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Message-ID: <063D6719AE5E284EB5DD2968C1650D6DCFFF7B93@AcuExch.aculab.com> Date: Wed, 17 May 2017 11:07:19 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Jakub Kicinski' <jakub.kicinski@...ronome.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org> CC: "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: 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. > 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. David
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