[PATCH net] net: stmmac: Guard against txfifosz=0

[Florian Fainelli]

After commit bfcb813203e619a8960a819bf533ad2a108d8105 ("net: dsa:
configure the MTU for switch ports") my Lamobo R1 platform which uses
an allwinner,sun7i-a20-gmac compatible Ethernet MAC started to fail
by rejecting a MTU of 1536. The reason for that is that the DMA
capabilities are not readable on this version of the IP, and there is
also no 'tx-fifo-depth' property being provided in Device Tree. The
property is documented as optional, and is not provided.

The minimum MTU that the network device accepts is ETH_ZLEN - ETH_HLEN,
so rejecting the new MTU based on the txfifosz value unchecked seems a
bit too heavy handed here.

Fixes: eaf4fac47807 ("net: stmmac: Do not accept invalid MTU values")
Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index e6898fd5223f..9c63ba6f86a9 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -3993,7 +3993,7 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 	new_mtu = STMMAC_ALIGN(new_mtu);
 
 	/* If condition true, FIFO is too small or MTU too large */
-	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
+	if ((txfifosz < new_mtu && txfifosz) || (new_mtu > BUF_SIZE_16KiB))
 		return -EINVAL;
 
 	dev->mtu = new_mtu;
-- 
2.19.1