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

[Guenter Roeck <linux@roeck-us.net>]

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