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

[Guenter Roeck]

On 10/17/2013 04:51 PM, Michael Bohan wrote:
> On Wed, Oct 16, 2013 at 09:54:27PM -0700, Guenter Roeck wrote:
> > On 10/16/2013 05:27 PM, Michael Bohan wrote:
> > > My motivation is actually to use the fdt format as a firmware.
> > > I have a requirement to express driver metadata that's loadable
> > >from the filesystem. This data is not reasonable to place in the
> > > system Device Tree, since it's application specific and does not
> > > actually describe hardware. The fact that the format chosen is
> > > 'flattened device tree' is merely just a coincidence.
> > 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).

> 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.

I also updated the devicetree discussion list address to get comments
from the experts.

Thanks,
Guenter

[1] https://lkml.org/lkml/2013/1/4/276

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