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Document requirements for disabling 3GB RAM limit #279

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d6554be
document disable 3GB RAM limit
markafarrell May 20, 2025
c15971a
add pcie-32bit-dma overlay to release zip
markafarrell May 20, 2025
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1 change: 1 addition & 0 deletions .github/workflows/linux_edk2.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,7 @@ jobs:
curl -O -L ${{ env.RPI_FIRMWARE_URL }}/raw/${{ env.DTB_VERSION }}/boot/bcm2711-rpi-400.dtb
curl -O -L ${{ env.RPI_FIRMWARE_URL }}/raw/${{ env.DTBO_VERSION }}/boot/overlays/miniuart-bt.dtbo
curl -O -L ${{ env.RPI_FIRMWARE_URL }}/raw/${{ env.DTBO_VERSION }}/boot/overlays/upstream-pi4.dtbo
curl -O -L ${{ env.RPI_FIRMWARE_URL }}/raw/${{ env.DTBO_VERSION }}/boot/overlays/pcie-32bit-dma.dtbo
mkdir overlays
mv *.dtbo overlays

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37 changes: 32 additions & 5 deletions Readme.md
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Expand Up @@ -12,14 +12,14 @@ This repository contains installable builds of the official

# Initial Notice

__PLEASE READ THE FOLLOWING:__
__PLEASE READ THE FOLLOWING:__
🔻🔻🔻🔻🔻🔻🔻🔻🔻

* Ethernet networking support in Linux requires a recent enough kernel (version 5.7 or
later)

* SD or wireless support in Linux also requires a recent enough kernel (version 5.12 or
later).
later).
Still, your mileage may vary as to whether these peripherals will actually be usable.

* Many drivers (GPIO, VPU, etc) are still likely to be missing from your OS, and will
Expand All @@ -28,12 +28,39 @@ __PLEASE READ THE FOLLOWING:__

* A 3 GB RAM limit is enforced __by default__, even if you are using a Raspberry Pi 4
model that has 4 GB or 8 GB of RAM, on account that the OS **must** patch DMA access,
to work around a hardware bug that is present in the Broadcom SoC.
to work around a hardware bug that is present in the Broadcom SoC.
For Linux this usually translates to using a recent kernel (version 5.8 or later) and
for Windows this requires the installation of a filter driver.
for Windows this requires the installation of a filter driver.
If you are running an OS that has been adequately patched, you can disable the 3 GB
limit by going to `Device Manager` → `Raspberry Pi Configuration` → `Advanced Settings`
in the UEFI settings.
or by doing the following:
```bash
pip install virt-firmware

virt-fw-vars \
--input RPI_EFI.fd \
--output RPI_EFI.fd \
--set-json <(cat <<EOF
{
"version": 2,
"variables": [
{
"name": "RamLimitTo3GB",
"guid": "cd7cc258-31db-22e6-9f22-63b0b8eed6b5",
"data": "00000000"
}
]
}
EOF
)
```

You also need to apply the `pcie-32bit-dma.dtbo` overlay (https://github.com/raspberrypi/firmware/tree/refs/heads/master/boot/overlays) so that pcie DMA addresses do not overlap system memory.

```bash
echo "dtoverlay=pcie-32bit-dma" >> config.txt
```

* This firmware is built from the
[official EDK2 repository](https://github.com/tianocore/edk2-platforms/tree/master/Platform/RaspberryPi/RPi4).
Expand All @@ -54,7 +81,7 @@ __PLEASE READ THE FOLLOWING:__
UEFI firmware and find that booting from USB or from ESP doesn't work, please visit
https://github.com/raspberrypi/rpi-eeprom/releases to update your EEPROM.

* Extract all the files from the archive onto the partition you created above.
* Extract all the files from the archive onto the partition you created above.
Note that outside of this `Readme.md`, which you can safely remove, you should not
change the names of the extracted files and directories.

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