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.
Regards,
Dean
--
Dean Jenkins
Embedded Software Engineer
Linux Transportation Solutions
Mentor Embedded Software Division
Mentor Graphics (UK) Ltd.
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