rt-thread模糊到清晰系列: thread切换相关

// 创建thread
tid = rt_thread_create("main", main_thread_entry, RT_NULL,
                           RT_MAIN_THREAD_STACK_SIZE, RT_MAIN_THREAD_PRIORITY, 20);

rt_thread_t rt_thread_create(const char *name,
                             void (*entry)(void *parameter),
                             void       *parameter,
                             rt_uint32_t stack_size,
                             rt_uint8_t  priority,
                             rt_uint32_t tick)
{
    struct rt_thread *thread;
    void *stack_start;

    thread = (struct rt_thread *)rt_object_allocate(RT_Object_Class_Thread,
                                                    name);
    if (thread == RT_NULL)
        return RT_NULL;

    stack_start = (void *)RT_KERNEL_MALLOC(stack_size);
    if (stack_start == RT_NULL)
    {
        /* allocate stack failure */
        rt_object_delete((rt_object_t)thread);

        return RT_NULL;
    }

    _rt_thread_init(thread,
                    name,
                    entry,
                    parameter,
                    stack_start,
                    stack_size,
                    priority,
                    tick);

    return thread;
}

thread的栈相关 （这是只是初始化时所指定的寄存器值，用于第一次调度是恢复给psp,  第二次以后的调度, psp会发生变化， 从而sp也会更新为和初始化时可能不一样的位置)

static rt_err_t _rt_thread_init(struct rt_thread *thread,
                                const char       *name,
                                void (*entry)(void *parameter),
                                void             *parameter,
                                void             *stack_start,
                                rt_uint32_t       stack_size,
                                rt_uint8_t        priority,
                                rt_uint32_t       tick)
{
    /* init thread list */
    rt_list_init(&(thread->tlist));

    thread->entry = (void *)entry;
    thread->parameter = parameter;

    /* stack init */
    thread->stack_addr = stack_start;
    thread->stack_size = stack_size;

    /* init thread stack */
    rt_memset(thread->stack_addr, '#', thread->stack_size);
#ifdef ARCH_CPU_STACK_GROWS_UPWARD
    thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
                                          (void *)((char *)thread->stack_addr),
                                          (void *)rt_thread_exit);
#else
    thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
                                          (rt_uint8_t *)((char *)thread->stack_addr + thread->stack_size - sizeof(rt_ubase_t)),
                                          (void *)rt_thread_exit);
#endif
// ...
}

rt_uint8_t *rt_hw_stack_init(void       *tentry,
                             void       *parameter,
                             rt_uint8_t *stack_addr,
                             void       *texit)
{
    struct stack_frame *stack_frame;
    rt_uint8_t         *stk;
    unsigned long       i;

// 8字节对齐，并预留末端记录寄存器结构
    stk  = stack_addr + sizeof(rt_uint32_t);
    stk  = (rt_uint8_t *)RT_ALIGN_DOWN((rt_uint32_t)stk, 8);
    stk -= sizeof(struct stack_frame);

    stack_frame = (struct stack_frame *)stk;

    /* init all register */
    for (i = 0; i < sizeof(struct stack_frame) / sizeof(rt_uint32_t); i ++)
    {
        ((rt_uint32_t *)stack_frame)[i] = 0xdeadbeef;
    }

    stack_frame->exception_stack_frame.r0  = (unsigned long)parameter; /* r0 : argument */
    stack_frame->exception_stack_frame.r1  = 0;                        /* r1 */
    stack_frame->exception_stack_frame.r2  = 0;                        /* r2 */
    stack_frame->exception_stack_frame.r3  = 0;                        /* r3 */
    stack_frame->exception_stack_frame.r12 = 0;                        /* r12 */
// 返回接退出

    stack_frame->exception_stack_frame.lr  = (unsigned long)texit;     /* lr */
// 入口

    stack_frame->exception_stack_frame.pc  = (unsigned long)tentry;    /* entry point, pc */
    stack_frame->exception_stack_frame.psr = 0x01000000L;              /* PSR */

    /* return task's current stack address */
    return stk;
}

切换thread

rt_hw_context_switch_interrupt
    EXPORT rt_hw_context_switch_interrupt
rt_hw_context_switch    PROC
    EXPORT rt_hw_context_switch

    ; set rt_thread_switch_interrupt_flag to 1
    LDR     r2, =rt_thread_switch_interrupt_flag
    LDR     r3, [r2]
    CMP     r3, #1
    BEQ     _reswitch
    MOV     r3, #1
    STR     r3, [r2]

    LDR     r2, =rt_interrupt_from_thread   ; set rt_interrupt_from_thread
    STR     r0, [r2]

_reswitch
    LDR     r2, =rt_interrupt_to_thread     ; set rt_interrupt_to_thread
    STR     r1, [r2]

    LDR     r0, =NVIC_INT_CTRL              ; trigger the PendSV exception (causes context switch)
    LDR     r1, =NVIC_PENDSVSET
    STR     r1, [r0]
    BX      LR
    ENDP

; r0 --> switch from thread stack
; r1 --> switch to thread stack
; psr, pc, lr, r12, r3, r2, r1, r0 are pushed into [from] stack
PendSV_Handler   PROC
    EXPORT PendSV_Handler

    ; disable interrupt to protect context switch
    MRS     r2, PRIMASK
    CPSID   I

    ; get rt_thread_switch_interrupt_flag
    LDR     r0, =rt_thread_switch_interrupt_flag
    LDR     r1, [r0]
    CBZ     r1, pendsv_exit         ; pendsv already handled

    ; clear rt_thread_switch_interrupt_flag to 0
    MOV     r1, #0x00
    STR     r1, [r0]

    LDR     r0, =rt_interrupt_from_thread
    LDR     r1, [r0]
    CBZ     r1, switch_to_thread    ; skip register save at the first time

    MRS     r1, psp                 ; get from thread stack pointer
    STMFD   r1!, {r4 - r11}         ; push r4 - r11 register
    LDR     r0, [r0]
    STR     r1, [r0]                ; update from thread stack pointer

switch_to_thread
    LDR     r1, =rt_interrupt_to_thread
    LDR     r1, [r1]
    LDR     r1, [r1]                ; load thread stack pointer

    LDMFD   r1!, {r4 - r11}         ; pop r4 - r11 register
    MSR     psp, r1                 ; update stack pointer

pendsv_exit
    ; restore interrupt
    MSR     PRIMASK, r2

    ORR     lr, lr, #0x04
    BX      lr
    ENDP

等价伪代码

// 设置thread的切换标志， 记录切换前后的thread的sp, 产生PendSV_Handler中断
void rt_hw_context_switch_interrupt(pp_from_thread_sp, pp_to_thread_sp)
{
    if (rt_thread_switch_interrupt_flag) {
        goto _reswitch;
    }
    rt_thread_switch_interrupt_flag = 1;
    rt_interrupt_from_thread = pp_from_thread_sp;

_reswitch:
    rt_interrupt_to_thread = pp_to_thread_sp;
    [NVIC_INT_CTRL] = NVIC_PENDSVSET;
}
    

// 1. 离开当前thread  >>> 把当前{r4-r11}入栈,并把栈位置记录给thread中的sp

// 2. 进入新的thread  >>> 1中的逆操作

void PendSV_Handler()
{
    r2 = PRIMASK;
    CPSID   I

    if (!rt_thread_switch_interrupt_flag) {
        goto pendsv_exit;
    }

    rt_thread_switch_interrupt_flag = 0;

    // 第一次调度，还没存在要离开的thread，因此不用压栈和记录栈顶位置给sp
    if (!pp_from_thread_sp) {
        goto switch_to_thread;
    }

    MRS     r1, psp                 ; get from thread stack pointer
// r1! 代表 r1最终指向最后一个入栈之后 (没有叹号代表r1不变，即指向第一个入栈位置之前)
    STMFD   r1!, {r4 - r11}         ; push r4 - r11 register
    // stack.push(r11)    stack.push(r10)    ...    stack.push(r4)
    **pp_from_thread_sp = r1

switch_to_thread:
    r1 = **pp_to_thread_sp
    LDMFD   r1!, {r4 - r11}
    MSR     psp, r1

pendsv_exit:
    PRIMASK, r2
}