Revert to assembly boot code

It is too risky to rely on the compiler to not insert any operations using the
stack.

Having a stack-setting call in Rust using the cortex-a crate as the first action
in a Rust-only _start() function does not work if you're subsequently using the
stack, because the compiler often inserts the operations to make room on the
stack to prepare a function call BEFORE the call to set the stack, which crashes
the boot process.

Hence, keep on using a small piece of assembly boot code throughout.

Author: andre-richter

.editorconfig

@@ -23,8 +23,9 @@ indent_size = 4
 [*.rs]
 indent_size = 4
 
-[*.S]
-indent_size = 4
+[*.s]
+indent_style = tab
+indent_size = 8
 
 [*.sh]
 indent_size = 4

01_wait_forever/Cargo.toml

@@ -4,6 +4,9 @@ version = "0.1.0"
 authors = ["Andre Richter <[email protected]>"]
 edition = "2018"
 
+[profile.release]
+lto = true
+
 [features]
 default = []
 bsp_rpi3 = []

01_wait_forever/README.md

@@ -23,7 +23,7 @@
     - Only `.text` section.
 - `main.rs`: Important [inner attributes]:
     - `#![no_std]`, `#![no_main]`
-- `boot.S`: Assembly `_start()` function that executes `wfe` (Wait For Event), halting all cores
+- `boot.s`: Assembly `_start()` function that executes `wfe` (Wait For Event), halting all cores
   that are executing `_start()`.
 - We (have to) define a `#[panic_handler]` function to make the compiler happy.
     - Make it `unimplemented!()` because it will be stripped out since it is not used.

01_wait_forever/src/_arch/aarch64/cpu/boot.S

@@ -11,5 +11,5 @@
 //!
 //! crate::cpu::boot::arch_boot
 
-// Assembly counterpart to this file. Includes function _start().
-global_asm!(include_str!("boot.S"));
+// Assembly counterpart to this file.
+global_asm!(include_str!("boot.s"));

01_wait_forever/src/_arch/aarch64/cpu/boot.rs

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2021 Andre Richter <[email protected]>
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+.section .text._start
+
+//------------------------------------------------------------------------------
+// fn _start()
+//------------------------------------------------------------------------------
+_start:
+	// Infinitely wait for events (aka "park the core").
+1:	wfe
+	b	1b
+
+.size	_start, . - _start
+.type	_start, function
+.global	_start

01_wait_forever/src/_arch/aarch64/cpu/boot.s

@@ -23,6 +23,5 @@ SECTIONS
     .text :
     {
         KEEP(*(.text._start))
-        *(.text*)
     } :segment_rx
 }

01_wait_forever/src/bsp/raspberrypi/link.ld

@@ -100,10 +100,8 @@
 //!
 //! # Boot flow
 //!
-//! 1. The kernel's entry point is the function [`cpu::boot::arch_boot::_start()`].
-//!     - It is implemented in `src/_arch/__arch_name__/cpu/boot.S`.
-//!
-//! [`cpu::boot::arch_boot::_start()`]: ../src/kernel/cpu/up/_arch/aarch64/cpu/boot.rs.html
+//! 1. The kernel's entry point is the function `cpu::boot::arch_boot::_start()`.
+//!     - It is implemented in `src/_arch/__arch_name__/cpu/boot.s`.
 
 #![feature(asm)]
 #![feature(global_asm)]

01_wait_forever/src/main.rs

@@ -17,3 +17,7 @@ bsp_rpi4 = []
 ##--------------------------------------------------------------------------------------------------
 
 [dependencies]
+
+# Platform specific dependencies
+[target.'cfg(target_arch = "aarch64")'.dependencies]
+cortex-a = { version = "5.x.x" }