netdev - [PATCH net-next 3/7] net: dsa: felix: unconditionally configure MAC speed to 1000Mbps

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Date:   Thu, 25 Jun 2020 18:23:27 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     davem@...emloft.net, netdev@...r.kernel.org
Cc:     andrew@...n.ch, f.fainelli@...il.com, vivien.didelot@...il.com,
        claudiu.manoil@....com, alexandru.marginean@....com,
        ioana.ciornei@....com, linux@...linux.org.uk
Subject: [PATCH net-next 3/7] net: dsa: felix: unconditionally configure MAC speed to 1000Mbps

From: Vladimir Oltean <vladimir.oltean@....com>

In VSC9959, the PCS is the one who performs rate adaptation (symbol
duplication) to the speed negotiated by the PHY. The MAC is unaware of
that and must remain configured for gigabit. If it is configured at
OCELOT_SPEED_10 or OCELOT_SPEED_100, it'll start transmitting PAUSE
frames out of control and never recover, _even if_ we then reconfigure
it at OCELOT_SPEED_1000 afterwards.

This patch fixes a bug that luckily did not have any functional impact.
We were writing 10, 100, 1000 etc into this 2-bit field in
DEV_CLOCK_CFG, but the hardware expects values in the range 0, 1, 2, 3.
So all speed values were getting truncated to 0, which is
OCELOT_SPEED_2500, and which also appears to be fine.

Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 drivers/net/dsa/ocelot/felix.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/net/dsa/ocelot/felix.c b/drivers/net/dsa/ocelot/felix.c
index 25b340e0a6dd..d229cb5d5f9e 100644
--- a/drivers/net/dsa/ocelot/felix.c
+++ b/drivers/net/dsa/ocelot/felix.c
@@ -240,9 +240,14 @@ static void felix_phylink_mac_config(struct dsa_switch *ds, int port,
 	u32 mac_fc_cfg;

 	/* Take port out of reset by clearing the MAC_TX_RST, MAC_RX_RST and
-	 * PORT_RST bits in CLOCK_CFG
+	 * PORT_RST bits in DEV_CLOCK_CFG. Note that the way this system is
+	 * integrated is that the MAC speed is fixed and it's the PCS who is
+	 * performing the rate adaptation, so we have to write "1000Mbps" into
+	 * the LINK_SPEED field of DEV_CLOCK_CFG (which is also its default
+	 * value).
 	 */
-	ocelot_port_writel(ocelot_port, DEV_CLOCK_CFG_LINK_SPEED(state->speed),
+	ocelot_port_writel(ocelot_port,
+			   DEV_CLOCK_CFG_LINK_SPEED(OCELOT_SPEED_1000),
 			   DEV_CLOCK_CFG);

 	/* Flow control. Link speed is only used here to evaluate the time
-- 
2.25.1