On 2016-05-04 03:58, Dean Jenkins wrote:
On 04/05/16 01:28, David B. Robins wrote:
Here is the code snippet from the patch with my annotations between #
#, I will try to explain my intentions. Feel free to point out any
flaws:
if (rx->remaining && (rx->remaining + sizeof(u32) <= skb->len)) {
# Only runs when rx->remaining !=0 and the end of the
Ethernet
frame + next 32-bit header word is within the URB buffer. #
# Therefore, this code does not run when the end of an
Ethernet frame has been reached in the previous URB #
# or when the end of the Ethernet frame + next 32-bit header
word will be in a later URB buffer #
It may well be. I don't have the setup with me now, but I can try
tomorrow to reproduce an environment where I can add some more
detailed logging.
Since the URB length has to be >= than the remaining data plus a u32,
the devices that John Stultz and I are using (AX88772B in my case) may
be adding some additional data/padding after an Ethernet frame,
expecting it to be discarded, and running into this check and its
consequences. This may mean the device is badly behaved, if it is
specified not to send anything extra; in any case, a well-intentioned
error correction has gone badly, but I better understand the intent
now. I am curious to know how often the device you are using benefits
from this block of code.
The issue is that the driver should be robust to cope with missing
URBs. Whilst testing with D-Link DUB-E100 C1 AX88772 USB to Ethernet
adaptor in our ARM embedded system which runs in hostile environments,
it was noticed that URBs could be lost (probably due to a bug
elsewhere or low memory issue). Without this patch, a missing URB
causes bad Ethernet frames to be passed up to the IP stack because
rx->remaining spans multiple URBs.
In the good case of an Ethernet frame spanning 2 URBs, the 1st URB is
processed and copies the 1st part of the Ethernet frame into the
netdev buffer, for the 2nd URB the remaining part of the Ethernet
frame is copied into the same netdev buffer to complete the Ethernet
frame. The netdev buffer is then sent up to the IP stack.
In the case of a missing URB, a bad Ethernet frame is created as
follows:
The 1st URB is processed and copies the 1st part of the Ethernet frame
into the netdev buffer, the 2nd URB is lost (somehow), the 3rd URB is
processed and blindly copies what it thinks is the remaining part of
the Ethernet frame in the same netdev buffer which corrupts the
Ethernet frame. The netdev buffer is then sent up to the IP stack. The
3rd URB and subsequent URBs are processed but synchronisation has been
lost so can misread data as a 32-bit header word. It is likely that
some good Ethernet frames get discarded whilst trying to
resynchronise.
A recovery strategy for regaining lock with the 32-bit header word is
necessary otherwise the driver will have difficulty in recovering from
a lost URB.
In the "olden days", the 32-bit header word was always at the start of
the URB buffer so previous URBs did not influence the current URB. So
no recovery strategy was needed at that time. But now we have to
remember what happened in the previous URB and a lost URB can cause a
discontinuity in the data stream because the data is not always
aligned to the start of the URB buffer.
I agree that your environment may never suffer from lost URBs so
removal of the patch would work OK.
I will try to find some time to setup a test environment.
I got the chance to add some more logging (and add back the header
synchronization code) to a system which is receiving video data on an
8-port POE device using AX88772Bs - it replaces the "Data header
synchronization was lost" log and logs a few more local items ("u32" =
rx->header):
[3105478.073908] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3105478.073918] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() Bad Header
Length 0xf05b7c5f, offset 4
[3105478.074022] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() skb->len =
2048
[3105478.074032] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() Bad Header
Length 0xffaae2a5, offset 4
[3105478.074146] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() skb->len =
1518
[3105478.074280] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() skb->len =
2048
[3105478.074291] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() Bad Header
Length 0xe1478400, offset 4
[3105478.074399] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() skb->len = 916
[3105478.074409] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() Bad Header
Length 0x9fa84eb5, offset 4
[3105478.074657] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() skb->len =
1518
Grepping for just the SYNC_LOSS messages shows a similar pattern to that
seen elsewhere in the thread:
[3104761.056033] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.058035] asix 1-1.4.1.3:1.0 poe2: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.076795] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.107904] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.114649] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.121152] asix 1-1.4.1.1:1.0 poe1: asix_rx_fixup() SYNC_LOSS
remain 988 len 1518 offset 992 u32 0xecf95000
[3104761.147784] asix 1-1.4.1.3:1.0 poe2: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.160535] asix 1-1.4.1.1:1.0 poe1: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.168024] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.168403] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.170654] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.180418] asix 1-1.4.1.1:1.0 poe1: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.221783] asix 1-1.4.1.1:1.0 poe1: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.248784] asix 1-1.4.2.2:1.0 poe7: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.252421] asix 1-1.4.1.2:1.0 poe5: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.262903] asix 1-1.4.2.4:1.0 poe8: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.266904] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.267281] asix 1-1.4.2.4:1.0 poe8: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.271665] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.273915] asix 1-1.4.2.4:1.0 poe8: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.280048] asix 1-1.4.2.4:1.0 poe8: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.287152] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.292157] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.295054] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.310915] asix 1-1.4.1.1:1.0 poe1: asix_rx_fixup() SYNC_LOSS
remain 988 len 2022 offset 992 u32 0xecf95000
[3104761.316918] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.324935] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.326665] asix 1-1.4.2.3:1.0 poe4: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
[3104761.336420] asix 1-1.4.1.4:1.0 poe6: asix_rx_fixup() SYNC_LOSS
remain 988 len 2048 offset 992 u32 0xecf95000
This is an Ubuntu 14.04.2 LTS i686 (32-bit) system running kernel
3.13.0-53-generic. I don't see any "low-speed" warnings, and lsusb -t
shows each device (and their hubs) running at 480M.
ifconfig poe1:
poe1 Link encap:Ethernet HWaddr 00:50:f9:ec:4f:d8
inet addr:192.168.201.1 Bcast:192.168.201.255
Mask:255.255.255.0
inet6 addr: fe80::250:f9ff:feec:4fd8/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:194441 errors:16649 dropped:15 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:267310738 (267.3 MB) TX bytes:10300282 (10.3 MB)
Without the "sanity test/synchronization" code:
poe1 Link encap:Ethernet HWaddr 00:50:f9:ec:4f:d8
inet addr:192.168.201.1 Bcast:192.168.201.255
Mask:255.255.255.0
inet6 addr: fe80::250:f9ff:feec:4fd8/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:398910 errors:1 dropped:14 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:513363997 (513.3 MB) TX bytes:13154142 (13.1 MB)
Regards,
Dean
David
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