Yocto variable not defined but set with _ operator - stm32

Question

I'm struggling with something I'm not sure to address correctly.
In a Yocto environment (for STM32MP1 by the way) I have to configure a new target.
Hence I added to meta-st/meta-st-stm32mp/conf/machine/include/st-machine-extlinux-config-stm32mp.inc this section, that looks like the other already available:
EXTLINUX_BOOTDEVICE_EMMC = "mmc1"
EXTLINUX_BOOTDEVICE_SDCARD = "mmc0"
EXTLINUX_ROOT_EMMC = "${#bb.utils.contains('ST_VENDORFS','1','root=/dev/mmcblk1p4','root=/dev/mmcblk1p3',d)}"
EXTLINUX_ROOT_NAND = "ubi.mtd=UBI rootfstype=ubifs root=ubi0:rootfs"
# Define available targets to use
UBOOT_EXTLINUX_CONFIGURED_TARGETS += "mp151a_sdcard"
UBOOT_EXTLINUX_CONFIGURED_TARGETS += "mp151a_emmc"
# Define bootprefix for each target
UBOOT_EXTLINUX_BOOTPREFIXES_mp151a_sdcard = "${EXTLINUX_BOOTDEVICE_SDCARD}_stm32mp151a_"
UBOOT_EXTLINUX_BOOTPREFIXES_mp151a_emcc = "${EXTLINUX_BOOTDEVICE_EMCC}_stm32mp151a_"
# Define labels for each target
UBOOT_EXTLINUX_LABELS_mp151a_sdcard = "stm32mp151a-sdcard"
UBOOT_EXTLINUX_LABELS_mp151a_emcc = "stm32mp151a-emcc"
# Define default boot config for each target
UBOOT_EXTLINUX_DEFAULT_LABEL_mp151a_sdcard ?= "stm32mp151a-sdcard"
UBOOT_EXTLINUX_DEFAULT_LABEL_mp151a_emcc ?= "stm32mp151a-emcc"
# Define FDT overrides for all labels
UBOOT_EXTLINUX_FDT_stm32mp151a-sdcard = "/stm32mp151a.dtb"
UBOOT_EXTLINUX_FDT_stm32mp151a-emcc = "/stm32mp151a.dtb"
# Define ROOT overrides for all labels
UBOOT_EXTLINUX_ROOT_stm32mp151a-sdcard = "${EXTLINUX_ROOT_SDCARD}"
UBOOT_EXTLINUX_ROOT_stm32mp151a-emcc = "${EXTLINUX_ROOT_EMCC}"
But when I bitbake <image> (that includes the file above) I get this output:
DEBUG: Executing python function update_extlinuxconf_targets
NOTE: UBOOT_EXTLINUX_CONFIGURED_TARGETS: mp157a-dk1_sdcard mp157a-dk1_sdcard-optee mp157c-dk2_sdcard mp157c-dk2_sdcard-optee mp157c-ed1_emmc mp157c-ed1_emmc-optee mp157c-ed1_sdcard mp157c-ed1_sdcard-optee mp157c-ev1_emmc mp157c-ev1_emmc-optee mp157c-ev1_nand mp157c-ev1_nor-sdcard mp157c-ev1_nor-emmc mp157c-ev1_sdcard mp157c-ev1_sdcard-optee mp151a_sdcard mp151a_emmc
NOTE: UBOOT_EXTLINUX_CONFIG_FLAGS: emmc sdcard
NOTE: *** Loop for config_label: emmc
NOTE: *** Loop for devicetree: stm32mp151a
NOTE: >>> New target label: mp151a_emmc
NOTE: >>> Append mp151a_emmc to UBOOT_EXTLINUX_TARGETS
NOTE: *** Loop for config_label: sdcard
NOTE: *** Loop for devicetree: stm32mp151a
NOTE: >>> New target label: mp151a_sdcard
NOTE: >>> Append mp151a_sdcard to UBOOT_EXTLINUX_TARGETS
NOTE: >>> UBOOT_EXTLINUX_TARGETS (updated): mp151a_emmc mp151a_sdcard
DEBUG: Python function update_extlinuxconf_targets finished
DEBUG: Executing python function do_create_multiextlinux_config
ERROR: UBOOT_EXTLINUX_ROOT not defined
DEBUG: Python function do_create_multiextlinux_config finished
ERROR: Function failed: do_create_multiextlinux_config
As you can see, the file is actually processed because it added the targets I've defined.
But it doesn't find the UBOOT_EXTLINUX_ROOT even if it's "set" with the _ operator:
UBOOT_EXTLINUX_ROOT_stm32mp151a-sdcard = "${EXTLINUX_ROOT_SDCARD}"
UBOOT_EXTLINUX_ROOT_stm32mp151a-emcc = "${EXTLINUX_ROOT_EMCC}"
I also tried to set the main variable to something like:
UBOOT_EXTLINUX_ROOT = ""
or
UBOOT_EXTLINUX_ROOT = "root=/dev/mmcblk1p4"
to see if it was the problem but it doesn't change nothing.
Is this something related to Yocto itself (I mean, something wrong in my syntax) or it's very specific to the SDK (meta-st) ?
The error above should be raised by this file:
root = localdata.getVar('UBOOT_EXTLINUX_ROOT')
if not root:
bb.fatal('UBOOT_EXTLINUX_ROOT not defined')
UPDATE
I checked the (huge) output of bitbake -e and among other targets I see:
# $UBOOT_EXTLINUX_ROOT [41 operations]
[...]
# "${EXTLINUX_ROOT_NOREMMC}"
# override[stm32mp157c-ev1-m4-examples-sdcard]:set /local/STM32MP15-Ecosystem-v1.1.0/Distribution-Package/openstlinux-4.19-thud-mp1-19-10-09/layers/meta-st/meta-st-stm32mp/conf/machine/include/st-machine-extlinux-config-stm32mp.inc:274
# "${EXTLINUX_ROOT_SDCARD}"
# override[stm32mp157c-ev1-m4-examples-sdcard-optee]:set /local/STM32MP15-Ecosystem-v1.1.0/Distribution-Package/openstlinux-4.19-thud-mp1-19-10-09/layers/meta-st/meta-st-stm32mp/conf/machine/include/st-machine-extlinux-config-stm32mp.inc:275
# "${EXTLINUX_ROOT_SDCARD_OPTEE}"
# override[stm32mp151a-sdcard]:set /local/STM32MP15-Ecosystem-v1.1.0/Distribution-Package/openstlinux-4.19-thud-mp1-19-10-09/layers/meta-st/meta-st-stm32mp/conf/machine/include/st-machine-extlinux-config-stm32mp.inc:296
# "${EXTLINUX_ROOT_SDCARD}"
# override[stm32mp151a-emcc]:set /local/STM32MP15-Ecosystem-v1.1.0/Distribution-Package/openstlinux-4.19-thud-mp1-19-10-09/layers/meta-st/meta-st-stm32mp/conf/machine/include/st-machine-extlinux-config-stm32mp.inc:297
[...]
# pre-expansion value:
# ""
UBOOT_EXTLINUX_ROOT=""
# $UBOOT_EXTLINUX_ROOT_cubemx-nor-sdcard
UBOOT_EXTLINUX_ROOT_cubemx-nor-sdcard="root=/dev/mmcblk0p3"
# $UBOOT_EXTLINUX_ROOT_cubemx-sdcard
UBOOT_EXTLINUX_ROOT_cubemx-sdcard="root=/dev/mmcblk0p6"
# $UBOOT_EXTLINUX_ROOT_stm32mp151a-emcc
UBOOT_EXTLINUX_ROOT_stm32mp151a-emcc="\${EXTLINUX_ROOT_EMCC}"
# $UBOOT_EXTLINUX_ROOT_stm32mp151a-sdcard
UBOOT_EXTLINUX_ROOT_stm32mp151a-sdcard="root=/dev/mmcblk0p6"
So far, if I understand correctly, the override values are correctly assigned (but not the ${EXTLINUX_ROOT_EMCC} - I don't understand where the \ comes from) but the main variable is still empty.
Adding UBOOT_EXTLINUX_ROOT = "root=/dev/mmcblk1p4" at the beginning of the above file, seems to do the trick (even if before I wrote the opposite, perhaps I forgot to clear the cache?) but I don't think it's the right way to do it.

Answer 1

You should specify the wanted name of the machine as a target to build, i.e.:
MACHINE=stm32mp151a-sdcard bitbake <image>
This way, the UBOOT_EXTLINUX_ROOT gets the non-empty value "root=/dev/mmcblk0p6" (from the UBOOT_EXTLINUX_ROOT_stm32mp151a-sdcard variant of the variable).