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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 39326C4363C for ; Wed, 7 Oct 2020 20:17:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E9ECD206F7 for ; Wed, 7 Oct 2020 20:17:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728334AbgJGURe (ORCPT ); Wed, 7 Oct 2020 16:17:34 -0400 Received: from netrider.rowland.org ([192.131.102.5]:33639 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728307AbgJGURd (ORCPT ); Wed, 7 Oct 2020 16:17:33 -0400 Received: (qmail 471404 invoked by uid 1000); 7 Oct 2020 16:17:32 -0400 Date: Wed, 7 Oct 2020 16:17:32 -0400 From: Alan Stern To: Matthias Kaehlcke Cc: Doug Anderson , Rob Herring , Greg Kroah-Hartman , Frank Rowand , "linux-kernel@vger.kernel.org" , Linux USB List , Bastien Nocera , Stephen Boyd , Ravi Chandra Sadineni , Krzysztof Kozlowski , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , Peter Chen Subject: Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs Message-ID: <20201007201732.GE468921@rowland.harvard.edu> References: <20201006141820.GA416765@rowland.harvard.edu> <20201006165957.GA191572@google.com> <20201006171524.GB423499@rowland.harvard.edu> <20201006192536.GB191572@google.com> <20201007010023.GA438733@rowland.harvard.edu> <20201007160336.GA620323@google.com> <20201007163838.GA457977@rowland.harvard.edu> <20201007172847.GB620323@google.com> <20201007192542.GA468921@rowland.harvard.edu> <20201007194229.GC620323@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201007194229.GC620323@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 07, 2020 at 12:42:29PM -0700, Matthias Kaehlcke wrote: > On Wed, Oct 07, 2020 at 03:25:42PM -0400, Alan Stern wrote: > > On Wed, Oct 07, 2020 at 10:28:47AM -0700, Matthias Kaehlcke wrote: > > > On Wed, Oct 07, 2020 at 12:38:38PM -0400, Alan Stern wrote: > > > > On Wed, Oct 07, 2020 at 09:03:36AM -0700, Matthias Kaehlcke wrote: > > > > > Ok, I wasn't sure if the hubs suspend asynchronously from each other. If they > > > > > do it should indeed not be a problem to have the "master" wait for its peers. > > > > > > > > Well, order of suspending is selectable by the user. It can be either > > > > asynchronous or reverse order of device registration, which might pose a > > > > problem. We don't know in advance which of two peer hubs will be > > > > registered first. It might be necessary to introduce some additional > > > > explicit synchronization. > > > > > > I'm not sure we are understanding each other completely. I agree that > > > synchronization is needed to have the primary hub wait for its peers, that > > > was one of my initial concerns. > > > > > > Lets use an example to clarify my secondary concern: a hub chip provides a > > > USB 3 and a USB 2 hub, lets say the USB 3 hub is the primary. > > > > > > Here is some pseudo-code for the suspend function: > > > > > > hub_suspend(hub) > > > ... > > > > > > if (hub->primary) { > > > device_pm_wait_for_dev(hub->peer) > > > > > > // check for connected devices and turn regulator off > > > } > > > > > > ... > > > } > > > > > > What I meant with 'asynchronous suspend' in this context: > > > > > > Can hub_suspend() of the peer hub be executed (asynchronously) while the > > > primary is blocked on device_pm_wait_for_dev(), > > > > Yes, that's exactly what would happen with async suspend. > > > > > or would the primary wait > > > forever if the peer hub isn't suspended yet? > > > > That wouldn't happen. device_pm_wait_for_dev is smart; it will return > > immediately if neither device uses async suspend. But in that case you > > could end up removing power from the peer hub before it had suspended. > > > > That's why I said you might need to add additional synchronization. The > > suspend routines for the two hubs could each check to see whether the > > other device had suspended yet, and the last one would handle the power > > regulator. The additional synchronization is for the case where the two > > checks end up being concurrent. > > That was exactly my initial concern and one of the reasons I favor(ed) a > platform instead of a USB driver: Clearly there's a tradeoff. > > otherwise all hubs need to know their peers and check in suspend if they > > are the last hub standing, only then the power can be switched off. > > To which you replied: > > > you just need to make the "master" hub wait for its peer to suspend, which > > is easy to do. > > However that apparently only works if async suspend is enabled, and we > can't rely on that. Yes, I had forgotten about the possibility of synchronous suspend. My mistake. > With the peers checking on each other you lose effectively the notion > of a primary. Well, you can still want to put the sysfs power-control attribute file into just one of the hubs' directories, and that one would be considered the primary. But I agree, it's a weak notion. > Going back to the binding: > > &usb_1_dwc3 { > hub_2_0: hub@1 { > compatible = "usbbda,5411"; > reg = <1>; > }; > > hub_3_0: hub@2 { > compatible = "usbbda,411"; > reg = <2>; > vdd-supply = <&pp3300_hub>; > companion-hubs = <&hub_2_0>; > }; > }; > > How does 'hub_2_0' know that its peer is hub_3_0 and that it has a regulator > (and potentially other resources)? The peering relation goes both ways, so it should be included in the hub_2_0 description too. Given that, the driver could check hub_2_0's peer's DT description for the appropriate resources. > All this mess can be avoided by having a single instance in control of the > resources which is guaranteed to suspend after the USB devices. Yes. At the cost of registering, adding a driver for, and making users aware of a fictitious platform device. Alan Stern