[OE-core] a few more picky questions about COMPATIBLE_MACHINE usage

[Robert P. J. Day]

  (i apologize for beating this issue to death, but i want to
understand it and, more often than not, what i'm puzzled about always
seems simple but i end up wondering, "ok, is that just sloppily coded,
or is there some subtle property that i'm missing?". anyway ...)

  from examples under meta-openembedded, let's first look at the
recipe meta-efl/recipes-efl/e17/elbow_git.bb, which opens with:

  # webkit-efl isn't available for < armv7a
  COMPATIBLE_MACHINE = "(-)"
  COMPATIBLE_MACHINE_x86 = "(.*)"
  COMPATIBLE_MACHINE_x86-64 = "(.*)"
  COMPATIBLE_MACHINE_armv7a = "(.*)"

  first, that comment seems out of date as it makes no mention of MIPS
or ppc, but that's just being picky.

  next, i assume the line:

  COMPATIBLE_MACHINE = "(-)"

is to initialize the set of compatible machines to the empty set, yes?
i'm more familiar with the construct:

  COMPATIBLE_MACHINE = "(^$)"

to do that, but i'm assuming setting that variable to any unmatchable
value from MACHINE_OVERRIDES will work just as well. (it would be nice
if that were consistent, unless there is something magic about the RE
"-").

  i next assume that lines of the form:

  COMPATIBLE_MACHINE_x86 = "(.*)"

are meant to indicate that if that MACHINE_OVERRIDES comparison
succeeds, then all possible targets are compatible, is that right?
however, given precisely those lines above, is it not equivalent to
just write:

  COMPATIBLE_MACHINE = "(x86|x86-64|armv7a)"

oddly, there is another recipe that seems to do the same thing, but in
a very different way, meta-oe/recipes-support/ne10/ne10_1.2.1.bb:

  COMPATIBLE_MACHINE_aarch64 = "(.*)"
  COMPATIBLE_MACHINE_armv7a = "(.*)"

  python () {
    if any(t.startswith('armv7') for t in d.getVar('TUNE_FEATURES').split()):
        d.setVar('NE10_TARGET_ARCH', 'armv7')
        bb.debug(2, 'Building Ne10 for armv7')
    elif any(t.startswith('aarch64') for t in d.getVar('TUNE_FEATURES').split()):
        d.setVar('NE10_TARGET_ARCH', 'aarch64')
        bb.debug(2, 'Building Ne10 for aarch64')
    else:
        raise bb.parse.SkipPackage("Incompatible with archs other than armv7 and aarch64")
  }

i'm confused ... what is the point of defining these compatibilites:

  COMPATIBLE_MACHINE_aarch64 = "(.*)"
  COMPATIBLE_MACHINE_armv7a = "(.*)"

without first starting with:

  COMPATIBLE_MACHINE = "(-)"

as is stands, isn't the above just saying this recipe is compatible
with everything?

  now, i can see that that anonymous python routine refines the
machine compatibility even further but, to speed things up, why not
first start with just:

  COMPATIBLE_MACHINE = "(aarch64|armv7a)"

or (once again), am i misunderstanding something?

  one more question coming on this, but i want to study it a bit more
carefully first.

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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