Custom DTS Overlay Memory Reservation Not Reflected After Boot on STM32MP157F-DK2

[VedantK33]

Hi everyone,

I am trying to reserve a block of RAM using a custom device tree overlay on my STM32MP157F-DK2 running Linux. Here's what I have done so far:

This is my custom DTS overlay

/dts-v1/;
/plugin/;
/ {
    compatible = "st,stm32mp157f-dk2", "st,stm32mp157";
    fragment@0 {
        target-path = "/reserved-memory";
        __overlay__ {
            sensor_shm: sensor_shm@c8000000 {
                compatible = "shared-dma-pool";
                reg = <0xc8000000 0x00040000>; /* 256KB */
                no-map;
                status = "okay";
            };
        };
    };
};

I compiled this to get custom-overlay.dtbo and placed it under /boot/overlays/.

Then, I edited the /boot/mmc0_extlinux/stm32mp157f-dk2_extlinux.conf configuration file to include the overlay during boot by adding this line inside the appropriate boot label:

# Generic Distro Configuration file generated by OpenEmbedded
menu title Select the boot mode
MENU BACKGROUND /splash_portrait.bmp
TIMEOUT 20
DEFAULT OpenSTLinux
LABEL OpenSTLinux
        KERNEL /uImage
        FDTDIR /
        INITRD /st-image-resize-initrd
        APPEND root=PARTUUID=e91c4e10-16e6-4c0e-bd0e-77becf4a3582 rootwait rw   console=${console},${baudrate}
        fdtoverlays /boot/overlays/custom-overlay.dtbo
LABEL stm32mp157f-dk2-a7-examples
        KERNEL /uImage
        FDT /stm32mp157f-dk2-a7-examples.dtb
        INITRD /st-image-resize-initrd
        APPEND root=PARTUUID=e91c4e10-16e6-4c0e-bd0e-77becf4a3582 rootwait rw   console=${console},${baudrate}
LABEL stm32mp157f-dk2-m4-examples
        KERNEL /uImage
        FDT /stm32mp157f-dk2-m4-examples.dtb
        INITRD /st-image-resize-initrd
        APPEND root=PARTUUID=e91c4e10-16e6-4c0e-bd0e-77becf4a3582 rootwait rw   console=${console},${baudrate}
~

Problem:
After reboot, the reserved memory node /reserved-memory/sensor_shm does not appear in the device tree (checked via /proc/device-tree), and my memory reservation does not take effect.

Could someone please help me identify what I might be missing or doing wrong with the overlay or boot configuration?

Thank you!

[Erwan SZYMANSKI]

Hello @VedantK33 ,
Few investigations path you can check at:

• Does your U-Boot has the necessary configuration to support DTBO ? I think we disable it by default for security reason, but need to double check. The config seems to be CONFIG_LIBFDT_OVERLAY
• Check if caps are not mandatory in your extconf file read by U-Boot. We have this example for X-LINUXTSNSWCH as extconf U-Boot file:

• LABEL stm32mp257f-ev1-tsn
          KERNEL /Image.gz
          FDTDIR /
          FDTOVERLAYS /devicetree/stm32mp25xx-ev1-swch.dtbo
          INITRD /st-image-resize-initrd
          APPEND root=PARTUUID=e91c4e10-16e6-4c0e-bd0e-77becf4a3582 rootwait rw   earlycon console=${console},${baudrate}
  

• Add logs in U-Boot in pxe_utils.c file to check if the overlay is well detected and applied.

Kind regards,
Erwan. 

[VedantK33]

How can I check my U-boot configuration ? I think I cant change it as the configuration file is baked at build time and now what we have on board is U-boot binary. So I might have to build U-boot with desired configuration. Please correct me if I am wrong. 

[Erwan SZYMANSKI · Accepted solution]

Hello @VedantK33 ,
If you built the default OpenSTLinux distribution as explained in the wiki, so the default U-Boot configuration might not have CONFIG_OF_LIBFDT_OVERLAY configured by default, regarding the stm32mp15_defconfig file in sources of U-Boot.

You have 2 different options:

• Creating a patch for U-Boot that modify the stm32mp15_defconfig and add the necessary configuration. Then add this patch as a bbappend in your Yocto build
• Cross compiling out of Yocto U-Boot with the modified configuration, and deploy it in your FIP used on your target (what we call the Developer Package method)

Kind regards,
Erwan.

[VedantK33]

Hey @Erwan SZYMANSKI 

Thanks for replying.

I used another way to overlay custom-overlay on the stm32mp157f.dts file. 

It is very simple and does not require developer package method.

I am pasting link below where I have mentioned steps to create reserved memory 

Guide to create reserved memory 

Thanks and regards,

Vedant.