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

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Date:	Fri, 18 Oct 2013 09:16:28 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Rob Herring <robherring2@...il.com>
Cc:	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Michael Bohan <mbohan@...eaurora.org>,
	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 Fri, Oct 18, 2013 at 10:57:24AM -0500, Rob Herring wrote:
> 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.
> 
I think Pantelis may have been asking about his devicetree overlay
support patch series.

Pantelis, did you have time to address the review comments you got earlier ?

Thanks,
Guenter
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