[yocto] supported IMAGE_FSTYPES

[Tim Bird]

I am in the process of creating my own distro, and trying to figure out
all the different build variables.  This is a pretty daunting task
for a yocto project/Poky newbie.

Is there some place where the possible values for IMAGE_FSTYPE are
documented?  The Poky reference manual has this helpful sentence:
"Formats of root filesystem images that you want to have created."
However, it does not describe the possible values this variable
should have.

At the root of the poky source dir, doing a 'find -type f -print 0 | xargs -0 grep IMAGE_FSTYPES'
seems to reveal the following used or checked-for image types:
 jffs2 sdimg tar.bz2 ubi ext3 ext2 cpio.gz live vmdk

It appears the cpio.gz corresponds to an image suitable for an initramfs.
jffs2, ext2, ext3, ubi and tar.bz2 appear self-explanatory.
I have a pretty good idea that vmdk is somehow related to the
virtual build image.

What does 'live' do?

Wait a sec...

Reading meta/classes/image_types.bbclass I just found IMAGE_TYPES,
which says it is "available to request which values are suitable
for IMAGE_FSTYPES".

This has the following definition in denzil:
IMAGE_TYPES = "jffs2 sum.jffs2 cramfs ext2 ext2.gz ext2.bz2 ext3 ext3.gz
ext2.lzma btrfs live squashfs squashfs-lzma ubi tar tar.gz tar.bz2 tar.xz
cpio cpio.gz cpio.xz cpio.lzma vmdk"

How does one query this variable?  Other than reading the source, is
there a way to know that such variables (which list the allowed values
for other variables) exist?

I apologize if these are newbie questions.

Thanks,
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment
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