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 <[email protected]>
> ---
> 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
