.

[ ... ]

+/* Not "port" - DSCP mapping is global */
+static int __maybe_unused
+yt921x_dsa_port_get_dscp_prio(struct dsa_switch *ds, int port, u8 dscp)
+{
+	struct yt921x_priv *priv = to_yt921x_priv(ds);
+	u32 val;
+	int res;
+
+	mutex_lock(&priv->reg_lock);
+	res = yt921x_reg_read(priv, YT921X_IPM_DSCPn(dscp), &val);
+	mutex_unlock(&priv->reg_lock);
+
+	if (res)
+		return res;
+
+	return FIELD_GET(YT921X_IPM_PRIO_M, val);
+}
+
+static int __maybe_unused
+yt921x_dsa_port_del_dscp_prio(struct dsa_switch *ds, int port, u8 dscp, u8 prio)
+{
+	struct yt921x_priv *priv = to_yt921x_priv(ds);
+	int res;
+
+	mutex_lock(&priv->reg_lock);
+	res = yt921x_reg_write(priv, YT921X_IPM_DSCPn(dscp),
+			       YT921X_IPM_PRIO(IEEE8021Q_TT_BK));
+	mutex_unlock(&priv->reg_lock);
+
+	return res;
+}

Should yt921x_dsa_port_del_dscp_prio() check the prio parameter before
resetting the DSCP mapping? The ocelot driver (ocelot_port_del_dscp_prio
in drivers/net/ethernet/mscc/ocelot.c) has a comment explaining that
during a "dcb app replace" command, the new entry is added first, then
the old entry is deleted. If the delete operation ignores the prio
parameter, it will incorrectly reset the newly-added mapping instead
of only deleting when the current priority matches the old one.

The ksz driver also checks this condition in ksz_port_del_dscp_prio().

Without this check, the sequence for "dcb app replace ... dscp X prio N"
followed by "dcb app replace ... dscp X prio M" would be:
  1. add dscp X -> prio M (correct)
  2. del dscp X, prio N (but code ignores prio N and resets to BK)
  3. result: dscp X -> BK instead of prio M

Perhaps something like this is needed:

  if (yt921x_dsa_port_get_dscp_prio(ds, port, dscp) != prio)
      return 0;