NCM buffer sizes are negotiated with the device independently of
the network device MTU. The RX buffers are allocated by the
usbnet framework based on the rx_urb_size value set by cdc_ncm. A
single RX buffer can hold a number of MTU sized packets.
The default usbnet change_mtu ndo only modifies rx_urb_size if it
is equal to hard_mtu. And the cdc_ncm driver will set rx_urb_size
and hard_mtu independently of each other, based on dwNtbInMaxSize
and dwNtbOutMaxSize respectively. It was therefore assumed that
usbnet_change_mtu() would never touch rx_urb_size. This failed to
consider the case where dwNtbInMaxSize and dwNtbOutMaxSize happens
to be equal.
Fix by implementing an NCM specific change_mtu ndo, modifying the
netdev MTU without touching the buffer size settings.
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>
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