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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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