From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg Kroah-Hartman Subject: Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize probe ordering Date: Thu, 1 Aug 2019 08:12:09 +0200 Message-ID: <20190801061209.GA3570@kroah.com> References: <20190731221721.187713-1-saravanak@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190731221721.187713-1-saravanak@google.com> Sender: linux-kernel-owner@vger.kernel.org To: Saravana Kannan Cc: Rob Herring , Mark Rutland , "Rafael J. Wysocki" , Frank Rowand , devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, David Collins , kernel-team@android.com List-Id: devicetree@vger.kernel.org On Wed, Jul 31, 2019 at 03:17:13PM -0700, Saravana Kannan wrote: > Add device-links to track functional dependencies between devices > after they are created (but before they are probed) by looking at > their common DT bindings like clocks, interconnects, etc. > > Having functional dependencies automatically added before the devices > are probed, provides the following benefits: > > - Optimizes device probe order and avoids the useless work of > attempting probes of devices that will not probe successfully > (because their suppliers aren't present or haven't probed yet). > > For example, in a commonly available mobile SoC, registering just > one consumer device's driver at an initcall level earlier than the > supplier device's driver causes 11 failed probe attempts before the > consumer device probes successfully. This was with a kernel with all > the drivers statically compiled in. This problem gets a lot worse if > all the drivers are loaded as modules without direct symbol > dependencies. > > - Supplier devices like clock providers, interconnect providers, etc > need to keep the resources they provide active and at a particular > state(s) during boot up even if their current set of consumers don't > request the resource to be active. This is because the rest of the > consumers might not have probed yet and turning off the resource > before all the consumers have probed could lead to a hang or > undesired user experience. > > Some frameworks (Eg: regulator) handle this today by turning off > "unused" resources at late_initcall_sync and hoping all the devices > have probed by then. This is not a valid assumption for systems with > loadable modules. Other frameworks (Eg: clock) just don't handle > this due to the lack of a clear signal for when they can turn off > resources. This leads to downstream hacks to handle cases like this > that can easily be solved in the upstream kernel. > > By linking devices before they are probed, we give suppliers a clear > count of the number of dependent consumers. Once all of the > consumers are active, the suppliers can turn off the unused > resources without making assumptions about the number of consumers. > > By default we just add device-links to track "driver presence" (probe > succeeded) of the supplier device. If any other functionality provided > by device-links are needed, it is left to the consumer/supplier > devices to change the link when they probe. All now queued up in my driver-core-testing branch, and if 0-day is happy with this, will move it to my "real" driver-core-next branch in a day or so to get included in linux-next. thanks for sticking with this! greg k-h