Message-ID: <1800e846-81d3-806e-3767-5839b2c14fe6@cogentembedded.com>
Date:   Fri, 13 Jan 2017 18:29:10 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Simon Horman <horms+renesas@...ge.net.au>,
        David Miller <davem@...emloft.net>
Cc:     Magnus Damm <magnus.damm@...il.com>, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Masaru Nagai <masaru.nagai.vx@...esas.com>
Subject: Re: [PATCH net] ravb: do not use zero-length alighment DMA request

Hello!

On 01/13/2017 04:09 PM, Simon Horman wrote:

> From: Masaru Nagai <masaru.nagai.vx@...esas.com>
>
> Due to alignment requirements of the hardware transmissions are split into
> two DMA descriptors, a small padding descriptor of 0 - 3 bytes in length

    OK but you forgot to use "descriptor" ISO "request" in the subject. :-)

> 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@...esas.com
> Signed-off-by: Simon Horman <horms+renesas@...ge.net.au>

Acked-by: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>

> ---
> v1 [Simon Horman]
> * don't handle skb->len < 4; it is always at least 60
> * add comment
> * dropped RFC designation
>
> v1 [Simon Horman]
> * rewrote changelog
> * handle skb->len < 4
>
> v0 [Masaru Nagai]
> ---
>  drivers/net/ethernet/renesas/ravb_main.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> index 5e5ad978eab9..1cd6ba986669 100644
> --- a/drivers/net/ethernet/renesas/ravb_main.c
> +++ b/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 terminate
> +	 * DMA requests. Avoid them by simply using a length of DPTR_ALIGN (4)

    I'd write "seem to terminate DMA transfers" rather.

[...]

MBR, Sergei

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