net: mscc: ocelot: divide watermark value by 60 when writing to SYS_ATOP

Tail dropping is enabled for a port when:

1. A source port consumes more packet buffers than the watermark encoded
   in SYS:PORT:ATOP_CFG.ATOP.

AND

2. Total memory use exceeds the consumption watermark encoded in
   SYS:PAUSE_CFG:ATOP_TOT_CFG.

The unit of these watermarks is a 60 byte memory cell. That unit is
programmed properly into ATOP_TOT_CFG, but not into ATOP. Actually when
written into ATOP, it would get truncated and wrap around.

Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mscc/ocelot.c

@@ -1253,7 +1253,7 @@ void ocelot_port_set_maxlen(struct ocelot *ocelot, int port, size_t sdu)
 	struct ocelot_port *ocelot_port = ocelot->ports[port];
 	int maxlen = sdu + ETH_HLEN + ETH_FCS_LEN;
 	int pause_start, pause_stop;
-	int atop_wm;
+	int atop, atop_tot;
 
 	if (port == ocelot->npi) {
 		maxlen += OCELOT_TAG_LEN;
@@ -1274,12 +1274,12 @@ void ocelot_port_set_maxlen(struct ocelot *ocelot, int port, size_t sdu)
 	ocelot_fields_write(ocelot, port, SYS_PAUSE_CFG_PAUSE_STOP,
 			    pause_stop);
 
-	/* Tail dropping watermark */
-	atop_wm = (ocelot->shared_queue_sz - 9 * maxlen) /
+	/* Tail dropping watermarks */
+	atop_tot = (ocelot->shared_queue_sz - 9 * maxlen) /
 		   OCELOT_BUFFER_CELL_SZ;
-	ocelot_write_rix(ocelot, ocelot->ops->wm_enc(9 * maxlen),
-			 SYS_ATOP, port);
-	ocelot_write(ocelot, ocelot->ops->wm_enc(atop_wm), SYS_ATOP_TOT_CFG);
+	atop = (9 * maxlen) / OCELOT_BUFFER_CELL_SZ;
+	ocelot_write_rix(ocelot, ocelot->ops->wm_enc(atop), SYS_ATOP, port);
+	ocelot_write(ocelot, ocelot->ops->wm_enc(atop_tot), SYS_ATOP_TOT_CFG);
 }
 EXPORT_SYMBOL(ocelot_port_set_maxlen);