net: dsa: introduce a dsa_port_from_netdev public helper

As its implementation shows, this is synonimous with calling
dsa_slave_dev_check followed by dsa_slave_to_port, so it is quite simple
already and provides functionality which is already there.

However there is now a need for these functions outside dsa_priv.h, for
example in drivers that perform mirroring and redirection through
tc-flower offloads (they are given raw access to the flow_cls_offload
structure), where they need to call this function on act->dev.

But simply exporting dsa_slave_to_port would make it non-inline and
would result in an extra function call in the hotpath, as can be seen
for example in sja1105:

Before:

000006dc <sja1105_xmit>:
{
 6dc:	e92d4ff0 	push	{r4, r5, r6, r7, r8, r9, sl, fp, lr}
 6e0:	e1a04000 	mov	r4, r0
 6e4:	e591958c 	ldr	r9, [r1, #1420]	; 0x58c <- Inline dsa_slave_to_port
 6e8:	e1a05001 	mov	r5, r1
 6ec:	e24dd004 	sub	sp, sp, #4
	u16 tx_vid = dsa_8021q_tx_vid(dp->ds, dp->index);
 6f0:	e1c901d8 	ldrd	r0, [r9, #24]
 6f4:	ebfffffe 	bl	0 <dsa_8021q_tx_vid>
			6f4: R_ARM_CALL	dsa_8021q_tx_vid
	u8 pcp = netdev_txq_to_tc(netdev, queue_mapping);
 6f8:	e1d416b0 	ldrh	r1, [r4, #96]	; 0x60
	u16 tx_vid = dsa_8021q_tx_vid(dp->ds, dp->index);
 6fc:	e1a08000 	mov	r8, r0

After:

000006e4 <sja1105_xmit>:
{
 6e4:	e92d4ff0 	push	{r4, r5, r6, r7, r8, r9, sl, fp, lr}
 6e8:	e1a04000 	mov	r4, r0
 6ec:	e24dd004 	sub	sp, sp, #4
	struct dsa_port *dp = dsa_slave_to_port(netdev);
 6f0:	e1a00001 	mov	r0, r1
{
 6f4:	e1a05001 	mov	r5, r1
	struct dsa_port *dp = dsa_slave_to_port(netdev);
 6f8:	ebfffffe 	bl	0 <dsa_slave_to_port>
			6f8: R_ARM_CALL	dsa_slave_to_port
 6fc:	e1a09000 	mov	r9, r0
	u16 tx_vid = dsa_8021q_tx_vid(dp->ds, dp->index);
 700:	e1c001d8 	ldrd	r0, [r0, #24]
 704:	ebfffffe 	bl	0 <dsa_8021q_tx_vid>
			704: R_ARM_CALL	dsa_8021q_tx_vid

Because we want to avoid possible performance regressions, introduce
this new function which is designed to be public.
Suggested-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -637,6 +637,7 @@ void dsa_devlink_resource_occ_get_register(struct dsa_switch *ds,
 					   void *occ_get_priv);
 void dsa_devlink_resource_occ_get_unregister(struct dsa_switch *ds,
 					     u64 resource_id);
+struct dsa_port *dsa_port_from_netdev(struct net_device *netdev);
 
 struct dsa_devlink_priv {
 	struct dsa_switch *ds;

net/dsa/dsa.c

@@ -412,6 +412,15 @@ void dsa_devlink_resource_occ_get_unregister(struct dsa_switch *ds,
 }
 EXPORT_SYMBOL_GPL(dsa_devlink_resource_occ_get_unregister);
 
+struct dsa_port *dsa_port_from_netdev(struct net_device *netdev)
+{
+	if (!netdev || !dsa_slave_dev_check(netdev))
+		return ERR_PTR(-ENODEV);
+
+	return dsa_slave_to_port(netdev);
+}
+EXPORT_SYMBOL_GPL(dsa_port_from_netdev);
+
 static int __init dsa_init_module(void)
 {
 	int rc;