[yocto] the apparent need for docbook <replaceable> tag in manual examples

[Christopher Larson]

On Sun, Jun 22, 2014 at 6:20 AM, Robert P. J. Day <rpjday at crashcourse.ca>
wrote:

>   as i said, it took me a few minutes to figure out what the
> conditional metadata example above was trying to demonstrate, but as
> soon as i saw a real example in the codebase, it was perfectly clear.
> to this end, i'm creating wiki pages that show stuff like that
> exactly, like this one for conditional overrides:
>

It's not directly related, but here's some background if it helps anyone at
all.

Conceptually, when we designed overrides, it was created to let us
implement layering of metadata (note: not layers as in yocto layers, but
conceptual layers), to let us always let more specific information be used
in preference to less specific information. Machine information will be
used in preference to Architecture which is used in preference to the
default. This lets us ensure that we're always using the most accurate
information available about the configuration in question.

I've always thought that thinking of it this way helps one see the purpose
behind the mechanism (and you can see that the order of the
includes/requires in bitbake.conf essentially implements the same ordering
where possible, to provide the same capability based on load order of the
config files). I'm not sure if any of our docs talk about it this way, and
I'm not sure if it helps anyone else wrap their heads around it, but I'm
throwing it out there since I've found that some folks seem to find it a
useful way of looking at it.
-- 
Christopher Larson
clarson at kergoth dot com
Founder - BitBake, OpenEmbedded, OpenZaurus
Maintainer - Tslib
Senior Software Engineer, Mentor Graphics
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