RE: [PATCH v2] usbnet: remove generic hard_header_len check

David Laight
2014-02-13T16:57:20Z

From: Of Emil Goode
> This patch removes a generic hard_header_len check from the usbnet
> module that is causing dropped packages under certain circumstances
> for devices that send rx packets that cross urb boundaries.
>
> One example is the AX88772B which occasionally send rx packets that
> cross urb boundaries where the remaining partial packet is sent with
> no hardware header. When the buffer with a partial packet is of less
> number of octets than the value of hard_header_len the buffer is
> discarded by the usbnet module.
...
> diff --git a/drivers/net/usb/ax88179_178a.c b/drivers/net/usb/ax88179_178a.c
> index d6f64da..955df81 100644
> --- a/drivers/net/usb/ax88179_178a.c
> +++ b/drivers/net/usb/ax88179_178a.c
> @@ -1118,6 +1118,10 @@ static int ax88179_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
> u16 hdr_off;
> u32 *pkt_hdr;
>
> + /* This check is no longer done by usbnet */
> + if (skb->len < dev->net->hard_header_len)
> + return 0;
> +

The ax88179 driver can also receive ethernet frames that cross the
end of rx URB.
It should have the code to save the last fragment (of a urb) until
the next data arrives, and then correctly merge the fragments.

It is likely that any sub-driver that sets the receive urb length
to a multiple of 1k (rather than leaving it at hard_hdr+mtu) can defrag
rx data that crosses urb boundaries.

David