RPi: update Pi3 and Pi4 readmes with correct info on FDT handling

Now that Pi 3 FDT handling is fixed, let's update the docs. Furthermore,
let's simplify - be proscriptive about the minimal supported
config.txt configuration. Also, fix some factual mistakes:
- device_tree= override is not mandatory when device_tree_address= is used.
  Not using it means start.elf can chose the right one for the variant.
- device_tree= override is not mandatory for dtoverlay use.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Pete Batard <pete@akeo.ie>

1 files changed, 25 insertions, 13 deletions

diff --git a/Platform/RaspberryPi/RPi3/Readme.md b/Platform/RaspberryPi/RPi3/Readme.md
index f5d25b50..dad58739 100644
--- a/Platform/RaspberryPi/RPi3/Readme.md
+++ b/Platform/RaspberryPi/RPi3/Readme.md
@@ -56,23 +56,30 @@ Build instructions from the top level edk2-platforms Readme.md apply.
 1. Format a uSD card as FAT32

 2. Copy the generated `RPI_EFI.fd` firmware onto the partition

 3. Download and copy the following files from https://github.com/raspberrypi/firmware/tree/master/boot

+  - `bcm2710-rpi-3-b.dtb` (for Pi 3B)

+  - `bcm2710-rpi-3-b-plus.dtb` (for Pi 3B+)

+  - `bcm2710-rpi-2-b.dtb` (for Pi 2B v1.2)

+  - `bcm2710-rpi-cm3.dtb` (for Pi CM3)

   - `bootcode.bin`

   - `fixup.dat`

   - `start.elf`

+  - `overlays/miniuart-bt.dtbo` or `overlays/disable-bt.dtbo` (Optional)

 4. Create a `config.txt` with the following content:

   ```

   arm_control=0x200

   enable_uart=1

   armstub=RPI_EFI.fd

-  disable_commandline_tags=1

+  disable_commandline_tags=2

+  device_tree_address=0x1f0000

+  device_tree_end=0x200000

   ```

-  Additionally, if you want to use PL011 instead of the miniUART, you can add the lines:

+  If you want to use PL011 instead of the miniUART on your Pi 3B/3B+, you can add the lines:

   ```

-  device_tree_address=0x10000

-  device_tree_end=0x20000

-  device_tree=bcm2710-rpi-3-b[-plus].dtb

   dtoverlay=miniuart-bt

   ```

+  Note: doing so requires `miniuart-bt.dbto` to have been copied into an `overlays/`

+  directory on the uSD card. Alternatively, you may use `disable-bt` instead of

+ `miniuart-bt` if you don't require Bluetooth.

 5. Insert the uSD card and power up the Pi.

 

 Note that if you have a model 3+ or a model 3 where you enabled USB boot through OTP

@@ -92,21 +99,26 @@ in the `TrustedFirmware/` directory from `edk2-non-osi`.
 

 ## Custom Device Tree

 

-The default Device Tree included in the firmware is the one for a Raspberry Pi 3 Model B (not B+).

-If you want to use a different Device Tree, to boot a Pi 3 Model B+ for instance (for which a

-DTB is also provided under `DeviceTree/`), you should copy the relevant `.dtb` into the root of

+By default, UEFI will use the device tree loaded by the VideoCore firmware. This

+depends on the model/variant, and relies on the presence on specific files on your boot media.

+E.g.:

+ - `bcm2710-rpi-3-b.dtb` (for Pi 3B)

+ - `bcm2710-rpi-3-b-plus.dtb` (for Pi 3B+)

+ - `bcm2710-rpi-2-b.dtb` (for Pi 2B v1.2)

+ - `bcm2710-rpi-cm3.dtb` (for Pi CM3)

+

+You can override the DTB and provide a custom one. Copy the relevant `.dtb` into the root of

 the SD or USB, and then edit your `config.txt` so that it looks like:

 

 ```

 (...)

 disable_commandline_tags=2

-device_tree_address=0x10000

-device_tree_end=0x20000

-device_tree=bcm2710-rpi-3-b-plus.dtb

+device_tree_address=0x1f0000

+device_tree_end=0x200000

+device_tree=your_fdt_file.dtb

 ```

 

-Note: the address range **must** be `[0x10000:0x20000]`.

-`dtoverlay` and `dtparam` parameters are also supported **when** providing a Device Tree`.

+Note: the address range **must** be `[0x1f0000:0x200000]`. `dtoverlay` and `dtparam` parameters are also supported.

 

 ## Custom `bootargs`