Return-path: Received: from mail-bk0-f42.google.com ([209.85.214.42]:47097 "EHLO mail-bk0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751513AbaAPKd3 convert rfc822-to-8bit (ORCPT ); Thu, 16 Jan 2014 05:33:29 -0500 Received: by mail-bk0-f42.google.com with SMTP id my12so1105005bkb.15 for ; Thu, 16 Jan 2014 02:33:27 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <1389864706.4345.1.camel@jlt4.sipsolutions.net> References: <1389787494-7361-1-git-send-email-michal.kazior@tieto.com> <1389787494-7361-10-git-send-email-michal.kazior@tieto.com> <1389792139.4338.6.camel@jlt4.sipsolutions.net> <1389864706.4345.1.camel@jlt4.sipsolutions.net> Date: Thu, 16 Jan 2014 11:33:27 +0100 Message-ID: (sfid-20140116_113332_847518_1FDB65A6) Subject: Re: [RFC 9/9] mac80211: implement multi-interface CSA From: Michal Kazior To: Johannes Berg Cc: linux-wireless Content-Type: text/plain; charset=UTF-8 Sender: linux-wireless-owner@vger.kernel.org List-ID: On 16 January 2014 10:31, Johannes Berg wrote: > On Thu, 2014-01-16 at 10:28 +0100, Michal Kazior wrote: >> On 15 January 2014 14:22, Johannes Berg wrote: >> > On Wed, 2014-01-15 at 13:04 +0100, Michal Kazior wrote: >> >> This implements a fairly simple multi-interface >> >> CSA. It doesn't support multiple channel >> >> contexts so it doesn't support multi-channel. >> > >> > It seems that this also means that somewhere you need to advertise the >> > multi-switch feature. I guess you need that anyway, but this makes it >> > obvious. >> >> I don't really understand. Do we really need it? You want to advertise >> it just to cfg80211 or all the way up to userspace? > > It seems it would be needed all the way to userspace so it knows whether > it can expect multi-vif CSA to be available? Or would you just want to > try & fail? Wouldn't that potentially impact channel selection since > you'd want to not pick radar channels for multi-vif? Not really sure ... > > Maybe the expectation is that internally there will be no drivers that > don't support it, i.e. if a driver implements the cfg80211 API at all > then it must support multi-vif CSA? Typically if you run more than one AP on a DFS channel and you detect a radar you want to move all APs - so you either succeed with multi-vif CSA or you fallback and re-start them. If you assume a driver supports multi-channel but isn't capable of multi-interface CSA you'll be running into timing issues. After you detect radar you have a limited time to quiesce. You might end up exceeding that limit if you move APs one-by-one. With my patches as they are new hostap will try sending new channel switch command but since it doesn't contain ifindex old kernel will respond with a -EINVAL and userspace has no way of knowing if the command was actually malformed or isn't supported. hostap could still, however, work in a best effort manner and fallback to the old command variant. Michał