Re: [PATCH net-next v6 02/10] dpaa_eth: add support for DPAA Ethernet

[David Miller <davem@davemloft.net>]

From: Madalin-Cristian Bucur <madalin.bucur@nxp.com>
Date: Mon, 7 Nov 2016 16:32:16 +0000

>> -----Original Message-----
>> From: David Miller [mailto:davem@davemloft.net]
>> Sent: Monday, November 07, 2016 5:55 PM
>> 
>> From: Madalin-Cristian Bucur <madalin.bucur@nxp.com>
>> Date: Mon, 7 Nov 2016 15:43:26 +0000
>> 
>> >> From: David Miller [mailto:davem@davemloft.net]
>> >> Sent: Thursday, November 03, 2016 9:58 PM
>> >>
>> >> Why?  By clearing this, you disallow an important fundamental way to do
>> >> performane testing, via pktgen.
>> >
>> > The Tx path in DPAA requires one to insert a back-pointer to the skb
>> into
>> > the Tx buffer. On the Tx confirmation path the back-pointer in the
>> buffer
>> > is used to release the skb. If Tx buffer is shared we'd alter the back-
>> pointer
>> > and leak/double free skbs. See also
>> 
>> Then have your software state store an array of SKB pointers, one for each
>> TX ring entry, just like every other driver does.
> 
> There is no Tx ring in DPAA. Frames are send out on QMan HW queues towards
> the FMan for Tx and then received back on Tx confirmation queues for cleanup.
> Array traversal would for sure cost more than using the back-pointer. Also,
> we can now process confirmations on a different core than the one doing Tx,
> we'd have to keep the arrays percpu and force the Tx conf on the same core.
> Or add locks.

Report back an integer index, like every scsi driver out there which
completes tagged queued block I/O operations asynchronously.  You can
associate the array with a specific TX confirmation queue.