[yocto] How to specify a default value for a variable in a recipe

[Gary Thomas]

On 2014-06-13 07:08, Patrick Doyle wrote:
> OK, I've got to ask this, because it totally confuses me.  I wrote a
> recipe to build a custom application (gstreamer plugin, actually) that
> I wanted to fetch from our SCM.  Bitbake told me I needed to specify a
> source revision and even gave me a clue of what to put in.  Looking at
> the development manual, I decided I wanted to use the latest revision
> from our repository by default, so I added the following line to my
> recipe:
>
> SRCREV_default_pn-gstxbac = "${AUTOREV}"
>
> But now I'm curious.  Why couldn't I use (I haven't tried this yet)
>
> SRCREV_pn-gstxbac ?= "${AUTOREV}"

If this is in the gstxbac recipe itself, use the much simpler
   SRCREV ?= "${AUTOREV}"

>
> and even more confusingly, why isn't the syntax something more like:
>
> SRCREV_pn-gstxbac_default = "${AUTOREV}"
>
> What's going on here?
>
> Does one syntax (BLAH_default vs BLAH ?=) get evaluated early in the
> bitbake process and the other get evaluated late?
>
> Does the location of the letters "default" in a variable name matter?
>
> At some point, I really need to wrap my head around Bitbake, it's
> syntax, and how it operates and internalize all of that.

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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