linux-kernel - Re: [RFD] Functional dependencies between devices

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Date:	Thu, 29 Oct 2015 10:31:08 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Mark Brown <broonie@...nel.og>, Rob Herring <robh@...nel.org>,
	Thierry Reding <treding@...dia.com>,
	Dmitry Torokhov <dtor@...gle.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [RFD] Functional dependencies between devices

Good grief, don't you guys ever trim unwanted material from your 
emails?  I had to erase more than 4 screens worth of useless stuff 
before getting to the relevant portions.

On Thu, 29 Oct 2015, Tomeu Vizoso wrote:

> >> Also, have you considered that not only drivers request resources? For
> >> example, the on-demand probing series would probe a device that is
> >> needed by an initcall, simplifying synchronization.

Did Rafael ever say that only drivers could create these functional
dependencies?  I don't recall seeing that anywhere.  Presumably any
part of the kernel will be allowed to do it.

> > You really need to explain what you mean here or maybe give an example.
> 
> There are initcalls that assume that a given resource is available.
> Because of async probes, or because the resource's driver being built
> as a module, or because the resource's driver gained a dependency
> (direct or not), those initcalls break unexpectedly at times.
> 
> If resource getters could probe dependencies on-demand, those
> initcalls would be more robust to changes in other parts of the
> codebase.
> 
> AFAIUI, your proposal would help with a device's dependencies being
> there when it's probed, but initcalls could still run into unfulfilled
> dependencies.

One possible approach is to have a "wait_for_driver" flag, along with a
timeout value (or perhaps using a fixed timeout value).  When a
dependency gets registered with this flag set, the function call
wouldn't return until the target device is bound to a driver or the
timeout has elapsed.

This would make it easy to insert dependencies at probe time without 
relying on deferred probing.

Alan Stern

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