Re: changing dev->needed_headroom/needed_tailroom?

Johannes Berg
2013-08-20T16:25:08Z

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