linux-kernel - Re: [PATCH] of/lib: Export fdt routines to modules

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Message-Id: <33103C16-6472-4AAE-ADB8-50807CC96C85@antoniou-consulting.com>
Date:	Fri, 18 Oct 2013 16:28:29 +0300
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Michael Bohan <mbohan@...eaurora.org>
Cc:	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

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?

> Mike
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation

Regards

-- Pantelis

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