net: fs_enet: Fix NETIF_F_SG feature for Freescale MPC5121

Commit 4fc9b87b ("net: fs_enet: Implement NETIF_F_SG feature")
brings a trouble to Freescale MPC512x: a kernel oops happens
during sending non-linear sk_buff with .data not aligned by 4.

Log quotation:

Unable to handle kernel paging request for data at address 0xe467c000
Faulting instruction address: 0xc000cd44
Oops: Kernel access of bad area, sig: 11 [#1]
MPC512x generic
Modules linked in:
CPU: 0 PID: 984 Comm: kworker/0:1H Not tainted 4.1.0-rc8-00024-gbb16140a #2
Workqueue: rpciod rpc_async_schedule
task: cf364a50 ti: cf362000 task.ti: cf362000
NIP: c000cd44 LR: c000c720 CTR: 00000206
REGS: cf363ac0 TRAP: 0300   Not tainted  (4.1.0-rc8-00024-gbb16140a)
MSR: 00009032 <EE,ME,IR,DR,RI>  CR: 42004082  XER: 00000000
DAR: e467c000 DSISR: 20000000
GPR00: c0279e24 cf363b70 cf364a50 e467c000 00000206 0000001f 00000001 00000001
GPR08: 00000000 e467c000 e46800be 000139a6 82002082 00000000 c002e46c cf3c3680
GPR16: c044cb30 c04b0000 cf363c48 00000000 00000001 fde0315c 00000000 0000000b
GPR24: 0000002c 000040be cf339aa0 0000000b 00000001 cf873210 00282f85 00000000
NIP [c000cd44] clean_dcache_range+0x1c/0x30
LR [c000c720] dma_direct_map_page+0x40/0x94
Call Trace:
[cf363b70] [cf339b60] 0xcf339b60 (unreliable)
[cf363b90] [c0279e24] fs_enet_start_xmit+0x1c8/0x42c
[cf363bd0] [c02ff710] dev_hard_start_xmit+0x2dc/0x3d4
[cf363c40] [c0319c60] sch_direct_xmit+0xcc/0x1cc
[cf363c70] [c02ff9c0] __dev_queue_xmit+0x1b8/0x47c
[cf363ca0] [c032a3e8] ip_finish_output+0x1fc/0x9a8
[cf363ce0] [c032c31c] ip_send_skb+0x1c/0xa4
[cf363cf0] [c035112c] udp_send_skb+0xe4/0x2e8
[cf363d10] [c0351368] udp_push_pending_frames+0x38/0x84
[cf363d20] [c03537b8] udp_sendpage+0x134/0x174
[cf363d70] [c0384fd4] xs_sendpages+0x21c/0x250
[cf363db0] [c03852bc] xs_udp_send_request+0x50/0xf8
[cf363de0] [c0382f08] xprt_transmit+0x64/0x280
[cf363e20] [c038017c] call_transmit+0x168/0x234
[cf363e40] [c0387918] __rpc_execute+0x88/0x2b0
[cf363e80] [c00296f8] process_one_work+0x124/0x2fc
[cf363ea0] [c0029a00] worker_thread+0x130/0x480
[cf363ef0] [c002e528] kthread+0xbc/0xd0
[cf363f40] [c000e4a8] ret_from_kernel_thread+0x5c/0x64
Instruction dump:
7c70faa6 60630800 7c70fba6 4c00012c 4e800020 38a0001f 7c632878 7c832050
7c842a14 5484d97f 4d820020 7c8903a6 <7c00186c> 38630020 4200fff8 7c0004ac
---[ end trace c846c1eceb513c85 ]---

The reason:

MPC5121 FEC requires 4-byte alignment for TX data buffer and calls
tx_skb_align_workaround() for copying sk_buff with not aligned .data to a new
sk_buff with aligned one. But tx_skb_align_workaround() uses
skb_copy_from_linear_data() which doesn't work for non-linear sk_buff:
a new sk_buff has non-zero nr_frags and zero .data_len.

So improve the condition of calling tx_skb_align_workaround() and use
skb_linearize() in it.
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c

@@ -490,6 +490,9 @@ static struct sk_buff *tx_skb_align_workaround(struct net_device *dev,
 {
 	struct sk_buff *new_skb;
 
+	if (skb_linearize(skb))
+		return NULL;
+
 	/* Alloc new skb */
 	new_skb = netdev_alloc_skb(dev, skb->len + 4);
 	if (!new_skb)
@@ -515,12 +518,27 @@ static int fs_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	cbd_t __iomem *bdp;
 	int curidx;
 	u16 sc;
-	int nr_frags = skb_shinfo(skb)->nr_frags;
+	int nr_frags;
 	skb_frag_t *frag;
 	int len;
-
 #ifdef CONFIG_FS_ENET_MPC5121_FEC
-	if (((unsigned long)skb->data) & 0x3) {
+	int is_aligned = 1;
+	int i;
+
+	if (!IS_ALIGNED((unsigned long)skb->data, 4)) {
+		is_aligned = 0;
+	} else {
+		nr_frags = skb_shinfo(skb)->nr_frags;
+		frag = skb_shinfo(skb)->frags;
+		for (i = 0; i < nr_frags; i++, frag++) {
+			if (!IS_ALIGNED(frag->page_offset, 4)) {
+				is_aligned = 0;
+				break;
+			}
+		}
+	}
+
+	if (!is_aligned) {
 		skb = tx_skb_align_workaround(dev, skb);
 		if (!skb) {
 			/*
@@ -532,6 +550,7 @@ static int fs_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	}
 #endif
+
 	spin_lock(&fep->tx_lock);
 
 	/*
@@ -539,6 +558,7 @@ static int fs_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	 */
 	bdp = fep->cur_tx;
 
+	nr_frags = skb_shinfo(skb)->nr_frags;
 	if (fep->tx_free <= nr_frags || (CBDR_SC(bdp) & BD_ENET_TX_READY)) {
 		netif_stop_queue(dev);
 		spin_unlock(&fep->tx_lock);