netdev - Re: [RFC v3 1/2] iwlwifi: pcie: allow to build an A-MSDU using TSO core

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Message-ID: <1445480807.22974.91.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 21 Oct 2015 19:26:47 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Grumbach, Emmanuel" <emmanuel.grumbach@...el.com>
Cc:	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Sharon, Sara" <sara.sharon@...el.com>,
	"ido@...ery.com" <ido@...ery.com>
Subject: Re: [RFC v3 1/2] iwlwifi: pcie: allow to build an A-MSDU using TSO
 core

On Thu, 2015-10-22 at 00:14 +0000, Grumbach, Emmanuel wrote:

> 
> Well. I guess I should at least check, but even with very small MSS, our
> device supports up to 20 pointers for the same 802.11 packet: 2 are for
> metadata. So basically, so leaves me only 18 pointers. for each MSS I
> need at least 2 (one for the headers and one for the payload), so I will
> have at most 9 of these for one packet, even with a tiny MSS.
> 

I did not see in your patch where you made the checks about 18 segs in a
TSO packet ?

> I agree that all this should be added to the code in a comment.
> Speaking of which...
> int tso_count_descs(struct sk_buff *skb)
> {
>         /* The Marvell Way */
>         return skb_shinfo(skb)->gso_segs * 2 + skb_shinfo(skb)->nr_frags;
> }
> 
> What if there is some payload in the header?
> To me it sounds safer to return:
> 
> skb_shinfo(skb)->gso_segs * 2 + skb_shinfo(skb)->nr_frags + 1;
> 
> or maybe to test if there is some payload in the header and then add 1?
> If there is payload in the header, it should be considered as another
> frag, shouldn't it?

Minimal count is gso_segs (one per MSS)

Then you have to add extra for the cases we have a mss spanning a frag
in skb.

Thats a max of (skb_shinfo(skb)->nr_frags - 1) + (data_in_head() ? 1 :
0);

So I believe formula would be correct.


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