From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lukas Wunner Subject: Re: [RFC/RFT][PATCH v2 2/7] driver core: Functional dependencies tracking support Date: Wed, 14 Sep 2016 10:28:24 +0200 Message-ID: <20160914082824.GA1597@wunner.de> References: <27296716.H9VWo8ShOm@vostro.rjw.lan> <20160911134058.GA189@wunner.de> <20160911204336.GA371@wunner.de> <2250917.Aes4ApWfhc@vostro.rjw.lan> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <2250917.Aes4ApWfhc@vostro.rjw.lan> Sender: linux-kernel-owner@vger.kernel.org To: "Rafael J. Wysocki" Cc: Linux PM list , Greg Kroah-Hartman , Alan Stern , Linux Kernel Mailing List , Tomeu Vizoso , Mark Brown , Marek Szyprowski , Kevin Hilman , Ulf Hansson , "Luis R. Rodriguez" List-Id: linux-pm@vger.kernel.org On Wed, Sep 14, 2016 at 03:21:27AM +0200, Rafael J. Wysocki wrote: > On Sunday, September 11, 2016 10:43:36 PM Lukas Wunner wrote: > > On Sun, Sep 11, 2016 at 03:40:58PM +0200, Lukas Wunner wrote: > > > On Thu, Sep 08, 2016 at 11:27:45PM +0200, Rafael J. Wysocki wrote: > > > > +/** > > > > + * device_is_dependent - Check if one device depends on another one > > > > + * @dev: Device to check dependencies for. > > > > + * @target: Device to check against. > > > > + * > > > > + * Check if @dev or any device dependent on it (its child or its consumer etc) > > > > + * depends on @target. Return 1 if that is the case or 0 otherwise. > > > > + */ > > > > +static int device_is_dependent(struct device *dev, void *target) > > > > +{ > > > > + struct device_link *link; > > > > + int ret; > > > > + > > > > + ret = device_for_each_child(dev, target, device_is_dependent); > > > > + list_for_each_entry(link, &dev->links_to_consumers, s_node) { > > > > + if (WARN_ON(link->consumer == target)) > > > > + return 1; > > > > + > > > > + ret = ret || device_is_dependent(link->consumer, target); > > > > + } > > > > + return ret; > > > > +} > > > > > > What happens if someone tries to add a device link from a parent > > > (as the consumer) to a child (as a supplier)? You're only checking > > > if target is a consumer of dev, for full correctness you'd also have > > > to check if target is a parent of dev. (Or grandparent, or great- > > > grandparent, ... you need to walk the tree up to the root.) > > > > > > > > > The function can be sped up by returning immediately if a match > > > is found instead of continuing searching and accumulating the > > > result in ret, i.e.: > > > > > > if (device_for_each_child(dev, target, device_is_dependent)) > > > return 1; > > > > > > and in the list_for_each_entry block: > > > > > > if (device_is_dependent(link->consumer, target)) > > > return 1; > > > > > > Then at the end of the function "return 0". > > > > > > > > > I'd move the WARN_ON() to the single invocation of this function in > > > device_link_add(), that way it's possible to use the function as a > > > helper elsewhere should the need arise. > > > > Oh I'm grasping only now, you want to emit a WARN for *every* > > infringing child/consumer. That could lead to a WARN flood if > > a developer accidentally does something really dumb, like linking > > the PCI root to some PCI endpoint device, but fair enough. > > > > The point about linking a parent to a child still stands however. > > I think a simple way to check this is to just add > > > > if (WARN_ON(dev == target)) > > return 1; > > > > at the top of the function, because when someone tries to link > > a parent to a child, when recursing from the parent downward > > one will eventually hit that child. This will also prevent > > someone from linking a device to itself. > > I actually would prefer to make it impossible to link a parent to > a child at all. Which is precisely what the code snippet above does. Thanks, Lukas