Re: changing dev->needed_headroom/needed_tailroom?

Stephen Hemminger
2013-08-20T16:29:42Z

On Tue, 20 Aug 2013 18:24:57 +0200
Johannes Berg <johannes@sipsolutions.net> wrote:

> On Tue, 2013-08-20 at 09:20 -0700, Stephen Hemminger wrote:
>
> > All code must check for needed headroom first, and copy packet
> > if space is not available. Since excess headroom is always safe,
> > most devices just always use the same worst case headroom.
> >
> > Even with your changes this will still be necessary since packets will
> > be still in flight while features change.
>
> I agree, it will always be necessary. I guess the question boils down to
> whether/what N bytes headroom more or less actually make a difference.
>
> The wireless stack currently sets needed_headroom to 34 or so I think,
> but I will need to increase by 8 which was why I had this question to
> start with. However, those 34 are an absolute worst case - in most cases
> it's much less...
>
> So I suppose the other question we should ask is will increasing from
> ~34 to ~42 make a significant different that it's worth thinking about
> avoiding it in the common cases at all?
>
> johannes
>

The only thing that might matter is alignment. but +/- 8 bytes isn't going
to change that.