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

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Date:	Thu, 22 Dec 2011 14:45:01 -0600
From:	Rob Herring <robherring2@...il.com>
To:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>
CC:	Greg KH <gregkh@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: Driver core support for early platform devices

On 12/22/2011 11:55 AM, Moffett, Kyle D wrote:
> 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

There was a proposal for DT support of early platform devices here:

http://lists.infradead.org/pipermail/linux-arm-kernel/2011-June/054529.html

Rob
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