How to force Linux to load a custom device tree instead of the built-in DTB?

Question

I'm trying to bring up a WWAN modem on a Dell Latitude 7455 laptop running Linux. The modem is connected via PCIe domain 0005, but that PCIe controller is currently disabled in the default device tree.

My goal: enable the PCIe5 controller so the modem shows up as 0005:01:00.0 under lspci.

What I've Done So Far

1. Created a custom device tree (merged-latitude7455-pcie5.dtb)

   • PCIe5 node has status = "okay".
   • Correct power supplies, GPIOs, and pinctrl are defined.
   • Validated syntax with dtc, no errors.

2. Updated GRUB to load our custom DTB

   • Added a custom entry in /boot/grub/grub.cfg:

     menuentry 'Fedora Custom DTB' {
         linux /boot/vmlinuz-6.17.0-8-qcom-x1e root=UUID=38fb7591-7ff3-4734-8533-f973f0f2b88a ro quiet
         devicetree /boot/merged-latitude7455-pcie5.dtb
         initrd /boot/initrd.img-6.17.0-8-qcom-x1e
     }
     

   • Set GRUB default to this entry.

   • Verified it is being used when booting.

3. Manual GRUB console test

   • At boot, pressed c to enter GRUB console.

   • Manually ran:

     insmod part_gpt
     insmod fat
     insmod fdt
     set root=(hd0,gpt5)
     linux /boot/vmlinuz-6.17.0-8-qcom-x1e root=UUID=38fb7591-7ff3-4734-8533-f973f0f2b88a ro
     devicetree /boot/merged-latitude7455-pcie5.dtb
     initrd /boot/initrd.img-6.17.0-8-qcom-x1e
     boot
     

   • System boots fine, but it still ignores our DTB.

4. Checked /proc/cmdline and runtime DTB

   • /proc/cmdline never shows a devicetree parameter.

   • Running cksum shows the loaded DTB does not match our custom DTB:

     sudo cksum /sys/firmware/fdt
     # 2753729362 <size>   <-- runtime DTB
     
     cksum /boot/merged-latitude7455-pcie5.dtb
     # 2195968948 212506   <-- our DTB
     

   • This proves Linux is booting with a different, built-in DTB, completely ignoring ours.

5. Other tests attempted

   • Replaced default DTBs under /boot/dtbs/ → no effect.
   • Tried different partition settings in GRUB.
   • Verified GRUB loads insmod fdt successfully.

The Problem

Despite everything I've tried, Linux always boots using the kernel’s built-in DTB, ignoring my custom file. Because of this:

• PCIe5 remains disabled at runtime:

  cat /proc/device-tree/soc@0/pci@1c00000/status
  # disabled
  

• PCIe domain 0005 never appears:

  lspci -Dnn | grep '^0005:'
  # no output
  

What I've Figured out

• Some ARM64 kernels can have a DTB compiled directly into the kernel image (vmlinuz).

• In my case, it looks like this is what’s happening:

  • Even when I pass devicetree /boot/merged-latitude7455-pcie5.dtb via GRUB, it is silently ignored.
  • The kernel never even tries to load it.

Questions

1. Is there any way to force Linux to use an external DTB instead of the built-in one?
2. If not, what is the correct process to replace the built-in DTB in the kernel?
3. Could there be a kernel configuration option like CONFIG_EFI_ARMSTUB_DTB_LOADER or something similar that we need to enable to allow external DTB loading?

System Details

• Laptop: Dell Latitude 7455
• CPU: Qualcomm Snapdragon X Elite (ARM64)
• Distro: Ubuntu 25
• Kernel: 6.17.0-8-qcom-x1e
• WWAN Modem: DW5932e-eSIM Snapdragon X62