confusing note in dev manual about search order for layers

confusing note in dev manual about search order for layers

[Robert P. J. Day]

  in section 3.1.9 of dev manual:

https://docs.yoctoproject.org/dev-manual/common-tasks.html#adding-a-layer-using-the-bitbake-layers-script

one reads the note:

"During a build, the OpenEmbedded build system looks in the layers
from the top of the list down to the bottom in that order."

ok, except it's not at all clear what the consequence of that is.
looks for what? what is the significance of that search order to the
developer?

rday

Re: [docs] confusing note in dev manual about search order for layers

[Quentin Schulz]

Hi Robert,

On Sun, Aug 29, 2021 at 08:52:42AM -0400, Robert P. J. Day wrote:
> 
>   in section 3.1.9 of dev manual:
> 
> https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.yoctoproject.org_dev-2Dmanual_common-2Dtasks.html-23adding-2Da-2Dlayer-2Dusing-2Dthe-2Dbitbake-2Dlayers-2Dscript&d=DwIBAg&c=_sEr5x9kUWhuk4_nFwjJtA&r=LYjLexDn7rXIzVmkNPvw5ymA1XTSqHGq8yBP6m6qZZ4njZguQhZhkI_-172IIy1t&m=mVZiXFV_cVQVdgl1QOxle8X44ftmg_wTdRhHpubvgnI&s=5pEBxWBXxmuUiL2gZv_9GyOl4bi2YEQSOwhj70Kxt9A&e= 
> 
> one reads the note:
> 
> "During a build, the OpenEmbedded build system looks in the layers
> from the top of the list down to the bottom in that order."
> 
> ok, except it's not at all clear what the consequence of that is.
> looks for what? what is the significance of that search order to the
> developer?
> 

My guess:
All layer.conf are parsed in the order specified in bblayers.conf. This
includes specifically BBPATH, to which layers usually append to with .=.
This variable defines the layer precedence for classes and conf files (e.g.
distro, machine, others?) selection when there's name conflict.
Apparently it is used for require/include with "relative-to-layer-root"
path (as opposed to "relative-to-recipe-file" path) too from what I read
on IRC.

Hope this helps and is not too far from the truth :)
Cheers,
Quentin

Re: [docs] confusing note in dev manual about search order for layers

[Peter Kjellerstedt]

> -----Original Message-----
> From: docs@lists.yoctoproject.org <docs@lists.yoctoproject.org> On Behalf
> Of Quentin Schulz
> Sent: den 30 augusti 2021 09:42
> To: Robert P. J. Day <rpjday@crashcourse.ca>
> Cc: YP docs mailing list <docs@lists.yoctoproject.org>
> Subject: Re: [docs] confusing note in dev manual about search order for
> layers
> 
> Hi Robert,
> 
> On Sun, Aug 29, 2021 at 08:52:42AM -0400, Robert P. J. Day wrote:
> >
> >   in section 3.1.9 of dev manual:
> >
> > https://urldefense.proofpoint.com/v2/url?u=https-
> 3A__docs.yoctoproject.org_dev-2Dmanual_common-2Dtasks.html-23adding-2Da-
> 2Dlayer-2Dusing-2Dthe-2Dbitbake-2Dlayers-
> 2Dscript&d=DwIBAg&c=_sEr5x9kUWhuk4_nFwjJtA&r=LYjLexDn7rXIzVmkNPvw5ymA1XTSq
> HGq8yBP6m6qZZ4njZguQhZhkI_-
> 172IIy1t&m=mVZiXFV_cVQVdgl1QOxle8X44ftmg_wTdRhHpubvgnI&s=5pEBxWBXxmuUiL2gZ
> v_9GyOl4bi2YEQSOwhj70Kxt9A&e=
> >
> > one reads the note:
> >
> > "During a build, the OpenEmbedded build system looks in the layers
> > from the top of the list down to the bottom in that order."
> >
> > ok, except it's not at all clear what the consequence of that is.
> > looks for what? what is the significance of that search order to the
> > developer?
> >
> 
> My guess:
> All layer.conf are parsed in the order specified in bblayers.conf. This
> includes specifically BBPATH, to which layers usually append to with .=.
> This variable defines the layer precedence for classes and conf files (e.g.
> distro, machine, others?) selection when there's name conflict.
> Apparently it is used for require/include with "relative-to-layer-root"
> path (as opposed to "relative-to-recipe-file" path) too from what I read
> on IRC.
> 
> Hope this helps and is not too far from the truth :)
> Cheers,
> Quentin

If you want bitbake to find configuration files etc as per Quentin's 
description above in the same order as it applies bbappend files, then 
you need to list the layers in bblayers.conf in reverse priority 
according to BBFILE_PRIORITY. This is because BBFILES is searched 
according to BBFILE_PRIORITY, whereas BBPATH is searched from left to 
right. I.e., the layers in bblayers.conf should look something like this
if you use poky and some openembedded layers:

BBLAYERS ?= " \
  .../meta-mylayer \
  .../meta-webserver \
  .../meta-multimedia \
  .../meta-filesystems \
  .../meta-networking \
  .../meta-python \
  .../meta-oe \
  .../meta-poky \
  .../meta \
  "

The only exception to this that I know about is meta-poky, which for 
unknown reasons uses =. instead of .= when it adds itself to BBPATH. 
This makes it very hard to exactly match the ordering, but as long as 
you don't need to override poky-sanity.bbclass in your own layer, this 
is normally not a problem.

//Peter