net: dsa: realtek: Rewrite RTL8366RB MTU handling

The MTU callbacks are in layer 1 size, so for example 1500
bytes is a normal setting. Cache this size, and only add
the layer 2 framing right before choosing the setting. On
the CPU port this will however include the DSA tag since
this is transmitted from the parent ethernet interface!

Add the layer 2 overhead such as ethernet and VLAN framing
and FCS before selecting the size in the register.

This will make the code easier to understand.

The rtl8366rb_max_mtu() callback returns a bogus MTU
just subtracting the CPU tag, which is the only thing
we should NOT subtract. Return the correct layer 1
max MTU after removing headers and checksum.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/drivers/net/dsa/realtek/rtl8366rb.c b/drivers/net/dsa/realtek/rtl8366rb.c
index 887afd1392cbb0f8c3c470a1c01cbc8659eebebf..e3b6a470ca67ff2b5150d0363cede1c4b9f2019d 100644 (file)
--- a/drivers/net/dsa/realtek/rtl8366rb.c
+++ b/drivers/net/dsa/realtek/rtl8366rb.c
@@ -15,6 +15,7 @@
 #include <linux/bitops.h>
 #include <linux/etherdevice.h>
 #include <linux/if_bridge.h>
+#include <linux/if_vlan.h>
 #include <linux/interrupt.h>
 #include <linux/irqdomain.h>
 #include <linux/irqchip/chained_irq.h>
@@ -929,15 +930,19 @@ static int rtl8366rb_setup(struct dsa_switch *ds)
        if (ret)
                return ret;
 
-       /* Set maximum packet length to 1536 bytes */
+       /* Set default maximum packet length to 1536 bytes */
        ret = regmap_update_bits(priv->map, RTL8366RB_SGCR,
                                 RTL8366RB_SGCR_MAX_LENGTH_MASK,
                                 RTL8366RB_SGCR_MAX_LENGTH_1536);
        if (ret)
                return ret;
-       for (i = 0; i < RTL8366RB_NUM_PORTS; i++)
-               /* layer 2 size, see rtl8366rb_change_mtu() */
-               rb->max_mtu[i] = 1532;
+       for (i = 0; i < RTL8366RB_NUM_PORTS; i++) {
+               if (i == priv->cpu_port)
+                       /* CPU port need to also accept the tag */
+                       rb->max_mtu[i] = ETH_DATA_LEN + RTL8366RB_CPU_TAG_SIZE;
+               else
+                       rb->max_mtu[i] = ETH_DATA_LEN;
+       }
 
        /* Disable learning for all ports */
        ret = regmap_write(priv->map, RTL8366RB_PORT_LEARNDIS_CTRL,
@@ -1442,24 +1447,29 @@ static int rtl8366rb_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
        /* Roof out the MTU for the entire switch to the greatest
         * common denominator: the biggest set for any one port will
         * be the biggest MTU for the switch.
-        *
-        * The first setting, 1522 bytes, is max IP packet 1500 bytes,
-        * plus ethernet header, 1518 bytes, plus CPU tag, 4 bytes.
-        * This function should consider the parameter an SDU, so the
-        * MTU passed for this setting is 1518 bytes. The same logic
-        * of subtracting the DSA tag of 4 bytes apply to the other
-        * settings.
         */
-       max_mtu = 1518;
+       max_mtu = ETH_DATA_LEN;
        for (i = 0; i < RTL8366RB_NUM_PORTS; i++) {
                if (rb->max_mtu[i] > max_mtu)
                        max_mtu = rb->max_mtu[i];
        }
-       if (max_mtu <= 1518)
+
+       /* Translate to layer 2 size.
+        * Add ethernet and (possible) VLAN headers, and checksum to the size.
+        * For ETH_DATA_LEN (1500 bytes) this will add up to 1522 bytes.
+        */
+       max_mtu += VLAN_ETH_HLEN;
+       max_mtu += ETH_FCS_LEN;
+
+       if (max_mtu <= 1522)
                len = RTL8366RB_SGCR_MAX_LENGTH_1522;
-       else if (max_mtu > 1518 && max_mtu <= 1532)
+       else if (max_mtu > 1522 && max_mtu <= 1536)
+               /* This will be the most common default if using VLAN and
+                * CPU tagging on a port as both VLAN and CPU tag will
+                * result in 1518 + 4 + 4 = 1526 bytes.
+                */
                len = RTL8366RB_SGCR_MAX_LENGTH_1536;
-       else if (max_mtu > 1532 && max_mtu <= 1548)
+       else if (max_mtu > 1536 && max_mtu <= 1552)
                len = RTL8366RB_SGCR_MAX_LENGTH_1552;
        else
                len = RTL8366RB_SGCR_MAX_LENGTH_16000;
@@ -1471,10 +1481,12 @@ static int rtl8366rb_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 
 static int rtl8366rb_max_mtu(struct dsa_switch *ds, int port)
 {
-       /* The max MTU is 16000 bytes, so we subtract the CPU tag
-        * and the max presented to the system is 15996 bytes.
+       /* The max MTU is 16000 bytes, so we subtract the ethernet
+        * headers with VLAN and checksum and arrive at
+        * 16000 - 18 - 4 = 15978. This does not include the CPU tag
+        * since that is added to the requested MTU by the DSA framework.
         */
-       return 15996;
+       return 16000 - VLAN_ETH_HLEN - ETH_FCS_LEN;
 }
 
 static int rtl8366rb_get_vlan_4k(struct realtek_priv *priv, u32 vid,