[OE-core] Consistency and use cases for IMAGE_FSTYPES

[Richard Purdie]

On Fri, 2012-03-09 at 14:39 -0700, Tom Rini wrote:
> Hey all,
> 
> Over in meta-ti I kicked off a discussion
> (https://lists.yoctoproject.org/pipermail/meta-ti/2012-March/000779.html)
> about if we should be using '?=' or '+=' with IMAGE_FSTYPES in the
> machine conf files.  This has been discussed a little bit before
> (http://thread.gmane.org/gmane.comp.handhelds.openembedded.core/2060/focus=2061).
>  The problem is we have the following and I believe ultimately
> conflicting use cases:

I've been under the impression that we decided upon:

> - The machine needs to say 'I need or support the following formats'

so the machine starts and sets:

IMAGE_FSTYPES = "xxxx"

> - The distro needs to say 'I always want format X'

so the distro can do:

IMAGE_FSTYPES += " yyy"

> - The user needs to say 'I know best, give me only format X'

So the user can do:

IMAGE_FSTYPES = "X"

> - The user needs to say 'I know best, give me what you support + X'

IMAGE_FSTYPES += " X"

> We achieve this today, but not very nicely.  For the last one you need
> to start playing order of operations games and that's just not nice.
> My suggestion is that we need to change what the machine.conf sets to
> another variable (SOMETHING_IMAGE_FSTYPES), and a default
> IMAGE_FSTYPES becomes ?= ${SOMETHING_IMAGE_FSTYPES}.  Distros, and
> users can then work more easily with their use cases.  Comments?

I think this is overcomplicating things. Yes the order is important but
lets just document the above?

Cheers,

Richard