[bitbake-devel] wanting clarification on overrides, _append, += and combinations of those

[Robert P. J. Day]

On Mon, 26 Nov 2012, Paul Eggleton wrote:

> On Saturday 24 November 2012 09:23:20 Robert P. J. Day wrote:

> > RDEPENDS_python-smartpm_append = " python-smartpm-backend-rpm python-codecs
> > python-textutils python-xml" RDEPENDS_python-smartpm_append += "
> > python-fcntl python-pickle python-crypt python-compression python-shell"
> > RDEPENDS_python-smartpm_append += " python-resource python-netclient
> > python-threading python-unixadmin"
>
> I would strongly discourage the use of += in conjunction with
> _append because the intent is unclear - I think we've discussed this
> before; this is just a case where it slipped through. Ideally this
> would just be done as a += split onto multiple lines and I will make
> sure it gets changed to that if someone else doesn't get to it
> first.

  two more bits of pedantry that i think would improve the
code/documentation, related to the above.

  first, as i understand it, the "_append" feature is used to do
appending at the *end* of the, what, parsing?  what do you call that
step, and how would you describe what happens when you use "_append"?
because it seems that, while it's a useful feature when ordering *is*
important, it's overkill when ordering doesn't matter.

  consider this snippet from linux-yocto_3.2.bb:

KERNEL_FEATURES_append=" features/netfilter"
KERNEL_FEATURES_append=" features/taskstats"
KERNEL_FEATURES_append_qemux86=" cfg/sound"
KERNEL_FEATURES_append_qemux86-64=" cfg/sound"

since the above is defining kernel features, is there anything
important about the ordering?  unless there's something tricky going
on, couldn't one just have coded:

KERNEL_FEATURES += "features/netfilter"
KERNEL_FEATURES += "features/taskstats"
KERNEL_FEATURES_qemux86 += "cfg/sound"
KERNEL_FEATURES_qemux86-64 +="cfg/sound"

  that second form is clearly simpler, as long as it represents
identical functionality.  in short, don't use "_append" unless it's
actually necessary.  which brings us to my second suggestion.

  use actual OE/yocto code to explain features.  from the bitbake user
manual, remember this snippet explaining how to use overrides and
appending:

===== start =====

OVERRIDES = "foo"
A_foo_append = "X"
In this case, X is unconditionally appended to the variable A_foo.
Since foo is an override, A_foo would then replace A.

OVERRIDES = "foo"
A = "X"
A_append_foo = "Y"
In this case, only when foo is in OVERRIDES, Y is appended to the
variable A so the value of A would become XY (NB: no spaces are
appended).

OVERRIDES = "foo"
A_foo_append = "X"
A_foo_append += "Y"

===== end =====

  but the above is not all that informative since the reader has no
clue what "A" or "foo" might represent.  so use excerpts from recipes
themselves that are immediately obvious; in fact, the above kernel
features can be used for that:

KERNEL_FEATURES += "features/netfilter"
KERNEL_FEATURES += "features/taskstats"
KERNEL_FEATURES_qemux86 += "cfg/sound"
KERNEL_FEATURES_qemux86-64 +="cfg/sound"

that at least makes it spectacularly clear that, given a particular
architecture, additional features will be selected for the kernel.
simply using "A" and "foo" and "X" and "Y" is fairly meaningless.

  i may shortly have another suggestion or two that have worked for me
in explaining stuff to students.

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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