Trait IScheduler

Source

pub trait IScheduler {
    // Required methods
    unsafe fn init_task_stack(
        stack_ptr: *mut usize,
        entry: *const usize,
        arg: *const usize,
        exit: *const usize,
    ) -> *mut usize;
    fn start_first_task(stack_ptr: *const usize);
    fn trigger_context_switch();
}

Expand description

Scheduler.

§Implementation

In addition to the trait the hardware context switch exception must be implemented. The exception must

Push the CPU registers to the stack
Call switch_context from the kernel
- in: current task stack pointer in first parameter register (e.g. r0)
- out: next task stack pointer in return register (e.g. r0)
Pop CPU register from new stack

On a ARM Cortex-M4 for example (simplified):

#[no_mangle]
#[unsafe(naked)]
extern "C" fn PendSV() {
    unsafe {
        asm!(
        "push    {{lr}}",
        "mrs     r0, psp",         // get process stack pointer
        "stmdb   r0!, {{r4-r11}}", // push registers to stack A
        "bl      switch_context",  // call kernel for context switch
        "pop     {{lr}}",
        "mov     r3, #3",
        "msr     control, r3",      // run in unprivileged mode
        "isb",
        "ldmia   r0!, {{r4-r11}}",  // pop registers from stack B
        "msr     psp, r0",          // set process stack pointer
        "bx      lr",
        options(noreturn),
        )
    }
}

The exception can call check_stack to check wheter the there is enough space left to store the registers. - in: stack pointer after stacking - out: 0 - stack would overflow, 1 - enought space left