Yocto notes - 2

Follow the yocto documentation to setup poky. I'm using kirkstone branch. Some details might vary depending on the branch. Always refer to the branch documentation.

MACHINE and DISTRO
==================

When you create a build, conf/local.conf specifies distro and the machine

dld@dld:~/yocto$ source poky/oe-init-build-env build-qemu

in this build, we have

MACHINE ??= "qemux86-64"
DISTRO ?= "poky"

distro comes from meta-poky/conf/distro/poky.conf

dld@dld:~/yocto/poky$ ls meta-poky/conf/distro/ -F
include/  poky-altcfg.conf  poky-bleeding.conf  poky.conf  poky-tiny.conf

machine comes from meta/conf/machine/qemux86-64.conf

dld@dld:~/yocto/poky$ ls meta/conf/machine -F
include/  qemuarm64.conf  qemuarm.conf  qemuarmv5.conf  qemumips64.conf  qemumips.conf  qemuppc64.conf  qemuppc.conf  qemuriscv32.conf  qemuriscv64.conf  qemux86-64.conf  qemux86.conf

What is a BSP layer
===================

- MACHINE comes from a BSP layer

- at a minimum, a BSP layer should have a machine configuration (see documentation/bsp-guide/bsp.rst for details)

- meta has machine configs in its conf/, so it's a BSP layer

- meta-poky isn't a BSP layer, it only has distro (check on your side or see its listed contents in the previous post)

- our build directory's conf/bblayers.conf has the following

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"
BBPATH = "${TOPDIR}"
BBFILES ?= ""
BBLAYERS ?= " \
  /home/dld/yocto/poky/meta \
  /home/dld/yocto/poky/meta-poky \
  /home/dld/yocto/poky/meta-yocto-bsp \
  "

Why is the meta-yocto-bsp there?

- it's a BSP layer

- if you check the conf/local.conf, you'll see that we can use the following for the MACHINE

# There are also the following hardware board target machines included for 
# demonstration purposes:
#
#MACHINE ?= "beaglebone-yocto"
#MACHINE ?= "genericx86"
#MACHINE ?= "genericx86-64"
#MACHINE ?= "edgerouter"
#

- these machines come from meta-yocto-bsp

dld@dld:~/yocto/poky$ tree meta-yocto-bsp/conf/
meta-yocto-bsp/conf/
├── layer.conf
└── machine
    ├── beaglebone-yocto.conf
    ├── edgerouter.conf
    ├── genericx86-64.conf
    ├── genericx86.conf
    └── include
        └── genericx86-common.inc

- so, if you build for one of these machines, you need the meta-yocto-bsp BSP layer

- when you are building for qemux86-64, you don't really need it

- so try removing that layer, and the build should work just fine

dld@dld:~/yocto/build-qemu$ bitbake-layers remove-layer /home/dld/yocto/poky/meta-yocto-bsp

dld@dld:~/yocto/build-qemu$ bitbake-layers show-layers
NOTE: Starting bitbake server...
layer                 path                                      priority
==========================================================================
meta                  /home/dld/yocto/poky/meta              5
meta-poky             /home/dld/yocto/poky/meta-poky         5

dld@dld:~/yocto/build-qemu$ bitbake core-image-minimal

bootloader and kernel specification
===================================

Where u-boot comes from
-----------------------

meta/recipes-bsp/u-boot

- this is the only u-boot specification in poky which is in meta BSP layer

- in meta/recipes-bsp/u-boot/u-boot.inc

PROVIDES = "virtual/bootloader"

- for example, if we build for beaglebone, we have in meta-yocto-bsp/conf/machine/beaglebone-yocto.conf

PREFERRED_PROVIDER_virtual/bootloader ?= "u-boot"

- see how PROVIDES and PREFERRED_PROVIDER work in documentation/ref-manual/variables.rst

Where linux kernel comes from
-----------------------------

- from the machine configuration file in the BSP layer, the machine we are building for

- for example, if we build for qemux86-64, which is the default, machine config will specify it (may get from an *.inc)

PREFERRED_PROVIDER_virtual/kernel ??= "linux-yocto"

- version comes from meta-poky/conf/distro/poky.conf for qemux86-64

PREFERRED_VERSION_linux-yocto ?= "5.15%"

- for beaglebone, we'll get them from the machine configuration beaglebone-yocto.conf

PREFERRED_PROVIDER_virtual/kernel ?= "linux-yocto"

PREFERRED_VERSION_linux-yocto ?= "5.15%"

- Q: where's the PROVIDES that specifies virtual/kernel?

- A: it's in the kernel.bbclass, and the kernel recipes inherit kernel.bbclass

- experiment commenting out the version and see what happens by querying the variables using bitbake -e (BitBake’s -e option is used to display variable values after parsing)

- lets check the linux version that'll be used

- for beaglebone

dld@dld:~/yocto/build-bb$ bitbake -e core-image-minimal | grep PREFERRED_VERSION_linux-yocto
# $PREFERRED_VERSION_linux-yocto
PREFERRED_VERSION_linux-yocto="5.15%"
# $PREFERRED_VERSION_linux-yocto-rt
PREFERRED_VERSION_linux-yocto-rt="5.15%"

- if you check the full bitbake -e (redirect to a file and search), you'll see from where it's set

for qemux86-64

#
# $PREFERRED_VERSION_linux-yocto
#   set? /home/dld/yocto/poky/meta-poky/conf/distro/poky.conf:23
#     "5.15%"
PREFERRED_VERSION_linux-yocto="5.15%"
#

for beaglebone

#
# $PREFERRED_VERSION_linux-yocto [2 operations]
#   set? /home/dld/yocto/poky/meta-yocto-bsp/conf/machine/beaglebone-yocto.conf:27
#     "5.15%"
#   set? /home/dld/yocto/poky/meta-poky/conf/distro/poky.conf:23
#     "5.15%"
# pre-expansion value:
#   "5.15%"
PREFERRED_VERSION_linux-yocto="5.15%"
#

- so, qemux86-64 will use

meta/recipes-kernel/linux/linux-yocto_5.15.bb

- beaglebone will use

meta-yocto-bsp/recipes-kernel/linux/linux-yocto_5.15.bbappend

- beaglebone is having the following in linux-yocto_5.15.bbappend

KBRANCH:beaglebone-yocto = "v5.15/standard/beaglebone"
KMACHINE:beaglebone-yocto ?= "beaglebone"
SRCREV_machine:beaglebone-yocto ?= "9aabbaa89fcb21af7028e814c1f5b61171314d5a"
COMPATIBLE_MACHINE:beaglebone-yocto = "beaglebone-yocto"
LINUX_VERSION:beaglebone-yocto = "5.15.54"