I'm reading the Zircon source code: https://fuchsia.googlesource.com/fuchsia/+/master/ and I'm trying to find how the kernel image is linked. That is, where is the start function placed?
For example, on raspberry pi, the kernel.img start function is placed at 0x80000 with a link script like here: https://github.com/bztsrc/raspi3-tutorial/blob/master/01_bareminimum/link.ld
There is the boards directory: https://fuchsia.googlesource.com/fuchsia/+/master/boards/ which has some dependencies for some boards. It adds some needed drivers. However, I couldn't find anywhere where the linking occurs for each board.
What if I want to support a new board? Suppose I want to port Zircon to raspberry pi, so I need to make the start.S, specifically the start function on it, start at 0x80000. How would I instruct the Zircon build system to do it?
There are a handful of layers to understand, some of which are a little different.
Here's the linker script you may be looking for, but it doesn't tell the whole story: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/phys/zbi_executable.gni
Kernel images included in ZBI's are generated around here, depending on configuration: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/BUILD.gn;l=465
There are also boot-shims, which are used to bridge various worlds to ZBI, some for arm64 are here: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/target/arm64/boot-shim/
These also have the kinds of build parameters you may be looking for: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/target/arm64/boot-shim/BUILD.gn;l=37
Multiboot is common in the Linux world, and this shim for x64 handles exactly that: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/target/pc/multiboot/
An example boot shim configuration with minimal dtb interaction is here: https://cs.opensource.google/fuchsia/fuchsia/+/master:zircon/kernel/target/arm64/board/msm8998/boot-shim-config.h;bpv=0