[dpdk-dev] : ixgbe: why bulk allocation is not used for a scattered Rx flow?

[Vlad Zolotarov <vladz@cloudius-systems.com>]

Hi, I have a question about the "scattered Rx" feature: why enabling it 
disabled "bulk allocation" feature?
There is some unclear comment in the ixgbe_recv_scattered_pkts():

		/*
		 * Descriptor done.
		 *
		 * Allocate a new mbuf to replenish the RX ring descriptor.
		 * If the allocation fails:
		 *    - arrange for that RX descriptor to be the first one
		 *      being parsed the next time the receive function is
		 *      invoked [on the same queue].
		 *
		 *    - Stop parsing the RX ring and return immediately.
		 *
		 * This policy does not drop the packet received in the RX
		 * descriptor for which the allocation of a new mbuf failed.
		 * Thus, it allows that packet to be later retrieved if
		 * mbuf have been freed in the mean time.
		 * As a side effect, holding RX descriptors instead of
		 * systematically giving them back to the NIC may lead to
		 * RX ring exhaustion situations.
		 * However, the NIC can gracefully prevent such situations
		 * to happen by sending specific "back-pressure" flow control
		 * frames to its peer(s).
		 */

Why the same "policy" can't be done in the bulk-context allocation? - 
Don't advance the RDT until u've refilled the ring. What do I miss here?

Another question is about the LRO feature - is there a reason why it's 
not implemented? I've implemented the LRO support in ixgbe PMD to begin 
with - I used a "scattered Rx" as a template and now I'm tuning it 
(things like the stuff above).

Is there any philosophical reason why it hasn't been implemented in 
*any* PMD so far? ;)

thanks,
vlad