linux-kernel - Re: Driver core support for early platform devices

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Date:	Thu, 22 Dec 2011 11:55:56 -0600
From:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>
To:	Greg KH <gregkh@...e.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: Driver core support for early platform devices

On Dec 22, 2011, at 12:45, Greg KH wrote:
> On Thu, Dec 22, 2011 at 11:15:06AM -0600, Moffett, Kyle D wrote:
>> Hi,
>> 
>> I'm tinkering with some improvements to the way that OpenPIC/MPIC are
>> detected and loaded on PowerPC platforms, and it seems like I am trying
>> to use the driver model before it is fully initialized.
>> 
>> In particular, it seems like it should be possible to simply declare an
>> OpenPIC in the device-tree and have it automatically bound to a platform
>> driver declaring the right OpenFirmware match strings.
>> 
>> Unfortunately, it needs to be bound by init_IRQ() time, while the driver
>> model does not get initialized until much later (after the scheduler is
>> up and running).
>> 
>> As far as I can tell, there seem to be 2 possible approaches to making
>> that possible:
>> 
>> (1) Split the driver-model initialization into "early" and "late" phases
>>    so that drivers can be registered and devices probed very early on
>>    and then replay the necessary scheduler-dependent things after the
>>    system is mostly started up (IE: devtmpfs, etc).
> 
> We already have that today with the "early_platform*" functions, right?
> Will those work for you, or do you need this for a bus you are creating
> and not using the platform bus?

Well, I can't figure out how "early_platform" is actually supposed to
integrate with the platform bus itself.  It seems designed mostly for
drivers like "earlyprintk" et. al. for which loading is controlled by
a kernel parameter.

Specifically, I don't see any "early_platform" logic to match devices in
the OF device-tree based on the driver "of_match" parameters, just based
on text strings in early_param().

Furthermore, if I register an "early_platform" device, it seems to get
unregistered when the normal driver model is brought up, instead of
being sucked in and promoted to a normal platform_device.  That code is
pretty poorly documented and only used in a couple places right now,
though, so it's possible I am misreading it.

Cheers,
Kyle Moffett
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