Date:	Tue, 26 May 2015 19:53:33 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stéphane Marchesin <stephane.marchesin@...il.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Alexander Holler <holler@...oftware.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 10/21] regulator: core: Probe regulators on demand

On 26 May 2015 at 18:54, Mark Brown <broonie@...nel.org> wrote:
> On Tue, May 26, 2015 at 05:08:38PM +0200, Tomeu Vizoso wrote:
>> On 26 May 2015 at 11:36, Mark Brown <broonie@...nel.org> wrote:
>
>> > Yes, x86 based embedded systems use ACPI (and we really ought to be
>> > trying to help systems using board files too for that matter).
>
>> Yes, I see how registering devices on-demand could be implemented for
>> all those, but what I don't see is how they would benefit from it.
>
> You'd need to be clearer about what problem you're trying to solve
> there, which is something you left us guessing at!
>
>> I can see an hypothetical maintenance benefit in sharing as much code
>> as possible between these different scenarios, but in this case,
>> because this feature is so closely tied to machine description I think
>> complexity would be actually bigger.
>
> We've now got abstractions for common firmware operations (look at the
> fwnode stuff) and this isn't exactly deep introspection here.
>
> If you're trying to solve the probe order problem you can probably get a
> long way by just doing something that boils down to "try to instantiate
> everything referenced from this node" which could probably even be
> kicked from the driver core prior to probe and cover most cases.  Or put
> this into the node lookup interface so we try to instantiate everything
> we reference.
>
>> On machines that have ACPI, most of those devices aren't exposed to
>> the kernel and few or no deferred probes happen (though I have only
>> tested on qemu and Minnowboard MAX, both with no deferred probes).
>
> On the machines that you happen to have looked at; I would rather expect
> that x86 based phones will be in a similar situation once they move to
> ACPI which they should be doing this year if they didn't already, and
> the embedded systems will doubtless run into this once they have any
> meaningful hardware on them (the base Minnoboard isn't really
> interesting here, it's once you build a system on top of that).
>
>> On machines with board files, devices are registered in a
>> predetermined order, presumably without any deferred probes.
>
> No, not in the least.  Quite aside from anything else as soon as you
> allow things to be built as modules userspace is free to load things in
> whatever order amuses it.  Think about what's going on here - it's not
> just registration of devices, it's also about the order in which
> subsystems and drivers register themselves.
>
>> My understanding is that the problem I'm addressing is specific of
>> machines in which the kernel is in charge of pretty much everything
>> and that the information about what devices are present is given in an
>> arbitrary order.
>
> I don't think you've fully understood the problem space here.

Fair enough, what's your understanding of it?

Thanks,

Tomeu
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