So I am on the quest of learning embedded Linux and have a few questions that I cannot seem to find an answer for.
1) Does the kernel depend on the dtb/dts files when compiling? I thought that the kernel only needs to know the chip architecture (i.e. arm) and the dtb file is loaded by the boot loader (uBoot) so therefore the kernel only needs to load its drivers which are configured by the dtb file.
2) Mixing and matching: I'm under the impression that I can mix and match any combination of boot loader, dtb, kernel, rootfs, and modules given the following
- kernel: must know which chip it is compiled for
- dtb: must know the board details and chip, i.e. how much ram, configure a GPIO for SPI
- boot loader: must know the chip and uEnv.txt must have params for the kernel and dtb location
- rootfs: completely independent
- modules: must be compiled with the specific version of kernel
3) Drivers: If I want to load a SPI driver do I need anything specific or will the kernel know how to operate this because the dtb file setup the required registers?
4) Modules: Are these just dependent on the kernel or do they need to know something about the chip and board (when I say chip what I mean is do they have to know more than a simple arm or x86 architecture)?
Thank you in advance, I know these are some basic questions but any help is appreciated.
The Linux kernel is compiled without any dependency on the Device Tree.
The compilation of the kernel does depend on the chip architecture, but which code modules that are compiled depends on the board configuration(s) and feature selection.
BTW it's U-Boot for Universal Boot, not microBoot.
Essentially correct, but typically one doesn't go overboard in trying to "mix-n-match". There are often optimal or preferred (or at least appropriate) choices.
By "rootfs" I'm assuming you mean type of filesystem for the rootfs, rather some image of a rootfs. (See Addendum below.)
There are two types of "SPI driver", the master and protocol.
The SPI master driver is for the SPI controller chip that serves as the one interface master. This is usually a platform driver and not have a device node in /dev.
For each SPI slave device there must be a protocol driver. This driver will typically have a device node in /dev.
The Device Tree must specify which driver is for which device and any/all resources allocated/assigned to each device.
The dtb file does not "setup" anything. It's only configuration data; there is no executable code. A device driver, typically during its probe or initialization phase, is responsible for acquiring/allocating its resources.
Your use of "modules" is ambiguous. Source code files are sometimes referred to as "modules". Presumably you really mean loadable kernel modules.
Although most people associate kernel modules (only) with device drivers, other kernel services such as filesystems and network protocol handlers can also be built as modules. The primary rationale for a kernel module versus static linkage (i.e. built in the kernel) is for runtime configurability (which in turn improves memory efficiency). Optional features, services and drivers can be left out of the kernel that is booted, but can still be loaded later when needed.
Loadable modules are "dependent" on the kernel simply because of linking requirements for proper execution. The degree of "chip and board knowledge" obviously depends on the functionality of the module, just like any other piece of kernel code.
Addendum
A kernel image and (prebuilt) rootfs image are not "completely independent".
The executable binaries and the shared libraries in the rootfs image must be compatible with the kernel features. More significantly, since kernel loadable modules are installed in the rootfs and not with the kernel image, and these modules can be strictly tied to a specific build of a kernel version, it makes sense to pair a kernel image with a rootfs image.