[yocto] creating global variables in a recipes

[Chris Larson]

On Wed, Feb 1, 2012 at 10:17 PM, Joshua Immanuel <josh at hipro.co.in> wrote:
> On Wed, 2012-02-01 at 11:50 -0500, William Mills wrote:
>> I am not sure I can give you a better option that local.conf but I can
>> explain why what your doing does not work.
>>
>> It may feel from your usage that the image recipe is the ancestor of
>> all the recipes but this is really not true.  The settings in a image
>> recipe effect the assembly of that image and not the packages that
>> they depend on.  An image could be assembled from packages that were
>> built long ago or inherited via shared state.
>>
>> settings in local,conf on the other hand affect all recipes.  If you
>> add something there it will be seen by all recipes.  Unfortunately
>> this means that all recipes are dependent on the settings and
>> everything will need to be rebuilt in case the new setting effects
>> them.  I believe this also means you will not be able to used share
>> state with someone with a different setting of (or unset)
>> SOME_VARIABLE.  (Well you can but you will both be rebuild
>> everything.)
>>
>> Alternatives to the whole approach are to make alternative packages
>> for the various cases (busybox-minimal vs busybox-full) or use an
>> image task to create an asset in the file system that then makes a
>> difference at run time.
>
> My use-case is that I am creating an /etc/fstab file via
> base-files.bbappend recipe. In that fstab, I use uuid (generated via
> uuidgen) to denote different partition filesystems. So, I have to use
> this uuid in the extended image_types.bbclass file to set them to the
> generated partition images appropriately (via tune2fs).
>
> One more approach I can think of is to echo the uuid to some file in the
> deploy images directory (while creating the fstab file) and read them
> back in image_types class. But this process looks messy.

I'd say generate them in the configuration metadata via inline python
or an anonymous python function, then use those variables both in the
base-files recipe and images.
-- 
Christopher Larson