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.kicin...@netronome.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().
