# Modify the device tree for CPU FVPs

## In this learning path

- [Introduction](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/)
- [Introduction to Arm Fixed Virtual Platforms (FVPs)](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/intro/)
- [Configure Trusted Firmware-A build flags to include cpu_ops support](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/steps/)
- [Modify the device tree for CPU FVPs](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/modify/)
- [Run the Linux software stack on an FVP](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/run/)
- [Debug the software stack](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/debug/)
- [Next steps](https://learn.arm.com/learning-paths/embedded-and-microcontrollers/linux-on-fvp/_next-steps/)

## Ensure the device tree matches your FVP model
To run Linux on Arm CPU FVPs, you need to adjust the device tree to match the hardware features of these platforms. This involves removing unsupported nodes, such as the System Memory Management Unit (SMMU) and Peripheral Component Interconnect (PCI), and ensuring that the CPU affinity values are set correctly.

### Remove PCI and SMMU nodes
CPU FVPs don’t support PCI or SMMU. If you leave these nodes in the device tree, Linux will crash at boot with a kernel panic.

So to workaround this, you need to remove PCI and SMMU nodes:

Open the device tree file in a text editor:
```bash
vim linux/arch/arm64/boot/dts/arm/fvp-base-revc.dts
```
Remove the following nodes:
- `pci@40000000`
- `iommu@2b400000`

> **Warning**  
> If you skip this step, you might encounter an error like:
> ```plaintext
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> ```

### Set CPU affinity values
Each FVP model uses specific CPU affinity values. If these don’t match the values in the device tree, some of the CPU cores won’t boot.

Find the correct affinities:
```bash
FVP_Base_Cortex-A55x4 -l | grep pctl.CPU-affinities
```
Example output:
```plaintext
pctl.CPU-affinities=0.0.0.0, 0.0.1.0, 0.0.2.0, 0.0.3.0
```
Convert each to hex for the `reg` field:
```plaintext
0x0, 0x0100, 0x0200, 0x0300
```
Update the CPU nodes in your device tree file to use these `reg` values.

> **Tip**  
> To avoid boot errors such as `psci: failed to boot CPUx (-22)`, make sure every `cpu@xxx` entry matches the FVP layout.

### Rebuild Linux
After editing the device tree, rebuild Linux:
```bash
./build-scripts/build-linux.sh -p aemfvp-a -f busybox clean
./build-scripts/build-linux.sh -p aemfvp-a -f busybox build
./build-scripts/aemfvp-a/build-test-busybox.sh -p aemfvp-a package
```
This regenerates the image with the updated device tree, ready for use with your FVP.