From: Ondrej Puzman <redacted>
Date: Thu, 28 Nov 2013 10:44:39 +0100

According to Documentation/networking/driver.txt the ndo_start_xmit method should not return NETDEV_TX_BUSY under normal circumstances.
Stop the transmit queue when tx_ring is full.

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Signed-off-by: Ondrej Puzman <redacted>

You should be instead preventing the transmit method from being invoked
when it might be possible that a request cannot be satisfied.

This means that at the end of a transmit request, you must stop the
queue if a packet with the maximum number of possible descriptors
cannot be satisfied.  Then it is impossible for the transmit function
to be invoked in a situation where it would need to fail for lack of
available transmit descriptors.

This is why drivers decided whether to stop their TX queues based upon
calculations involving MAX_SKB_FRAGS.