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

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Date:	Thu, 17 Oct 2013 19:54:06 -0700
From:	Michael Bohan <mbohan@...eaurora.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	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,
	Pantelis Antoniou <panto@...oniou-consulting.com>
Subject: Re: [PATCH] of/lib: Export fdt routines to modules

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.

Mike

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