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Message-ID: <20170517103637.2dd8b49d@cakuba.netronome.com> Date: Wed, 17 May 2017 10:36:37 -0700 From: Jakub Kicinski <kubakici@...pl> To: David Laight <David.Laight@...LAB.COM> 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 On Wed, 17 May 2017 11:07:19 +0000, David Laight wrote: > From: Jakub Kicinski > > Sent: 16 May 2017 01:55 > > Given that our rings are always a power of 2, we can simplify the > > calculation of number of completed TX descriptors by using masking > > instead of if statement based on whether the index have wrapped > > or not. > > > > Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com> > > --- > > drivers/net/ethernet/netronome/nfp/nfp_net_common.c | 10 ++-------- > > 1 file changed, 2 insertions(+), 8 deletions(-) > > > > diff --git a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c > > b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c > > index c64514f8ee65..da83e17b8b20 100644 > > --- a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c > > +++ b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c > > @@ -940,10 +940,7 @@ static void nfp_net_tx_complete(struct nfp_net_tx_ring *tx_ring) > > if (qcp_rd_p == tx_ring->qcp_rd_p) > > return; > > > > - if (qcp_rd_p > tx_ring->qcp_rd_p) > > - todo = qcp_rd_p - tx_ring->qcp_rd_p; > > - else > > - todo = qcp_rd_p + tx_ring->cnt - tx_ring->qcp_rd_p; > > + todo = D_IDX(tx_ring, qcp_rd_p + tx_ring->cnt - tx_ring->qcp_rd_p); > > I'm not sure you need to add tx_ring->cnt here. > I bet D_IDX() masks it away. True, feel free to send a fix, or I will queue up a correction after other work I have pending. > > 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().
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