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4.8 KiB
4.8 KiB
Tutorial 04 - Zero Overhead Abstraction
tl;dr
All hand-written assembly is replaced by Rust code from the cortex-a crate,
which provides zero-overhead abstractions and wraps the unsafe
parts.
bsp::wait_forever()
is introduced.
Diff to previous
diff -uNr 03_hacky_hello_world/Cargo.toml 04_zero_overhead_abstraction/Cargo.toml
--- 03_hacky_hello_world/Cargo.toml
+++ 04_zero_overhead_abstraction/Cargo.toml
@@ -10,7 +10,10 @@
# The features section is used to select the target board.
[features]
default = []
-bsp_rpi3 = []
+bsp_rpi3 = ["cortex-a"]
[dependencies]
r0 = "0.2.*"
+
+# Optional dependencies
+cortex-a = { version = "2.*", optional = true }
diff -uNr 03_hacky_hello_world/src/bsp/rpi3/panic_wait.rs 04_zero_overhead_abstraction/src/bsp/rpi3/panic_wait.rs
--- 03_hacky_hello_world/src/bsp/rpi3/panic_wait.rs
+++ 04_zero_overhead_abstraction/src/bsp/rpi3/panic_wait.rs
@@ -15,9 +15,5 @@
println!("Kernel panic!");
}
- unsafe {
- loop {
- asm!("wfe" :::: "volatile")
- }
- }
+ super::wait_forever()
}
diff -uNr 03_hacky_hello_world/src/bsp/rpi3/start.S 04_zero_overhead_abstraction/src/bsp/rpi3/start.S
--- 03_hacky_hello_world/src/bsp/rpi3/start.S
+++ 04_zero_overhead_abstraction/src/bsp/rpi3/start.S
@@ -1,21 +0,0 @@
-// SPDX-License-Identifier: MIT
-//
-// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
-
-.section ".text._start"
-
-.global _start
-
-_start:
- mrs x1, mpidr_el1 // Read Multiprocessor Affinity Register
- and x1, x1, #3 // Clear all bits except [1:0], which hold core id
- cbz x1, 2f // Jump to label 2 if we are core 0
-1: wfe // Wait for event
- b 1b // In case an event happend, jump back to 1
-2: // If we are here, we are core0
- ldr x1, =_start // Load address of function "_start()"
- mov sp, x1 // Set start of stack to before our code, aka first
- // address before "_start()"
- bl init // Jump to the "init()" kernel function
- b 1b // We should never reach here. But just in case,
- // park this core aswell
diff -uNr 03_hacky_hello_world/src/bsp/rpi3.rs 04_zero_overhead_abstraction/src/bsp/rpi3.rs
--- 03_hacky_hello_world/src/bsp/rpi3.rs
+++ 04_zero_overhead_abstraction/src/bsp/rpi3.rs
@@ -8,8 +8,34 @@
use crate::interface;
use core::fmt;
+use cortex_a::{asm, regs::*};
-global_asm!(include_str!("rpi3/start.S"));
+/// The entry of the `kernel` binary.
+///
+/// The function must be named `_start`, because the linker is looking for this
+/// exact name.
+///
+/// # Safety
+///
+/// - Linker script must ensure to place this function at `0x80_000`.
+#[no_mangle]
+pub unsafe extern "C" fn _start() -> ! {
+ use crate::runtime_init;
+
+ const CORE_0: u64 = 0;
+ const CORE_MASK: u64 = 0x3;
+ const STACK_START: u64 = 0x80_000;
+
+ if CORE_0 == MPIDR_EL1.get() & CORE_MASK {
+ SP.set(STACK_START);
+ runtime_init::init()
+ } else {
+ // if not core0, infinitely wait for events
+ loop {
+ asm::wfe();
+ }
+ }
+}
/// A mystical, magical device for generating QEMU output out of the void.
struct QEMUOutput;
@@ -37,6 +63,13 @@
// Implementation of the kernel's BSP calls
////////////////////////////////////////////////////////////////////////////////
+/// Park execution on the calling CPU core.
+pub fn wait_forever() -> ! {
+ loop {
+ asm::wfe()
+ }
+}
+
/// Returns a ready-to-use `console::Write` implementation.
pub fn console() -> impl interface::console::Write {
QEMUOutput {}
diff -uNr 03_hacky_hello_world/src/main.rs 04_zero_overhead_abstraction/src/main.rs
--- 03_hacky_hello_world/src/main.rs
+++ 04_zero_overhead_abstraction/src/main.rs
@@ -13,9 +13,7 @@
//!
//! [`kernel::interface`]: interface/index.html
-#![feature(asm)]
#![feature(format_args_nl)]
-#![feature(global_asm)]
#![feature(panic_info_message)]
#![no_main]
#![no_std]
@@ -33,7 +31,8 @@
/// Entrypoint of the `kernel`.
fn kernel_entry() -> ! {
- println!("Hello from Rust!");
+ println!("[0] Hello from pure Rust!");
- panic!("Stopping here.")
+ println!("[1] Stopping here.");
+ bsp::wait_forever()
}
diff -uNr 03_hacky_hello_world/src/runtime_init.rs 04_zero_overhead_abstraction/src/runtime_init.rs
--- 03_hacky_hello_world/src/runtime_init.rs
+++ 04_zero_overhead_abstraction/src/runtime_init.rs
@@ -13,7 +13,7 @@
///
/// - Only a single core must be active and running this function.
#[no_mangle]
-pub unsafe extern "C" fn init() -> ! {
+pub unsafe fn init() -> ! {
extern "C" {
// Boundaries of the .bss section, provided by the linker script
static mut __bss_start: u64;