[yocto] What are _virtual providers? and other Suffixes?

[Brad Litterell]

Hi Paul,

Thanks for taking the time to explain, very enlightening.  I think I understood it, please allow me to play back my understanding:

1.  _virtual is part of the variable name, and is not a special type of override.
2.  PREFERRED_PROVIDER_virtual/kernel_am335x-evm breaks into: "PREFERRED_PROVIDER_virtual/kernel" with an override condition of  "am335x-evm"
3. So for the jpeg case:

>> PREFERRED_PROVIDER_jpeg = "libjpeg-turbo"
>> 
>> PREFERRED_PROVIDER_jpeg_armv5te = "jpeg"

Could this have also been _virtual/jpeg?  It's just that some components use the _virtual convention and others don't?


Do I have it right?

Thanks,
Brad



On Aug 20, 2013, at 3:27 PM, Paul Eggleton <paul.eggleton at linux.intel.com>
 wrote:

> Hi Brad,
> 
> On Monday 19 August 2013 22:51:04 Brad Litterell wrote:
>> I searched the Yocto Mega Manual, but am still somewhat mystified by the
>> suffix formatting of various variable - especially virtual ones like this:
>> 
>> 
>> PREFERRED_PROVIDER_virtual/gettext = "gettext"
>> 
>> PREFERRED_PROVIDER_virtual/kernel_am335x-evm = "linux-ti-staging"
>> 
>> PREFERRED_PROVIDER_virtual/bootloader_am335x-evm = "u-boot-ti-staging"
> 
> These are setting PREFERRED_PROVIDER for a number of targets. "virtual/xyz" is
> a convention we use where there are multiple providers for a particular
> component, so a recipe that requires that component can have virtual/xyz in
> its DEPENDS, each provider declares virtual/xyz in its PROVIDES, and then in
> configuration the desired recipe to provide that can be selected via 
> PREFERRED_PROVIDER_virtual/xyz = "...".
> 
> The second two examples have an additional override which makes them only
> applicable for a specific machine (am335x-evm). If you want to know how this
> mechanism works have a look at this:
> 
> http://docs.openembedded.org/bitbake/html/ch02.html#id439023
> 
> (We're in the process of updating the BitBake manual, so this old version will have to suffice.)
> 
>> Also, there seems to be some sort of prioritization going on with the
>> multiple levels underscore characters in places like this:
>> 
>> 
>> # Set the PREFERRED_PROVIDER for jpeg functionality based on the MACHINE
>> 
>> # architecture.  For armv7a devices libjpeg-turbo should be used to take
>> 
>> # advantage of the SIMD instructions.
>> 
>> PREFERRED_PROVIDER_jpeg = "libjpeg-turbo"
>> 
>> PREFERRED_PROVIDER_jpeg_armv5te = "jpeg"
>> 
>> 
>> (all examples from the arago project in arago-prefs.inc)
>> 
>> Is there a succinct way of viewing these suffixes that will allow me to wrap
>> my head around their various & myriad versions?
> 
> OVERRIDES is listed in order so that overrides for the later items are
> preferred. If you run bitbake -e | less and search for OVERRIDES (with the /
> key) you can see what the value is for the global environment; here is an
> example for my current configuration:
> 
> # $OVERRIDES [3 operations]
> #   set conf/bitbake.conf:670
> #     "${TARGET_OS}:${TRANSLATED_TARGET_ARCH}:build-${BUILD_OS}:pn-${PN}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${CLASSOVERRIDE}:forcevariable"
> #   set conf/bitbake.conf:671
> #     [vardepsexclude] "MACHINEOVERRIDES"
> #   postdot /home/paul/poky/poky/meta/conf/distro/include/tclibc-eglibc.inc:9
> #     "${LIBCOVERRIDE}"
> # computed:
> #   "${TARGET_OS}:${TRANSLATED_TARGET_ARCH}:build-${BUILD_OS}:pn-${PN}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${CLASSOVERRIDE}:forcevariable${LIBCOVERRIDE}"
> OVERRIDES="linux:i586:build-linux:pn-bblayers:x86:qemuall:qemux86:poky:class-target:forcevariable:libc-glibc"
> 
> Putting the values of all these variables into OVERRIDES gives us a way of
> setting variable values per-machine, per-architecture etc. Based on the
> above value, if I had:
> 
> SOMEVAR = "value1"
> SOMEVAR_qemux86 = "anothervalue"
> SOMEVAR_i586 = "yetanothervalue"
> 
> The computed value of SOMEVAR would be "anothervalue". If MACHINE were
> changed to, say, "qemumips", then OVERRIDES would change and the value 
> of SOMEVAR would become "value1".
> 
>> Am I right in assuming the "variable name" consists of the uppercase parse
>> (regardless of number of underscores)?
> 
> That is our (fairly strict) convention in OpenEmbedded, but FWIW I don't
> think bitbake itself enforces that; it's all dependent on what is in OVERRIDES
> and where the _ character appears in the left hand side of the assignment
> statement.
> 
> Cheers,
> Paul
> 
> -- 
> 
> Paul Eggleton
> Intel Open Source Technology Centre