[OE-core] defining a new image: inherit "image" vs "image_types"?

[Robert P. J. Day]

  possibly a distinction with no distinction but i wanted to throw
together a page on how one defines a new "image type" -- most
commonly, an SD card image -- and i started with what comes with the
meta-raspberrypi layer, which seems to work just fine.

  what i noticed is that that layer introduces the new
"sdcard_image-rpi.bbclass" class file, which opens with:

inherit image_types

  but then i remembered that the meta-ti layer does the same thing,
providing the new class file "sdcard_image.bbclass" which opens
instead with:

inherit image

  i realize that the oe-core image.bbclass contains this snippet:

IMAGE_CLASSES ?= "image_types"
inherit ${IMAGE_CLASSES}

so it's clear that inheriting "image" is sufficient, but the
alternative *isn't* clear.

  what's the preferred construction here?  before digging further to
see if there's something subtle or equivalent happening, should it be
sufficient for new image definitions to simply:

inherit image_types

??  have i just not RTFS far enough to see that there's no difference?

rday

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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