Hi Dave,
We are writing you as the maintainer for the usbnet.c driver and have questions
re: function usbnet_start_xmit() in usbnet.c. We found a problem in the way our
modems handle USB packets of specific size which we fixed by adding the line
urb->transfer_flags |= USB_ZERO_PACKET;
inside function usbnet_start_xmit() right after the call to usb_fill_bulk_urb().
We are curious to know the reasons, if any, why this flag is not always set
before submitting the urb?
We also found that the next section of code:
/* don't assume the hardware handles USB_ZERO_PACKET
* NOTE: strictly conforming cdc-ether devices should expect
* the ZLP here, but ignore the one-byte packet.
*/
if (!(info->flags &FLAG_SEND_ZLP) && (length % dev->maxpacket) == 0) {
urb->transfer_buffer_length++;
if (skb_tailroom(skb)) {
skb->data[skb->len] = 0;
__skb_put(skb, 1);
}
}
produces non-deterministic sized packets depending on whether the skb buffer
has an extra space for 1 byte or not. When we commented out the whole section
and set the urb transfer flag USB_ZERO_PACKET instead we did observe the
zero-length byte being sent correctly and the problem with our modem went away.
We would like to offer to patch this driver to replace the code section above
with a single line to set the USB_ZERO_PACKET flag. Please let us know your
view.
Thanks,
Elina
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