Increase the BAR memory address space for PCIe devices on the Raspberry Pi Compute Module 4

increase-pci-bar-space.sh

#!/bin/bash

# The default BAR address space available on the CM4 may be too small to allow
# some devices to initialize correctly. To avoid 'failed to assign memory'
# errors on boot, you can increase the range of the PCIe bus in the Raspberry
# Pi's Device Tree (a .dtb file specific to each Pi model).
#
# You should probably read up on Device Trees if you don't know what they are:
# https://www.raspberrypi.org/documentation/configuration/device-tree.md
#
# NOTE: The default BAR allocation was increased to 1 GB in this commit:
# https://github.com/raspberrypi/linux/commit/54db4b2fa4d17251c2f6e639f849b27c3b553939

# Download the trial firmware from this post:
# https://www.raspberrypi.org/forums/viewtopic.php?p=1761834#p1761834
# and replace the corresponding files in the boot volume.

# Back up current Device Tree for the CM4.
sudo cp /boot/bcm2711-rpi-cm4.dtb /boot/bcm2711-rpi-cm4.dtb.bak

# Decompile the current Device Tree to a dts (source) file.
dtc -I dtb -O dts /boot/bcm2711-rpi-cm4.dtb -o ~/test.dts

# Edit the file and change the PCIe bus range as mentioned in:
# https://www.raspberrypi.org/forums/viewtopic.php?p=1746665#p1746665
nano ~/test.dts

# Replace the line that allocates 1 GB of RAM in the pci section:
#   ranges = <0x02000000 0x0 0xc0000000 0x6 0x00000000 0x0 0x40000000>;
#
# with the following line, which provides up to 8 GB of RAM:
#   ranges = <0x02000000 0x0 0x00000000 0x6 0x00000000 0x2 0x00000000>;

# Also replace the scb ranges to:
#    ranges = <0x0 0x7c000000  0x0 0xfc000000  0x0 0x03800000>,
#             <0x0 0x40000000  0x0 0xff800000  0x0 0x00800000>,
#             <0x6 0x00000000  0x6 0x00000000  0x2 0x00000000>,
#             <0x0 0x00000000  0x0 0x00000000  0x0 0xfc000000>;

# Recompile the Device Tree from the dts (source) file.
dtc -I dts -O dtb ~/test.dts -o ~/test.dtb

# Copy the new Device Tree into place.
sudo mv ~/test.dtb /boot/bcm2711-rpi-cm4.dtb

# Reboot.
sudo reboot

This may be useful on the Pi 5 as well (to your question @5erv3), since some larger graphics cards are setting down to 256 MB of BAR space as they have 12+ GB of VRAM: geerlingguy/raspberry-pi-pcie-devices#680

followed the same process for bcm2711-rpi-4-b.dtb it booted

Please suggest the way forward to test it on the Raspberry pi 4b

dmesg | grep -i brcm-pcie
[ 0.500225] brcm-pcie fd500000.pcie: host bridge /scb/pcie@7d500000 ranges:
[ 0.500249] brcm-pcie fd500000.pcie: No bus range found for /scb/pcie@7d500000, using [bus 00-ff]
[ 0.500267] brcm-pcie fd500000.pcie: MEM 0x0600000000..0x07ffffffff -> 0x00c0000000
[ 0.500283] brcm-pcie fd500000.pcie: IB MEM 0x0000000000..0x00bfffffff -> 0x0400000000
[ 0.500295] brcm-pcie fd500000.pcie: Memory resource size exceeds max for 32 bits
[ 0.501377] brcm-pcie fd500000.pcie: PCI host bridge to bus 0000:00
[ 0.607152] brcm-pcie fd500000.pcie: clkreq-mode set to default
[ 0.609219] brcm-pcie fd500000.pcie: link up, 5.0 GT/s PCIe x1 (SSC)

The line:
MEM 0x0600000000..0x07ffffffff -> 0x00c0000000
indicates that the PCIe Root Complex has successfully mapped an 8 GB window (from 0x600000000 to 0x7ffffffff).

The "0x00c0000000" part: This is the internal translation address. It tells the SoC that any data sent to the 8 GB range in CPU memory should be translated to the PCIe bus starting at the 0xc0000000 offset.