USB device debugging

[Bjørn Mork]

Narasimha M <mnarasimha786 at gmail.com> writes:

> Thanks for explanation. Here data is getting corrupted before it comes
> to the usbnet itself, so after it reaches to usbnet and go through
> network stack, it is failing in tcp checksum and packet is getting
> dropped. Same driver is working with linux-3.10.20 but not working
> with linux-2.6.32.Not able to find the exact function in driver which
> sends receive packet to usbnet, not able to proceed further. Please
> suggest some pointers to proceed further.

This is no surprise.  There is no "send receive packet to usbnet".

usbnet allocates a receive buffer and hands it to the USB host
controller. This happens in rx_submit().  The host controller calls the
rx_complete() callback when it gets data from the device.  This callback
will trigger further handling in the usbnet_bh() tasklet, calling
rx_process().  This again calls the minidriver specific rx_fixup()
callback if there is any, which is GobiNet's only chance of inspecting
and possibly modifying the buffer.  But normally it will not touch the
buffer, since there is no fixup necessary for Gobi devices in 802.3 mode
(they transmit plain ethernet packets). rx_process() ends up calling
usbnet_skb_return() which hands the buffer over the the networking stack
using netif_rx().

So, if we ignore the possible firmware bug workarounds in rx_fixup(),
then nothing ever touches the receive buffer in usbnet. It's just a
handle being passed around.

Note that I don't remember, or care, what 2.6.32 might have done. It's too
outdated to be relevant.  But the usbnet design is much older and
haven't changed drastically, so I assume most of the above is valid
there too.


Bjørn