Date:	Fri, 18 Oct 2013 10:57:24 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Pantelis Antoniou <panto@...oniou-consulting.com>
CC:	Michael Bohan <mbohan@...eaurora.org>,
	Guenter Roeck <linux@...ck-us.net>,
	David Daney <ddaney@...iumnetworks.com>,
	David Gibson <david@...son.dropbear.id.au>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	rob.herring@...xeda.com, ralf@...ux-mips.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	david.daney@...ium.com, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] of/lib: Export fdt routines to modules

On 10/18/2013 08:28 AM, Pantelis Antoniou wrote:
> Hi Michael,
> 
> On Oct 18, 2013, at 5:54 AM, Michael Bohan wrote:
> 
>> On Thu, Oct 17, 2013 at 05:44:07PM -0700, Guenter Roeck wrote:
>>> On 10/17/2013 04:51 PM, Michael Bohan wrote:
>>>> On Wed, Oct 16, 2013 at 09:54:27PM -0700, Guenter Roeck wrote:
>>>>> Still, what prevents you from unflattening it and just using the
>>>>> normal device tree functions as David suggested ?
>>>>
>>>> I'm assuming you're suggesting to use of_fdt_unflatten_tree()?
>>>
>>> Yes, that was the idea.
>>>
>>>> That's an interesting thought. I was planning to scan the fdt
>>>> only once and populate my own structures, but I suppose I could
>>>> use the of_* APIs equivalently.
>>>>
>>>> It seems there are some problems though.  of_fdt_unflatten_tree()
>>>> does not return errors, and so for the purposes of my driver it
>>>> would not be sufficient to detect an invalid firmware image.
>>>>
>>> It does so, at least partially. If there is an error, it won't set
>>> the nodes pointer. Granted, that is not perfect, but it is at least
>>> a start. Ultimately, I considered it 'good enough' for my purpose
>>> (for devicetree overlays - see [1] below), as any missing mandatory
>>> properties or nodes are detected later when trying to actually read
>>> the properties. In my case, I also have a couple of validation
>>> properties to ensure that the overlay is acceptable (specifically
>>> I use 'compatible' and 'assembly-ids', but that is really a detail).
>>
>> That's certainly better than nothing, but I think it would be
>> useful to make a distinction between a malformed fdt and a fdt
>> that's simply missing the right information. Without error
>> codes, I think we lose this aspect.
>>
>>>> Would people entertain changing this API
>>>> (and implicitly __unflatten_device_tree) to return errors? I'm
>>>> guessing the reason it's coded that way is because the normal
>>>> usecase is 'system boot', at which time errors aren't that
>>>> meaningful.
>>>>
>>>> Also, there's no way to free the memory that was allocated from
>>>> the unflatten process. May I add one?
>>>>
>>>
>>> The patchset submitted by Pantelis Antoniou to add support for
>>> devicetree overlays adds this and other related functionality.
>>> See [1], specifically the patch titled "OF: Introduce utility
>>> helper functions". Not sure where that is going, though.
>>> It may need some cleanup to be accepted upstream.
>>> Copying Pantelis for comments.
>>> Guenter
>>>
>>> [1] https://lkml.org/lkml/2013/1/4/276
>>
>> Thanks. So it seems that Pantelis's __of_free_tree() is what I'm
>> looking for.
>>
> 
> I guess it's time for another try to getting it in?
> 
> DT maintainers, which one of you will want to review?

This falls in Grant's and my plate since we are talking kernel DT
support code rather than bindings. Unflattening is definitely the right
direction to go here.

Rob

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