ravb: do not use zero-length alignment DMA descriptor

Due to alignment requirements of the hardware transmissions are split into
two DMA descriptors, a small padding descriptor of 0 - 3 bytes in length
followed by a descriptor for rest of the packet.

In the case of IP packets the first descriptor will never be zero due to
the way that the stack aligns buffers for IP packets. However, for non-IP
packets it may be zero.

In that case it has been reported that timeouts occur, presumably because
transmission stops at the first zero-length DMA descriptor and thus the
packet is not transmitted. However, in my environment a BUG is triggered as
follows:

[   20.381417] ------------[ cut here ]------------
[   20.386054] kernel BUG at lib/swiotlb.c:495!
[   20.390324] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[   20.395805] Modules linked in:
[   20.398862] CPU: 0 PID: 2089 Comm: mz Not tainted 4.10.0-rc3-00001-gf13ad2db193f #162
[   20.406689] Hardware name: Renesas Salvator-X board based on r8a7796 (DT)
[   20.413474] task: ffff80063b1f1900 task.stack: ffff80063a71c000
[   20.419404] PC is at swiotlb_tbl_map_single+0x178/0x2ec
[   20.424625] LR is at map_single+0x4c/0x98
[   20.428629] pc : [<ffff00000839c4c0>] lr : [<ffff00000839c680>] pstate: 800001c5
[   20.436019] sp : ffff80063a71f9b0
[   20.439327] x29: ffff80063a71f9b0 x28: ffff80063a20d500
[   20.444636] x27: ffff000008ed5000 x26: 0000000000000000
[   20.449944] x25: 000000067abe2adc x24: 0000000000000000
[   20.455252] x23: 0000000000200000 x22: 0000000000000001
[   20.460559] x21: 0000000000175ffe x20: ffff80063b2a0010
[   20.465866] x19: 0000000000000000 x18: 0000ffffcae6fb20
[   20.471173] x17: 0000ffffa09ba018 x16: ffff0000087c8b70
[   20.476480] x15: 0000ffffa084f588 x14: 0000ffffa09cfa14
[   20.481787] x13: 0000ffffcae87ff0 x12: 000000000063abe2
[   20.487098] x11: ffff000008096360 x10: ffff80063abe2adc
[   20.492407] x9 : 0000000000000000 x8 : 0000000000000000
[   20.497718] x7 : 0000000000000000 x6 : ffff000008ed50d0
[   20.503028] x5 : 0000000000000000 x4 : 0000000000000001
[   20.508338] x3 : 0000000000000000 x2 : 000000067abe2adc
[   20.513648] x1 : 00000000bafff000 x0 : 0000000000000000
[   20.518958]
[   20.520446] Process mz (pid: 2089, stack limit = 0xffff80063a71c000)
[   20.526798] Stack: (0xffff80063a71f9b0 to 0xffff80063a720000)
[   20.532543] f9a0:                                   ffff80063a71fa30 ffff00000839c680
[   20.540374] f9c0: ffff80063b2a0010 ffff80063b2a0010 0000000000000001 0000000000000000
[   20.548204] f9e0: 000000000000006e ffff80063b23c000 ffff80063b23c000 0000000000000000
[   20.556034] fa00: ffff80063b23c000 ffff80063a20d500 000000013b1f1900 0000000000000000
[   20.563864] fa20: ffff80063ffd18e0 ffff80063b2a0010 ffff80063a71fa60 ffff00000839cd10
[   20.571694] fa40: ffff80063b2a0010 0000000000000000 ffff80063ffd18e0 000000067abe2adc
[   20.579524] fa60: ffff80063a71fa90 ffff000008096380 ffff80063b2a0010 0000000000000000
[   20.587353] fa80: 0000000000000000 0000000000000001 ffff80063a71fac0 ffff00000864f770
[   20.595184] faa0: ffff80063b23caf0 0000000000000000 0000000000000000 0000000000000140
[   20.603014] fac0: ffff80063a71fb60 ffff0000087e6498 ffff80063a20d500 ffff80063b23c000
[   20.610843] fae0: 0000000000000000 ffff000008daeaf0 0000000000000000 ffff000008daeb00
[   20.618673] fb00: ffff80063a71fc0c ffff000008da7000 ffff80063b23c090 ffff80063a44f000
[   20.626503] fb20: 0000000000000000 ffff000008daeb00 ffff80063a71fc0c ffff000008da7000
[   20.634333] fb40: ffff80063b23c090 0000000000000000 ffff800600000037 ffff0000087e63d8
[   20.642163] fb60: ffff80063a71fbc0 ffff000008807510 ffff80063a692400 ffff80063a20d500
[   20.649993] fb80: ffff80063a44f000 ffff80063b23c000 ffff80063a69249c 0000000000000000
[   20.657823] fba0: 0000000000000000 ffff80063a087800 ffff80063b23c000 ffff80063a20d500
[   20.665653] fbc0: ffff80063a71fc10 ffff0000087e67dc ffff80063a20d500 ffff80063a692400
[   20.673483] fbe0: ffff80063b23c000 0000000000000000 ffff80063a44f000 ffff80063a69249c
[   20.681312] fc00: ffff80063a5f1a10 000000103a087800 ffff80063a71fc70 ffff0000087e6b24
[   20.689142] fc20: ffff80063a5f1a80 ffff80063a71fde8 000000000000000f 00000000000005ea
[   20.696972] fc40: ffff80063a5f1a10 0000000000000000 000000000000000f ffff00000887fbd0
[   20.704802] fc60: fffffff43a5f1a80 0000000000000000 ffff80063a71fc80 ffff000008880240
[   20.712632] fc80: ffff80063a71fd90 ffff0000087c7a34 ffff80063afc7180 0000000000000000
[   20.720462] fca0: 0000ffffcae6fe18 0000000000000014 0000000060000000 0000000000000015
[   20.728292] fcc0: 0000000000000123 00000000000000ce ffff0000088d2000 ffff80063b1f1900
[   20.736122] fce0: 0000000000008933 ffff000008e7cb80 ffff80063a71fd80 ffff0000087c50a4
[   20.743951] fd00: 0000000000008933 ffff000008e7cb80 ffff000008e7cb80 000000100000000e
[   20.751781] fd20: ffff80063a71fe4c 0000ffff00000300 0000000000000123 0000000000000000
[   20.759611] fd40: 0000000000000000 ffff80063b1f0000 000000000000000e 0000000000000300
[   20.767441] fd60: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[   20.775271] fd80: 0000000000000000 0000000000000000 ffff80063a71fda0 ffff0000087c8c20
[   20.783100] fda0: 0000000000000000 ffff000008082f30 0000000000000000 0000800637260000
[   20.790930] fdc0: ffffffffffffffff 0000ffffa0903078 0000000000000000 000000001ea87232
[   20.798760] fde0: 000000000000000f ffff80063a71fe40 ffff800600000014 ffff000000000001
[   20.806590] fe00: 0000000000000000 0000000000000000 ffff80063a71fde8 0000000000000000
[   20.814420] fe20: 0000000000000000 0000000000000000 0000000000000000 0000000000000001
[   20.822249] fe40: 0000000203000011 0000000000000000 0000000000000000 ffff80063a68aa00
[   20.830079] fe60: ffff80063a68aa00 0000000000000003 0000000000008933 ffff0000081f1b9c
[   20.837909] fe80: 0000000000000000 ffff000008082f30 0000000000000000 0000800637260000
[   20.845739] fea0: ffffffffffffffff 0000ffffa07ca81c 0000000060000000 0000000000000015
[   20.853569] fec0: 0000000000000003 000000001ea87232 000000000000000f 0000000000000000
[   20.861399] fee0: 0000ffffcae6fe18 0000000000000014 0000000000000300 0000000000000000
[   20.869228] ff00: 00000000000000ce 0000000000000000 00000000ffffffff 0000000000000000
[   20.877059] ff20: 0000000000000002 0000ffffcae87ff0 0000ffffa09cfa14 0000ffffa084f588
[   20.884888] ff40: 0000000000000000 0000ffffa09ba018 0000ffffcae6fb20 000000001ea87010
[   20.892718] ff60: 0000ffffa09b9000 0000ffffcae6fe30 0000ffffcae6fe18 000000000000000f
[   20.900548] ff80: 0000000000000003 000000001ea87232 0000000000000000 0000000000000000
[   20.908378] ffa0: 0000000000000000 0000ffffcae6fdc0 0000ffffa09a7824 0000ffffcae6fdc0
[   20.916208] ffc0: 0000ffffa0903078 0000000060000000 0000000000000003 00000000000000ce
[   20.924038] ffe0: 0000000000000000 0000000000000000 ffffffffffffffff ffffffffffffffff
[   20.931867] Call trace:
[   20.934312] Exception stack(0xffff80063a71f7e0 to 0xffff80063a71f910)
[   20.940750] f7e0: 0000000000000000 0001000000000000 ffff80063a71f9b0 ffff00000839c4c0
[   20.948580] f800: ffff80063a71f840 ffff00000888a6e4 ffff80063a24c418 ffff80063a24c448
[   20.956410] f820: 0000000000000000 ffff00000811cd54 ffff80063a71f860 ffff80063a24c458
[   20.964240] f840: ffff80063a71f870 ffff00000888b258 ffff80063a24c418 0000000000000001
[   20.972070] f860: ffff80063a71f910 ffff80063a7b7028 ffff80063a71f890 ffff0000088825e4
[   20.979899] f880: 0000000000000000 00000000bafff000 000000067abe2adc 0000000000000000
[   20.987729] f8a0: 0000000000000001 0000000000000000 ffff000008ed50d0 0000000000000000
[   20.995560] f8c0: 0000000000000000 0000000000000000 ffff80063abe2adc ffff000008096360
[   21.003390] f8e0: 000000000063abe2 0000ffffcae87ff0 0000ffffa09cfa14 0000ffffa084f588
[   21.011219] f900: ffff0000087c8b70 0000ffffa09ba018
[   21.016097] [<ffff00000839c4c0>] swiotlb_tbl_map_single+0x178/0x2ec
[   21.022362] [<ffff00000839c680>] map_single+0x4c/0x98
[   21.027411] [<ffff00000839cd10>] swiotlb_map_page+0xa4/0x138
[   21.033072] [<ffff000008096380>] __swiotlb_map_page+0x20/0x7c
[   21.038821] [<ffff00000864f770>] ravb_start_xmit+0x174/0x668
[   21.044484] [<ffff0000087e6498>] dev_hard_start_xmit+0x8c/0x120
[   21.050407] [<ffff000008807510>] sch_direct_xmit+0x108/0x1a0
[   21.056064] [<ffff0000087e67dc>] __dev_queue_xmit+0x194/0x4cc
[   21.061807] [<ffff0000087e6b24>] dev_queue_xmit+0x10/0x18
[   21.067214] [<ffff000008880240>] packet_sendmsg+0xf40/0x1220
[   21.072873] [<ffff0000087c7a34>] sock_sendmsg+0x18/0x2c
[   21.078097] [<ffff0000087c8c20>] SyS_sendto+0xb0/0xf0
[   21.083150] [<ffff000008082f30>] el0_svc_naked+0x24/0x28
[   21.088462] Code: d34bfef7 2a1803f3 1a9f86d6 35fff878 (d4210000)
[   21.094611] ---[ end trace 5bc544ad491f3814 ]---
[   21.099234] Kernel panic - not syncing: Fatal exception in interrupt
[   21.105587] Kernel Offset: disabled
[   21.109073] Memory Limit: none
[   21.112126] ---[ end Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 2f45d1902acf ("ravb: minimize TX data copying")
Signed-off-by: Masaru Nagai <masaru.nagai.vx@renesas.com
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/ravb_main.c

@@ -1504,6 +1504,19 @@ static netdev_tx_t ravb_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	buffer = PTR_ALIGN(priv->tx_align[q], DPTR_ALIGN) +
 		 entry / NUM_TX_DESC * DPTR_ALIGN;
 	len = PTR_ALIGN(skb->data, DPTR_ALIGN) - skb->data;
+	/* Zero length DMA descriptors are problematic as they seem to
+	 * terminate DMA transfers. Avoid them by simply using a length of
+	 * DPTR_ALIGN (4) when skb data is aligned to DPTR_ALIGN.
+	 *
+	 * As skb is guaranteed to have at least ETH_ZLEN (60) bytes of
+	 * data by the call to skb_put_padto() above this is safe with
+	 * respect to both the length of the first DMA descriptor (len)
+	 * overflowing the available data and the length of the second DMA
+	 * descriptor (skb->len - len) being negative.
+	 */
+	if (len == 0)
+		len = DPTR_ALIGN;
+
 	memcpy(buffer, skb->data, len);
 	dma_addr = dma_map_single(ndev->dev.parent, buffer, len, DMA_TO_DEVICE);
 	if (dma_mapping_error(ndev->dev.parent, dma_addr))