[OpenWrt-Devel] [PATCH 3/4] ramips: Fix too small rx buffer

[Sven Eckelmann]

The driver assumes that the maximum received buffer for non-jumbo frames is
1536 bytes. But the allocation of the rx fragment doesn't reflect that. It
currently allocates fragments which will only be large enough to be used as
rx buffer with the size of 1534 bytes. This is problematic because the GMAC
will now try to write to 2 bytes which don't belong to its receive buffer
when a large enough ethernet frame is received.

This may already be a problem on existing chips but will at least become a
problem when the 1536 byte rx modus is enabled on MT7621a. It is required
on this SoC to receive ethernet frames which use their full 1500 bytes MTU
and a VLAN header next to the switch VLAN tag.

Signed-off-by: Sven Eckelmann <sven at open-mesh.com>
---
 .../files/drivers/net/ethernet/ralink/ralink_soc_eth.c   | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/target/linux/ramips/files/drivers/net/ethernet/ralink/ralink_soc_eth.c b/target/linux/ramips/files/drivers/net/ethernet/ralink/ralink_soc_eth.c
index 2691cfb..05b810a 100644
--- a/target/linux/ramips/files/drivers/net/ethernet/ralink/ralink_soc_eth.c
+++ b/target/linux/ramips/files/drivers/net/ethernet/ralink/ralink_soc_eth.c
@@ -32,6 +32,7 @@
 #include <linux/reset.h>
 #include <linux/tcp.h>
 #include <linux/io.h>
+#include <linux/bug.h>
 
 #include <asm/mach-ralink/ralink_regs.h>
 
@@ -41,8 +42,8 @@
 #include "ralink_ethtool.h"
 
 #define	MAX_RX_LENGTH		1536
-#define FE_RX_HLEN		(NET_SKB_PAD + VLAN_ETH_HLEN + VLAN_HLEN + \
-		+ NET_IP_ALIGN + ETH_FCS_LEN)
+#define FE_RX_ETH_HLEN		(VLAN_ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN)
+#define FE_RX_HLEN		(NET_SKB_PAD + FE_RX_ETH_HLEN + NET_IP_ALIGN)
 #define DMA_DUMMY_DESC		0xffffffff
 #define FE_DEFAULT_MSG_ENABLE    \
         (NETIF_MSG_DRV      | \
@@ -172,14 +173,21 @@ static int fe_set_mac_address(struct net_device *dev, void *p)
 
 static inline int fe_max_frag_size(int mtu)
 {
+	/* make sure buf_size will be at least MAX_RX_LENGTH */
+	if (mtu + FE_RX_ETH_HLEN < MAX_RX_LENGTH)
+		mtu = MAX_RX_LENGTH - FE_RX_ETH_HLEN;
+
 	return SKB_DATA_ALIGN(FE_RX_HLEN + mtu) +
 		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 }
 
 static inline int fe_max_buf_size(int frag_size)
 {
-	return frag_size - NET_SKB_PAD - NET_IP_ALIGN -
-		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+	int buf_size = frag_size - NET_SKB_PAD - NET_IP_ALIGN -
+		       SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
+	BUG_ON(buf_size < MAX_RX_LENGTH);
+	return buf_size;
 }
 
 static inline void fe_get_rxd(struct fe_rx_dma *rxd, struct fe_rx_dma *dma_rxd)
-- 
2.5.3
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