From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6A1B4C433ED for ; Thu, 15 Apr 2021 11:23:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3E9DA611CD for ; Thu, 15 Apr 2021 11:23:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232130AbhDOLXu (ORCPT ); Thu, 15 Apr 2021 07:23:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:48162 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230202AbhDOLXt (ORCPT ); Thu, 15 Apr 2021 07:23:49 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1618485804; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=qA/aGS5Izv8h7RrYKhh2H6WVWgB2jT16mRzPvW2sj98=; b=b81DE0jNBJkhrxIZkNLzAsgZ2pMD+t8wc+tRP87f3PF7xO0j4mLIqGfSKd5y39n2WMVyFu ouXZZLmOIUbXefqdteEkmUqSlaTQu0usecdEGCoqrtXSt55VVk+YFGXdo5ZsIJtaXW1LsV 2cEDIwIrwwDQAsdlda7+c7wkxcANZzQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 9789BABED; Thu, 15 Apr 2021 11:23:24 +0000 (UTC) Message-ID: <72d092726448607af2fd453c48be5b0ba69e617a.camel@suse.com> Subject: Re: [RFC]extension of the anchor API From: Oliver Neukum To: Alan Stern Cc: linux-usb@vger.kernel.org Date: Thu, 15 Apr 2021 13:23:21 +0200 In-Reply-To: <20210414145608.GB1493067@rowland.harvard.edu> References: <5b3c30d268ea2d13d303759ef3dfee8d72830084.camel@suse.com> <20210325150657.GC785961@rowland.harvard.edu> <5d3852dca69ff194017c806078e996c50ee621be.camel@suse.com> <20210325183856.GA799855@rowland.harvard.edu> <20210408150725.GC1296449@rowland.harvard.edu> <8c11f03b08a0bdfd2761a74f5a7964067dc4b98b.camel@suse.com> <20210412150628.GA1420451@rowland.harvard.edu> <30abed362c4b2e6af33078505ac9985389ad39bb.camel@suse.com> <20210414145608.GB1493067@rowland.harvard.edu> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.34.4 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org Am Mittwoch, den 14.04.2021, 10:56 -0400 schrieb Alan Stern: > On Wed, Apr 14, 2021 at 10:12:01AM +0200, Oliver Neukum wrote: > > Am Montag, den 12.04.2021, 11:06 -0400 schrieb Alan Stern: > > > On Mon, Apr 12, 2021 at 11:58:16AM +0200, Oliver Neukum wrote: > > > > That presumes that the URBs will finish in order. I don't think such > > > > an assumption can be made. > > > > > > I don't understand -- I can't detect any such presumption. > > > > OK, this shows that I am bad at explaining. > > > As far as I can tell, the only reason for maintaining the URBs in any > > > particular order on the anchor list is so that usb_kill_anchored_urbs > > > and usb_poison_anchored_urbs can kill them in reverse order of > > > submission. THat's why the current code moves completed URBs to the end > > > of the list. > > > > No longer strictly true, as the API has a call to submit everything > > on an anchor, but I think it boils down to the same thing. > > > > > If you keep a pointer to the most recently submitted URB, killing them > > > easy enough to do. Start with that URB, then go backward through the > > > list (wrapping to the end when you reach the beginning of the list). > > > > Yes, but that supposes that the next on the list has not been > > resubmitted _before_ the one after it. > > > > If you do not keep the list ordered, but in the initial order, > > we can have the situation that A (happens most recently submitted) > > is followed by B and C, but C was submitted before B. > > I think the only reasonable alternative is to move an URB to the end of > the list when it is submitted, rather than when it completes. Have you > considered doing it that way? No, that did not occur to me. Back to the drawing board. Still I have to put it somewhere when I anchor an URB. Head or tail? > The real problem with usb_submit_anchored_urbs is that the core can't > know in what order the caller wants the URBs to be submitted. If the I think the reasonable assumption is that they need to be submitted in the order they were anchored. > In the kerneldoc you can explain that if the anchor has not been used > since its URBs were added then the URBs will be submitted in the order > they were added to the anchor, but otherwise they will be submitted in > an unspecified order, which may not be suitable. Yes. > > > The order in which the URBs complete doesn't matter, because trying to > > > unlink a completed URB won't cause any harm. > > > > As long as it stays completed. > > Rather, as long as they complete in order of submission. > > > > The only assumption here > > > is that URBs get submitted in the list's order (possibly circularly) -- > > > this should always be true. > > > > I am afraid we cannot guarantee that. It might intuitively seem so, > > but nothing guarantees that all URBs are going to the same endpoint. > > I hadn't thought of that. Do anchors get used that way anywhere? I haven't found an example, but I thought it could not be ruled out. So you think that that case should be discouraged in documentation and henceforth ignored? So we do agree that we need the following: a - submit in the order you anchored b - kill or poison in the reverse order c - unpoison does not really matter but better do it in the submit order? Does that mean that the list needs to be kept ordered by sequence of submission? I think so. Regards Oliver