Re: [RFC v2] mac80211: assign needed_headroom/tailroom for netdevs

David Miller
2008-05-05T00:30:51Z

From: Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
Date: Sun, 04 May 2008 23:32:25 +0200

> --- everything.orig/net/mac80211/iface.c 2008-05-04 23:24:17.000000000 +0200
> +++ everything/net/mac80211/iface.c 2008-05-04 23:31:46.000000000 +0200
> @@ -53,6 +53,15 @@ int ieee80211_if_add(struct net_device *
> if (!ndev)
> return -ENOMEM;
>
> + ndev->needed_headroom = local->tx_headroom +
> + 4*6 /* four MAC addresses */
> + + 2 + 2 + 2 + 2 /* ctl, dur, seq, qos */
> + + 6 /* mesh */
> + + 8 /* rfc1042/bridge tunnel */
> + - ETH_HLEN /* ethernet hard_header_len */
> + + IEEE80211_ENCRYPT_HEADROOM;
> + ndev->needed_tailroom = IEEE80211_ENCRYPT_TAILROOM;
> +
> ret = dev_alloc_name(ndev, ndev->name);
> if (ret < 0)
> goto fail;

Where is the patch that adds these new members to struct netdevice,
and where is the code that uses these new values?

I even searched the netdev archives and couldn't find it.

Anyways I see your basic idea and this may be the best way to handle
the problem. The invariants would be:

1) LL_MAX_HEADER has to include all of these bits.

2) LL_RESERVED_SPACE*() has to take the new needed_headroom
into account.

Your patch which I can't find, which adds netdev->needed_*, probably
does all of that. But I'm just making sure :-)

Note that what would be really nice is if we could assert, in
dev_queue_xmit, that the SKB has all of the necessary headroom, and
give a WARN_ON_ONCE() backtrace if not.

If we can ensure that, things like mac80211 and others will not need
to skb_realloc_headroom() or anything like that unless they need to
modify packet contents after skb->data and the packet is shared
(ie. the pskb_expand_headroom(skb, 0, 0, GFP_*) case the TSO drivers
use).
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