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Arm drivers: are device tree descriptions required for every device?

I do not fully understand how the kernel attaches drivers for embedded boards.  Clearly, the device tree guides this process and the device tree loaded by the boot loader needs to include the relevant information.

What I don’t understand is whether _every_ driver must be described in the device tree or if the kernel probes for other devices at any point.

I am mostly trying to figure out how USB devices are attached to embedded boards.

For example, the amd64 kernels include drivers for USB devices (e.g., disks, network adapters, wifi, etc.) and they are attached to a usbbus by the kernel probing.  Those drivers can also be compiled into armv7hf kernels.  

However, I am not sure if such a kernel is expected to attach those drivers if a corresponding USB device is present, even though it might not be in the device tree.  Must devices be described in the device tree before the kernel will attach them, or can the kernel itself probe for connected USB devices as it does with amd64?

Sorry if this is a simple question, but I’m finding it a challenge to decipher exactly how device attachment and kernel configuration are supposed to work for embedded NetBSD.

Thanks for your help.


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