Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757545AbbLBIpW (ORCPT ); Wed, 2 Dec 2015 03:45:22 -0500 Received: from mga11.intel.com ([192.55.52.93]:62337 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755253AbbLBIpV (ORCPT ); Wed, 2 Dec 2015 03:45:21 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,372,1444719600"; d="scan'208";a="611158437" Message-ID: <565EB0D8.5050908@linux.intel.com> Date: Wed, 02 Dec 2015 10:50:32 +0200 From: Mathias Nyman User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.8.0 MIME-Version: 1.0 To: Alan Stern CC: daniel@quora.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [TESTPATCH] xhci: fix usb2 resume timing and races. References: In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3051 Lines: 78 On 01.12.2015 17:47, Alan Stern wrote: > On Mon, 30 Nov 2015, Mathias Nyman wrote: > >> usb2 ports need to signal resume for 20ms before moving to U0 state. >> Both device and host can initiate resume. >> >> On host initated resume port is set to resume state, sleep 20ms, >> and finally set port to U0 state. > > That's an odd approach. The assumption in usbcore is that the HCD will > not sleep here. > >> On device initated resume a port status interrupt with a port in resume >> state in issued. The interrupt handler tags a resume_done[port] >> timestamp with current time + 20ms, and kick roothub timer. >> Root hub timer requests for port status, finds the port in resume state, >> checks if resume_done[port] timestamp passed, and set port to U0 state. > > ehci-hcd does the same thing, except that it also uses this resume_done > timestamp with host-initiated resumes. > >> There are a few issues with this approach, >> 1. A host initated resume will also generate a resume event, the event >> handler will find the port in resume state, believe it's a device >> initated and act accordingly. >> >> 2. A port status request might cut the 20ms resume signalling short if a >> get_port_status request is handled during the 20ms host resume. >> The port will be found in resume state. The timestamp is not set leading >> to time_after_eq(jiffoes, timestamp) returning true, as timestamp = 0. >> get_port_status will proceed with moving the port to U0. >> >> 3. If an error, or anything else happends to the port during device >> initated 20ms resume signalling it will leave all device resume >> parameters hanging uncleared preventing further resume. >> >> Fix this by using the existing resuming_ports bitfield to indicate if >> resume signalling timing is taken care of. >> Also check if the resume_done[port] is set before using it in time >> comparison. Also clear out any resume signalling related variables if port >> is not in U0 or Resume state. > > Wouldn't it be better to change the host-initiated resume mechanism to > be consisten with device-initiated resumes? Or would that be too big a > change for the time being? > > Alan Stern > Yes, changing host initiated resume code would make sense. Hence the comment in the testpatch: /* Host initated resume doesn't time the resume * signalling using resume_done[]. * It manually sets RESUME state, sleeps 20ms * and sets U0 state. This should probably be * changed, but not right now, do nothing */ I was focusing more on clearing the stale resume related variables and didn't want to dig into the history of host initiated resume code at that moment. If ehci-hcd is using the timestamp + kick roothub approach for host resume, then I don't see why xhci can't do the same. -Mathias -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/