[yocto] MACHINE_EXTRA_* and MACHINE_ESSENTIAL_* documentation

[Darren Hart]

This is something I've run into frequently and have been putting off
trying to resolve. Hopefully the following will help illustrate the path
a BSP developer might go down trying to use the MACHINE_ESSENTIAL* and
MACHINE_EXTRA* variables. If someone with more knowledge on the subject
could fill in the gaps noted below, I'll be happy to test and work with
Scott to improve the documentation.

I am unable to discern from the following text in the reference manual,
which variable I should be using to include a firmware package in the
images for a given BSP.

From
http://www.yoctoproject.org/docs/1.1/poky-ref-manual/poky-ref-manual.html:

"
MACHINE_ESSENTIAL_RDEPENDS

    List of packages required to boot device
MACHINE_ESSENTIAL_RRECOMMENDS

    List of packages required to boot device (usually additional kernel
modules)
MACHINE_EXTRA_RDEPENDS

    List of packages required to use device
MACHINE_EXTRA_RRECOMMNEDS  (<-- typo still present)

    List of packages useful to use device (for example additional kernel
modules)
"

>From the above, I don't learn anything about what the impact of using
each of these variables is. Which ones add the packages to the rootfs,
which only build them, do they impact the initrd (one might assume a
package required for boot should be in the initrd). The difference
between RDEPENDS and RRECOMMENDS is unclear. Three of the four use the
term "required" while the other uses "useful" in the description.
Neither semantic split (RDEPENDS vs. RRECOMMENDS, ESSENTIAL vs. EXTRA)
can be understood from the available descriptions.

The firmware I want to add is needed for wireless functionality. This is
likely not required for boot, so I added it to MACHINE_EXTRA_RRECOMMENDS
as it is only needed if the wireless driver is used. This resulted in
the package being built, but not installed. In practice this would mean
that people building the BSP would need to manually add the firmware
package to IMAGE_INSTALL or install it manually later. This strikes me
as unnecessarily complex.

This was also unexpected given the definition of RRECOMMENDS in the
manual, which would have led me to believe the package should have been
installed:

"
RRECOMMENDS

    List of packages which extend usability of the package. Those
packages will be automatically installed but can be removed by user.
"

I wasn't able to find a general meaning for the term "EXTRA" by
searching through the manual.

I then tried the remaining 3 variables, doing the following after
setting each:

$ bitbake -c cleanall task-machine-base task-core-boot
linux-firmware-core-image-sato
$ bitbake -u depexp -g core-image-sato
$ bitbake core-image-sato

I then determined if the linux-firmware package appeared in the list
presented by the depexp UI and whether or not the linux-firmware package
was built and installed. The results follow:

MACHINE_EXTRA_RDEPENDS = "linux-firmware-iwlwifi-6000g2a-5"
# appears in depexp with no rdepends, builds, does not install

#MACHINE_EXTRA_RECOMMENDS += "linux-firmware-iwlwifi-6000g2a-5"
# does not appear in depexp, does not build

#MACHINE_ESSENTIAL_RDEPENDS = "linux-firmware-iwlwifi-6000g2a-5"
# does not appear in depexp, does not build


I've tried searching the sources for these variables, and quickly lose
their scent. Here is what I did find:

task-base.bb:
#
# packages added by machine config
#
RDEPENDS_task-machine-base = "${MACHINE_EXTRA_RDEPENDS}"
RRECOMMENDS_task-machine-base = "${MACHINE_EXTRA_RRECOMMENDS}"

task-core-boot.bb:
RDEPENDS_task-core-boot = "\
    ...
    ${MACHINE_ESSENTIAL_EXTRA_RDEPENDS}"

RRECOMMENDS_task-core-boot = "\
    ${MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS}"


How these should impact the final image is not clear to me.


Thanks,

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel