From: Bjørn Mork
> David Laight <[email protected]> writes:
> > From: Bjørn Mork
> >> It doesn't matter whether the buffer size goes up or down. We have to
> >> keep usbnet and device syncronized to be able to split transfers at the
> >> correct boundaries. The spec allow skipping short packets when using
> >> max sized transfers. If we don't tell usbnet about our new expected rx
> >> buffer size, then it will merge and/or split NTBs. The driver does not
> >> support this, and the result will be lots of framing errors.
> >>
> >> Fix by always reallocating usbnet rx buffers when the rx_max value
> >> changes.
> >
> > I'm guessing that the rx_max value is the maximum size of the USB bulk
> > data 'message' that the device generates?
> >
> > As such the URB only need to be longer that it.
>
> So did I think too at first. That's how I added the bug fixed by this
> commit :-)
>
> The problem with NCM is that it explicitly allows (and encourage) using
> transfers which are multiples of the USB packet size, *without* any
> terminating short packet (0 or 1 byte). This means that the USB core
> won't know or care about the end of one transfer and the beginning of
> the next. Which is fine. But the cdc_ncm driver has to know, because
> it must split the transfers into frames it can decode.
>
> Now, the current cdc_ncm implementation has a built-in assumption that
> the size of the URB == rx_max. This lets it simplify the splitting into
> frames to nearly nothing: Any received URB contains exactly one frame.
> Therefore we need to keep the rx URB size strictly syncronized the
> rx_max.
Hmmm....
So there is an ethernet packet size somewhere near 500 that exactly fills
a 512 byte USB2 frame.
If you receive one of those does the hardware send a 0 length terminating
fragment?
If not the then URB won't complete until after the next ethernet frame
arrives.
Receive three of them followed by a longer frame and you'll overrun the
end of the URB.
At least one path through usbnet ensures that the rx buffers aren't
a multiple of the USB frame size. I'm not sure whether that really helps.
Sounds like the hardware expects you to receive each USB bulk buffer
separately.
David
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