Adding a GPIO port expander (mcp23017) to a M64

Banana Pi SBCs design BPI-M64(A64)
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With a Raspberry Pi (raspbian), when one adds a port expander (like an i2c mcp23017), one can use dtoverlay to load a device tree which will load and configure the device driver to make the additional GPIO pins become available under sysfs (/sys/class/gpio). What does one do under Armbian Focal?

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overlays that exits are available in armbian-config -> system -> hardware

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Not partitularly helpful. /boot/dtb/overlay does not exist – I cheated and looked at the source code for armbian-config – I don’t really want to manually use armbian-config for this sort of thing – I want something I can include in a script.

I guess what I am looking for is the source code for overlays for armbian for the M64. Where is that?

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Check if this helps you:

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OK, that looks like it will be helpful. I need to create an overlay for the mcp230XX chips – I think I can craft something using the sunxi-DT-overlay repo and the Raspbian overlays as a guide.