net: stmmac: Fix zero-division error when disabling tc cbs

The commit b8c4336 ("net: stmmac: No need to calculate speed divider
when offload is disabled") allows the "port_transmit_rate_kbps" to be
set to a value of 0, which is then passed to the "div_s64" function when
tc-cbs is disabled. This leads to a zero-division error.

When tc-cbs is disabled, the idleslope, sendslope, and credit values the
credit values are not required to be configured. Therefore, adding a return
statement after setting the txQ mode to DCB when tc-cbs is disabled would
prevent a zero-division error.

Fixes: b8c4336 ("net: stmmac: No need to calculate speed divider when offload is disabled")
Cc: <[email protected]>
Co-developed-by: Choong Yong Liang <[email protected]>
Signed-off-by: Choong Yong Liang <[email protected]>
Signed-off-by: KhaiWenTan <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: KhaiWenTan

drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c

@@ -386,6 +386,7 @@ static int tc_setup_cbs(struct stmmac_priv *priv,
 			return ret;
 
 		priv->plat->tx_queues_cfg[queue].mode_to_use = MTL_QUEUE_DCB;
+		return 0;
 	}
 
 	/* Final adjustments for HW */