JMS_MIN_SIZE is the full ethernet frame length, while mtu is just the
data payload size. Comparing these two meant that mtus between 1500 and
1518 did not trigger enabling jumbo frames.

So instead compare the set mtu ETH_DATA_LEN, which is equal to
JMS_MIN_SIZE - ETH_HLEN - ETH_FCS_LEN;

Also do a check that the requested mtu is actually greater than the
minimum length, else we do not need to enable jumbo frames.

In practice this only introduced a very small range of mtus that did not
work properly. Newer chips allow 2000 byte large frames by default, and
older chips allow 1536 bytes long, which is equivalent to an mtu of
1514. So effectivly only mtus of 1515~1517 were broken.

Fixes: 6ae5834b983a ("net: dsa: b53: add MTU configuration support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
---
 drivers/net/dsa/b53/b53_common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/dsa/b53/b53_common.c b/drivers/net/dsa/b53/b53_common.c
index 0783fc121bbbf979abe6c9985b10cf4379bf2a9b..57df00ad9dd4cedfe9e959ea779d48e3f8f36142 100644
--- a/drivers/net/dsa/b53/b53_common.c
+++ b/drivers/net/dsa/b53/b53_common.c
@@ -2259,7 +2259,7 @@ static int b53_change_mtu(struct dsa_switch *ds, int port, int mtu)
 	if (!dsa_is_cpu_port(ds, port))
 		return 0;
 
-	enable_jumbo = (mtu >= JMS_MIN_SIZE);
+	enable_jumbo = (mtu > ETH_DATA_LEN);
 	allow_10_100 = (dev->chip_id == BCM583XX_DEVICE_ID);
 
 	return b53_set_jumbo(dev, enable_jumbo, allow_10_100);

-- 
2.43.0

On 10/4/2024 1:47 AM, Jonas Gorski wrote:
> JMS_MIN_SIZE is the full ethernet frame length, while mtu is just the
> data payload size. Comparing these two meant that mtus between 1500 and
> 1518 did not trigger enabling jumbo frames.
> 
> So instead compare the set mtu ETH_DATA_LEN, which is equal to
> JMS_MIN_SIZE - ETH_HLEN - ETH_FCS_LEN;
> 
> Also do a check that the requested mtu is actually greater than the
> minimum length, else we do not need to enable jumbo frames.
> 
> In practice this only introduced a very small range of mtus that did not
> work properly. Newer chips allow 2000 byte large frames by default, and
> older chips allow 1536 bytes long, which is equivalent to an mtu of
> 1514. So effectivly only mtus of 1515~1517 were broken.
> 
> Fixes: 6ae5834b983a ("net: dsa: b53: add MTU configuration support")
> Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
-- 
Florian