linux-kernel - Re: [PATCH v3 02/18] of/platform: add of_platform

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Date:	Tue, 11 Aug 2015 11:37:38 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Javier Martinez Canillas <javier@....samsung.com>,
	Thierry Reding <thierry.reding@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-acpi@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 02/18] of/platform: add of_platform_probe

On 7 August 2015 at 14:19, Mark Brown <broonie@...nel.org> wrote:
> On Thu, Aug 06, 2015 at 04:11:39PM +0200, Tomeu Vizoso wrote:
>
>> Walks the OF tree up and finds the closest ancestor that has a platform
>> device associated with it, probing it if isn't bound to a driver yet.
>
>> The above should ensure that the dependency represented by the passed OF
>> node is available, because probing a platform device should cause its
>> descendants to be probed as well.
>
> This sounds like it's going to break in the case where we have MFDs that
> represent their functions in DT (not a pattern I'm a fan of but it's a
> thing people do).  We'll walk back to the platform device for the MFD
> function, try to probe it and then give up.  Perhaps that's good enough
> anyway but it's not clear to me why we don't just try every parent we
> find?

Agreed. In the attempt at probing dependencies before a device is
probed, I considered that a device's parent is also a dependency and
that worked well. From what I saw, few devices will defer their probe
if their parent hasn't been probed yet, assuming that it will have
probed already. But with simple-mfd and simple-bus that shouldn't be
relied upon as things will break if their parents defer their probe.
With async probing enabled this failure scenario becomes more
probable.

> I'm also not a fan of the fact that the interface here is explicitly
> saying that we want to probe a platform device, that's an implementation
> detail that callers shouldn't need to know about.  From the point of
> view of the callers what they're trying to do is kick any dependencies
> into being instantiated, the fact that we currently try to accomplish it
> with platform devices isn't something they care about.

Agreed.

Thanks,

Tomeu
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