When booting a kernel in an embedded device, you need to supply a device tree to the Linux kernel, while booting a kernel on a regular x86 pc doesn't require a device tree — why?
As I understand, on an x86 pc the kernel "probes" for hardware (correct me if I'm wrong), so why can't the kernel probe for hardware in and embedded system?
Best Answer
Peripherals are connected to the main processor via a bus. Some bus protocols support enumeration (also called discovery), i.e. the main processor can ask “what devices are connected to this bus?” and the devices reply with some information about their type, manufacturer, model and configuration in a standardized format. With that information, the operating system can report the list of available devices and decide which device driver to use for each of them. Some bus protocols don't support enumeration, and then the main processor has no way to find out what devices are connected other than guessing.
All modern PC buses support enumeration, in particular PCI (the original as well as its extensions and successors such as AGP and PCIe), over which most internal peripherals are connected, USB (all versions), over which most external peripherals are connected, as well as Firewire, SCSI, all modern versions of ATA/SATA, etc. Modern monitor connections also support discovery of the connected monitor (HDMI, DisplayPort, DVI, VGA with EDID). So on a PC, the operating system can discover the connected peripherals by enumerating the PCI bus, and enumerating the USB bus when it finds a USB controller on the PCI bus, etc. Note that the OS has to assume the existence of the PCI bus and the way to probe it; this is standardized on the PC architecture (“PC architecture” doesn't just mean an x86 processor: to be a (modern) PC, a computer also has to have a PCI bus and has to boot in a certain way).
Many embedded systems use less fancy buses that don't support enumeration. This was true on PC up to the mid-1990s, before PCI overtook ISA. Most ARM systems, in particular, have buses that don't support enumeration. This is also the case with some embedded x86 systems that don't follow the PC architecture. Without enumeration, the operating system has to be told what devices are present and how to access them. The device tree is a standard format to represent this information.
The main reason PC buses support discovery is that they're designed to allow a modular architecture where devices can be added and removed, e.g. adding an extension card into a PC or connecting a cable on an external port. Embedded systems typically have a fixed set of devices¹, and an operating system that's pre-loaded by the manufacturer and doesn't get replaced, so enumeration is not necessary.
¹ If there's an external bus such as USB, USB peripherals are auto-discovered, they wouldn't be mentioned in the device tree.