Re: [RFD] Functional dependencies between devices

[Alan Stern]

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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