sunxi-dt-overlays's Introduction

Device Tree overlays for sunxi devices

📙 End user documentation

💬 Forum thread for issues and suggestions

Version

As of now overlays in this repository should work with the kernel version 5.0.x. They may need changes/adjustments to work on older versions and eventually they will be updated to newer kernel versions.

Technical info

Requirements

mainline u-boot 2017.03 or newer with CONFIG_OF_LIBFDT_OVERLAY enabled
latest version of appropriate boot script
existing armbianEnv.txt with correct overlay_prefix value
Device Tree compiler with overlays support for compiling the overlays

Notes:

Older u-boot versions require this patch to fix endiannes of values obtained with fdt get value command

Implementation details

Boot script reads /boot/armbianEnv.txt which may contain following environment variables:

overlay_prefix
overlays
user_overlays
overlay specific parameters

Overlay files referenced by overlays and user_overlays variables are loaded and applied using fdt apply command. After applying all overlays a SoC specific fixup script is executed to process overlay specific parameters.

Limitations

U-boot fdt command does not support "tuples" values which limits implementing things like GPIO SPI chip selects with variable GPIO pins that require this type of values:

cs-gpios = <0>, <&pio 0 1 0>, <&pio 7 7 0>; /* Native, PA1, PH7 */

U-boot does not support overlay parameters, so changing values is implemented via executing a "fixup" script after all overlays were applied. This script uses environment variables loaded from /boot/armbianEnv.txt to change the live tree using fdt command.
Since SoCs have multiple controllers of the same type (I2C, SPI) that can be exposed in different combinations on different boards, slave devices (I2C, I2S, SPI, ...) are provided not as ready to use overlays but as examples that require minimal editing to match the used configuration.

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