netdev - Re: [PATCH v2 net-next] mlx4: optimize xmit path

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Message-ID: <CAMEtUuw0UjJuDqSDrK9L-CUWaVdKni1p9cib+VGCOYtBbrCY4Q@mail.gmail.com>
Date:	Mon, 29 Sep 2014 10:46:00 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Or Gerlitz <gerlitz.or@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Amir Vadai <amirv@...lanox.com>,
	Or Gerlitz <or.gerlitz@...il.com>
Subject: Re: [PATCH v2 net-next] mlx4: optimize xmit path

On Sun, Sep 28, 2014 at 9:19 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>         send_doorbell = !skb->xmit_more || netif_xmit_stopped(ring->tx_queue);
>
>         if (ring->bf_enabled && desc_size <= MAX_BF && !bounce &&
>             !vlan_tx_tag_present(skb) && send_doorbell) {

This patch is good,

but I've been thinking more about bf+xmit_more and want
to double check my understanding in that scenario:
xmit_more=true will queue descriptors normally and
the last xmit_more=false packet will write into BF.
I guess BF suppose to pick up the earlier ones from
the queue, otherwise the whole thing is broken.
So if indeed BF can pick up the whole chain, then
it should be the faster way than doing iowrite32(), right?
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