netdev - RE: [PATCH net-next 9/9] nfp: eliminate an if statement in calculation of completed frames

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Date:   Fri, 19 May 2017 09:18:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jakub Kicinski' <kubakici@...pl>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oss-drivers@...ronome.com" <oss-drivers@...ronome.com>
Subject: RE: [PATCH net-next 9/9] nfp: eliminate an if statement in
 calculation of completed frames

From: Jakub Kicinski
> Sent: 17 May 2017 18:37
..
> > >  	while (todo--) {
> > >  		idx = D_IDX(tx_ring, tx_ring->rd_p++);
> >
> > That '++' looks suspicious.
> > I think you need to decide whether you are incrementing pointers into the ring
> > or indexes into it.
> > Sometimes it is safer to use a non-wrapping index and mask when accessing the entry.
> > 	entry_ptr = &ring[idx & (RING_SIZE - 1)]
> > Ring full is then (read_idx == write_idx + RING_SIZE),
> > ring empty (read_idx == write_idx).
> > So the index just wrap at (probably)_2^32.
> 
> I may be missing the point.  I use a mix of the two, actually, the
> software pointers are free running (non-wrapping) but the HW QCP
> pointers wrap.  Because HW pointers wrap I always keep one entry on
> the rings empty, see nfp_net_tx_full().

Ah, I'd assumed that rd_p was a pointer, not an index.

	David