Help me understand Openembedded variable expansion

Help me understand Openembedded variable expansion

[Matt Schuckmann]

So there is something I just don't get about how variables get created, expanded, interpreted, etc in Bitbake

For example I'm trying to copy some of the techniques in the Poky Tiny distribution to reduce my image size and build time. 
One of the fragments is this one. 

# The mtrace script included by eglibc is a perl script. This means the system
# will build perl in case this package is installed. Since we don't care about
# this script, remove the dependency from here.
RDEPENDS_${PN}-mtrace_pn-eglibc = ""

I put this in my distribution .conf file and I doesn't seem to do anything perl is still getting built. 
Of course it doesn't help that I don't understand what does, especially in the context of the distribution .conf file. 

I also see hints in the code in debug output that the machine, image, or distribution names can automatically get appended to paths, or variables but I don't really understand how that works. 

I'm not even sure how to phrase a question around this, immediately I'd like to figure out why Perl is still getting built but ultimately I'd like to understand the variable rules better so I that I can better take advantage of this power tool. 

Thanks,
Matt S. 

Re: Help me understand Openembedded variable expansion

[Richard Purdie]

On Tue, 2014-10-28 at 10:11 -0700, Matt Schuckmann wrote:
> So there is something I just don't get about how variables get
> created, expanded, interpreted, etc in Bitbake
> 
> For example I'm trying to copy some of the techniques in the Poky Tiny
> distribution to reduce my image size and build time. 
> One of the fragments is this one. 
> 
> # The mtrace script included by eglibc is a perl script. This means
> the system
> # will build perl in case this package is installed. Since we don't
> care about
> # this script, remove the dependency from here.
> RDEPENDS_${PN}-mtrace_pn-eglibc = ""
> 
> I put this in my distribution .conf file and I doesn't seem to do
> anything perl is still getting built. 
> Of course it doesn't help that I don't understand what does,
> especially in the context of the distribution .conf file. 
> 
> I also see hints in the code in debug output that the machine, image,
> or distribution names can automatically get appended to paths, or
> variables but I don't really understand how that works. 
> 
> I'm not even sure how to phrase a question around this, immediately
> I'd like to figure out why Perl is still getting built but ultimately
> I'd like to understand the variable rules better so I that I can
> better take advantage of this power tool. 

Perhaps have a look at "overrides" in the bitbake manual.

You can split RDEPENDS_${PN}-mtrace_pn-eglibc into components. Starting
on the RHS, "_pn-eglibc" is an override that applies to recipes where PN
= "eglibc". In those recipes, the variable becomes:

RDEPENDS_${PN}-mtrace = ""

and is basically overriding a dependency on perl that would other be
there, something like:

RDEPENDS_${PN}-mtrace = "perl".

Does that help?

Cheers,

Richard

Re: Help me understand Openembedded variable expansion

[Matt Schuckmann]

> -----Original Message-----
> From: Richard Purdie [mailto:richard.purdie@linuxfoundation.org]
> Sent: Thursday, October 30, 2014 6:52 AM
> To: Matt Schuckmann
> Cc: Yocto Project
> Subject: Re: [yocto] Help me understand Openembedded variable expansion
> 
> On Tue, 2014-10-28 at 10:11 -0700, Matt Schuckmann wrote:
> > So there is something I just don't get about how variables get
> > created, expanded, interpreted, etc in Bitbake
> >
> > For example I'm trying to copy some of the techniques in the Poky
> Tiny
> > distribution to reduce my image size and build time.
> > One of the fragments is this one.
> >
> > # The mtrace script included by eglibc is a perl script. This means
> > the system # will build perl in case this package is installed. Since
> > we don't care about # this script, remove the dependency from here.
> > RDEPENDS_${PN}-mtrace_pn-eglibc = ""
> >
> > I put this in my distribution .conf file and I doesn't seem to do
> > anything perl is still getting built.
> > Of course it doesn't help that I don't understand what does,
> > especially in the context of the distribution .conf file.
> >
> > I also see hints in the code in debug output that the machine, image,
> > or distribution names can automatically get appended to paths, or
> > variables but I don't really understand how that works.
> >
> > I'm not even sure how to phrase a question around this, immediately
> > I'd like to figure out why Perl is still getting built but ultimately
> > I'd like to understand the variable rules better so I that I can
> > better take advantage of this power tool.
> 
> Perhaps have a look at "overrides" in the bitbake manual.
> 
> You can split RDEPENDS_${PN}-mtrace_pn-eglibc into components. Starting
> on the RHS, "_pn-eglibc" is an override that applies to recipes where
> PN = "eglibc". In those recipes, the variable becomes:
> 
> RDEPENDS_${PN}-mtrace = ""
> 
> and is basically overriding a dependency on perl that would other be
> there, something like:
> 
> RDEPENDS_${PN}-mtrace = "perl".
> 
> Does that help?
> 
> Cheers,
> 
> Richard

Thank you to both Peter Bergin and Richard Purdie, 

I'm "slightly" more knowledgeable now, Peter's suggestion of using bitbake -g -u depexp {image} and looking at the reverse depends for perl lead me to see that it was the external-linaro-toolchain recipe from the Arago layer that was bringing in perl. 
Doing some digging around I could see that meta-arago-extras/recipes-core/meta/external-arago-toolchain.bb is including recipes-core/eglibc/eglibc-package.inc which has the RDEPENDS line perl on mtrace. 

Richards explanation of the dependency override lead me to the following line in my own distribution.conf file. 
RDEPENDS_${PN}-mtrace_pn-external-linaro-toolchain = ""

Now when I run bitbake -g -u depexp {image} I no longer see perl in the list of packages. 

I'm still not sure why it's "pn-external-linaro-toolchain" since the recipe file that includes eglibc-package.inc is called external-arago-toolchain.bb. 

I also tried to take a look in the manual for a description of overrides of this form and I didn't immediately see anything that covered this _pn-foo-bar syntax. I did find this section but it is a bit light and doesn't seem to cover this case
http://www.yoctoproject.org/docs/1.6/bitbake-user-manual/bitbake-user-manual.html#conditional-syntax-overrides

Am I missing something? 

Thanks, 
Matt S.