netdev - Re: [PATCH 3/3] net: mvneta: Introduce a software TSO implementation

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Date:	Mon, 5 May 2014 11:47:02 -0300
From:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
To:	netdev@...r.kernel.org
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>, cmetcalf@...era.com,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Simon Guinot <simon.guinot@...uanux.org>,
	Willy Tarreau <w@....eu>, Tawfik Bayouk <tawfik@...vell.com>,
	Lior Amsalem <alior@...vell.com>,
	Simon Guinot <sguinot@...ie.com>
Subject: Re: [PATCH 3/3] net: mvneta: Introduce a software TSO implementation

Hi all,

On 10 Apr 07:58 PM, Ezequiel Garcia wrote:
[..]
> +
> +	/* Calculate expected number of TX descriptors */
> +	desc_count = skb_shinfo(skb)->gso_segs * 2 + skb_shinfo(skb)->nr_frags;
> +	if ((txq->count + desc_count) >= txq->size)
> +		return 0;
> +

Is this calculus correct? Does it give the accurate number of needed
descriptors or is it an approximation?

Tilera's tilegx driver does a much stricter descriptor count (see
tso_count_edescs). This functions loops through the skb_frag_t fragments
as it's done later in the data egress, hence strictly counting the
needed descriptors.

However, as it's a much heavier routine than the one shown above,
I'm wondering if we can get away without it.

Willy, Any ideas here?
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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