STM32MP2: "stm32_sdmmc2_mmc_init: DT error" after build of custom board with emmc booting

I'm working on adding a custom STM32MP257 board based on the openstlinux distribution package, and I'm currently using the STM32MP257F-EV1 board. I'm having problems with booting on eMMC with a stm32cubemx-generated device tree. What I've done:

* Created a STM32MP257F-EV1 project in CubeMX. I've only changed the project name, no pinning etc (based on instructions from https://wiki.st.com/stm32mpu/wiki/How_to_create_STM32MPU_projects_with_STM32CubeMX#Create_an_STM32MP2_series_project)

* Copied the generated output into layers/meta-st/meta-st-stm32mp-addons/mx

* Copied stm32mp25-mx.conf to a custom one and edited (per
https://wiki.st.com/stm32mpu/wiki/How_to_create_your_own_machine)

BOOTSCHEME_LABELS += "optee"
[...]
BOOTDEVICE_LABELS += "sdcard"
BOOTDEVICE_LABELS += "emmc"

* Built the image using

DISTRO=openstlinux-weston MACHINE=stm32mp257f-simon source layers/meta-st/scripts/envsetup.sh
bitbake st-image-weston

which finishes successfully (CUBEMX_DTB, CUBEMX_PROJECT etc being set in the conf. I had them wrong first, and then the build fails).

* Flashed using

STM32_Programmer_CLI -c port=usb1 -w flashlayout_st-image-weston/optee/FlashLayout_emmc_stm32mp257f-stm32mp257f-simon-mx-optee.tsv

and started with the boot switches set to emmc booting.


However, it doesn't boot. On the serial terminal, I see

PANIC at PC : 0x000000000e0276ac
NOTICE: CPU: STM32MP257FAI Rev.Y
NOTICE: Model: STMicroelectronics STM32MP257F-EV1 STM32CubeMX board - openstlinux-6.6-yocto-scarthgap-mpu-v26.02.18
NOTICE: Board: MB1936 Var1.0 Rev.D-01
NOTICE: Reset reason: Power-on reset (por_rstn) (0x2035)
INFO: PMIC2 version = 0x11
INFO: PMIC2 product ID = 0x20
INFO: FCONF: Reading TB_FW firmware configuration file from: 0xe011000
INFO: FCONF: Reading firmware configuration information for: stm32mp_fuse
INFO: FCONF: Reading firmware configuration information for: stm32mp_io
INFO: Using EMMC
INFO: Instance 2
INFO: Boot used partition fsbl1
ERROR: stm32_sdmmc2_mmc_init: DT error
ERROR: SDMMC2 init failed
BACKTRACE: START: boot_mmc
0: EL3: 0xe017bac
1: EL3: 0xe0276a4
2: EL3: 0xe0170dc
BACKTRACE: END: boot_mmc
PANIC at PC : 0x000000000e0276ac

 
So how do I get booting via emmc working? What works:

* Flashing on the sdcard from the same build (with the boot switches set to sdcard boot).
* Building with

DISTRO=openstlinux-weston MACHINE=stm32mp2 source layers/meta-st/scripts/envsetup.sh

instead, and flashing the -ev1 board for emmc, i.e., without my cubemx stuff and custom conf.



What I want is basically to prepare for pinning changes via CubeMX, and setup an environment where I can modify u-boot, the kernel, device trees etc for my custom design. I need to boot on emmc, which is all I'll have. Any clues to what to do to get this running?