From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from s3.sipsolutions.net ([144.76.43.152]:55694 "EHLO sipsolutions.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751255Ab3HTQZI (ORCPT ); Tue, 20 Aug 2013 12:25:08 -0400 Message-ID: <1377015897.13829.16.camel@jlt4.sipsolutions.net> (sfid-20130820_182517_364693_7BE6C4BC) Subject: Re: changing dev->needed_headroom/needed_tailroom? From: Johannes Berg To: Stephen Hemminger Cc: Florian Fainelli , Eric Dumazet , Ben Hutchings , netdev , linux-wireless@vger.kernel.org Date: Tue, 20 Aug 2013 18:24:57 +0200 In-Reply-To: <20130820092013.6ca909e2@nehalam.linuxnetplumber.net> (sfid-20130820_182019_441833_A12DE7FE) References: <1374850210.8248.59.camel@jlt4.sipsolutions.net> <1375433758.24371.20.camel@deadeye.wl.decadent.org.uk> <1375449082.3927.5.camel@edumazet-glaptop> <1375711240.8120.11.camel@jlt4.sipsolutions.net> <20130820092013.6ca909e2@nehalam.linuxnetplumber.net> (sfid-20130820_182019_441833_A12DE7FE) Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Sender: linux-wireless-owner@vger.kernel.org List-ID: 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