Few questions about <machine>.conf

Few questions about <machine>.conf

[Christian Gagneraud]

Hi,

I'm trying to find the best way to add machine specific dependencies 
packages to the various images i can build (i'm mainly interested in 
base-image and console-image). I'm using the origin/stable/2009 branch 
with angstrom-2008.1 distro.

To find what oe offered to me, i went through the oe user's manual, 
conf/documentation.conf and start greping all around the place.

The list of variables I get by greping the machine conf files was 
really bigger than what i could find in the documentation, so to 
clarify my mind and the user manual if it needs too, here are some 
simple questions:

When writing a new machine conf file, are these variable the only ones 
which are required?
TARGET_ARCH
MACHINE_FEATURES
PREFERRED_PROVIDER_virtual/kernel

When writing a new machine conf file, are these variable allowed, do 
they really have effect, does that make sense to use them here, and 
finally can/should we use "=", "+=" or "?="  to set them?

  * Extra image commands
EXTRA_IMAGECMD_<image_type>
EXTRA_IMAGECMD_<image_type>_<machine>

The first one is documented in the manual, but is the second form 
allowed? if so is it recommended to use this form?

  * Image dependency
EXTRA_IMAGEDEPENDS
EXTRA_IMAGE_DEPENDS
BOOTSTRAP_EXTRA_RDEPENDS
MACHINE_ESSENTIAL_EXTRA_RDEPENDS
MACHINE_EXTRA_RDEPENDS
MACHINE_EXTRA_RDEPENDS_append
MACHINE_EXTRA_RRECOMMENDS

MACHINE_ESSENTIAL_EXTRA_RDEPENDS sounds like the best one to use for 
machine specific dependencies, what about the other ones?

  * PREFERRED_VERSION_<package>

Can this one be used to force a specific linux kernel version for 
example? Or does it have to be managed in linux-<version> recipe via 
DEFAULT_PREFERENCE_<machine>?

  * VENDOR, VOLATILE_STORAGE_SIZE

These 2 ones seems useless


Any comments welcome!

Regards,
Chris