linux-2.6.26内核中ARM中断实现详解（2）

作者：刘老师，华清远见嵌入式学院金牌讲师，ARM公司ATC授权培训讲师。

三、中断处理过程

这一节将以S3C2410为例，描述linux-2.6.26内核中，从中断开始，中断是如何一步一步执行到我们注册函数的。

3.1 中断向量表 arch\arm\kernel\entry-armv.S

__vectors_start:
             swi SYS_ERROR0
             b    vector_und + stubs_offset
             ldr pc, .LCvswi + stubs_offset
             b    vector_pabt + stubs_offset
             b    vector_dabt + stubs_offset
             b    vector_addrexcptn + stubs_offset
             b    vector_irq + stubs_offset
             b    vector_fiq + stubs_offset
             .globl   __vectors_end
        __vectors_end:

中断发生后，跳转到b vector_irq + stubs_offset的位置执行。注意现在的向量表的初始位置是0xffff0000。

3.2 中断跳转的入口位置 arch\arm\kernel\entry-armv.S

      .globl   __stubs_start
    __stubs_start:
     /*
     * Interrupt dispatcher
     */
             vector_stub    irq, IRQ_MODE, 4 @IRQ_MODE在include\asm\ptrace.h中定义：0x12
             .long    __irq_usr @ 0 (USR_26 / USR_32)
             .long    __irq_invalid @ 1 (FIQ_26 / FIQ_32)
             .long    __irq_invalid @ 2 (IRQ_26 / IRQ_32)
             .long    __irq_svc @ 3 (SVC_26 / SVC_32)
             .long    __irq_invalid @ 4
             .long    __irq_invalid @ 5
             .long    __irq_invalid @ 6
             .long    __irq_invalid @ 7
             .long    __irq_invalid @ 8
             .long    __irq_invalid @ 9
             .long __irq_invalid @ a
             .long __irq_invalid @ b
             .long __irq_invalid @ c
             .long __irq_invalid @ d
             .long __irq_invalid @ e
             .long __irq_invalid @ f

上面代码中vector_stub宏的定义为：

.macro vector_stub, name, mode, correction=0
             .align 5
        vector_\name:
             .if \correction
             sub lr, lr, #\correction
             .endif
             @
             @ Save r0, lr_<exception> (parent PC) and spsr_<exception>
             @ (parent CPSR)
             @
             stmia sp, {r0, lr} @ save r0, lr
             mrs lr, spsr
             str lr, [sp, #8] @ save spsr
             @
             @ Prepare for SVC32 mode. IRQs remain disabled.
             @
             mrs r0, cpsr
             eor r0, r0, #(\mode ^ SVC_MODE)
             msr spsr_cxsf, r0 @为后面进入svc模式做准备

@
             @ the branch table must immediately follow this code
              @
             and lr, lr, #0x0f @进入中断前的mode的后4位
             @#define USR_MODE 0x00000010
             @#define FIQ_MODE 0x00000011
             @#define IRQ_MODE 0x00000012
             @#define SVC_MODE 0x00000013
             @#define ABT_MODE 0x00000017
             @#define UND_MODE 0x0000001b
             @#define SYSTEM_MODE 0x0000001f
             mov r0, sp
             ldr lr, [pc, lr, lsl #2] @如果进入中断前是usr，则取出PC+4*0的内容，即__irq_usr @如果进入中断前是svc，则取出PC+4*3的内容，即__irq_svc
             movs pc, lr @ 当指令的目标寄存器是PC，且指令以S结束，则它会把@ spsr的值恢复给cpsr branch to handler in SVC mode
             .endm
             .globl __stubs_start
       __stubs_start:
       /*
       * Interrupt dispatcher
       */
             vector_stub irq, IRQ_MODE, 4
             .long __irq_usr @ 0 (USR_26 / USR_32)
             .long __irq_invalid @ 1 (FIQ_26 / FIQ_32)
             .long __irq_invalid @ 2 (IRQ_26 / IRQ_32)
             .long __irq_svc @ 3 (SVC_26 / SVC_32)

用“irq, IRQ_MODE, 4”代替宏vector_stub中的“name, mode, correction”，找到了我们中断处理的入口位置为vector_irq（宏里面的vector_\name）。
从上面代码中的注释可以看出，根据进入中断前的工作模式不同，程序下一步将跳转到_irq_usr 、或__irq_svc等位置。我们先选择__irq_usr作为下一步跟踪的目标。

3.3 __irq_usr的实现 arch\arm\kernel\entry-armv.S

__irq_usr:
             usr_entry @后面有解释
             kuser_cmpxchg_check
       #ifdef CONFIG_TRACE_IRQFLAGS
             bl trace_hardirqs_off
       #endif
             get_thread_info tsk @获取当前进程的进程描述符中的成员变量thread_info的地址，并将该地址保存到寄存器tsk等于r9（在entry-header.S中定义）
       #ifdef CONFIG_PREEMPT//如果定义了抢占，增加抢占数值
             ldr r8, [tsk, #TI_PREEMPT] @ get preempt count
             add r7, r8, #1 @ increment it
             str r7, [tsk, #TI_PREEMPT]
       #endif

irq_handler @中断处理，我们最关心的地方，3.4节有实现过程。
      #ifdef CONFIG_PREEMPT
             ldr r0, [tsk, #TI_PREEMPT]
             str r8, [tsk, #TI_PREEMPT]
             teq r0, r7
             strne r0, [r0, -r0]
      #endif
      #ifdef CONFIG_TRACE_IRQFLAGS
             bl trace_hardirqs_on
       #endif

mov why, #0

b ret_to_user @中断处理完成，返回中断产生的位置，3.7节有实现过程

上面代码中的usr_entry是一个宏，主要实现了将usr模式下的寄存器、中断返回地址保存到堆栈中。

.macro usr_entry
       sub sp, sp, #S_FRAME_SIZE @ S_FRAME_SIZE的值在arch\arm\kernel\asm-offsets.c
       @ 中定义 DEFINE(S_FRAME_SIZE, sizeof(struct pt_regs));实际上等于72

     stmib sp, {r1 - r12}
             ldmia r0, {r1 - r3}
             add r0, sp, #S_PC @ here for interlock avoidance
             mov r4, #-1 @ "" "" "" ""

             str r1, [sp] @ save the "real" r0 copied
             @ from the exception stack

     @
             @ We are now ready to fill in the remaining blanks on the stack:
             @
             @ r2 - lr_<exception>, already fixed up for correct return/restart
             @ r3 - spsr_<exception>
             @ r4 - orig_r0 (see pt_regs definition in ptrace.h)
             @
             @ Also, separately save sp_usr and lr_usr
             @
             stmia r0, {r2 - r4}
             stmdb r0, {sp, lr}^

      @
             @ Enable the alignment trap while in kernel mode
             @
alignment_trap r0

     @
             @ Clear FP to mark the first stack frame
             @
             zero_fp
             .endm

上面的这段代码主要在填充结构体pt_regs ，这里提到的struct pt_regs，在include/asm/ptrace.h中定义。此时sp指向struct pt_regs。

      struct pt_regs {
                          long uregs[18];
             };
       #define ARM_cpsr uregs[16]
       #define ARM_pc uregs[15]
       #define ARM_lr uregs[14]
       #define ARM_sp uregs[13]
       #define ARM_ip uregs[12]
       #define ARM_fp uregs[11]
       #define ARM_r10 uregs[10]
       #define ARM_r9 uregs[9]
       #define ARM_r8 uregs[8]
       #define ARM_r7 uregs[7]
       #define ARM_r6 uregs[6]
       #define ARM_r5 uregs[5]
       #define ARM_r4 uregs[4]
       #define ARM_r3 uregs[3]
       #define ARM_r2 uregs[2]
       #define ARM_r1 uregs[1]
       #define ARM_r0 uregs[0]
       #define ARM_ORIG_r0 uregs[17]

3.4 irq_handler的实现过程，arch\arm\kernel\entry-armv.S

  .macro irq_handler
              get_irqnr_preamble r5, lr
              @在include/asm/arch-s3c2410/entry-macro.s中定义了宏get_irqnr_preamble为空操作，什么都不做
              1: get_irqnr_and_base r0, r6, r5, lr @判断中断号，通过R0返回，3.5节有实现过程
              movne r1, sp
              @
              @ routine called with r0 = irq number, r1 = struct pt_regs *
              @
              adrne lr, 1b
              bne asm_do_IRQ @进入中断处理。
       ……
              .endm

3.5 get_irqnr_and_base中断号判断过程，include/asm/arch-s3c2410/entry-macro.s

.macro get_irqnr_and_base, irqnr, irqstat, base, tmp
              mov \base, #S3C24XX_VA_IRQ
              @@ try the interrupt offset register, since it is there
              ldr \irqstat, [ \base, #INTPND ]
              teq \irqstat, #0
              beq 1002f
              ldr \irqnr, [ \base, #INTOFFSET ] @通过判断INTOFFSET寄存器得到中断位置
              mov \tmp, #1
              tst \irqstat, \tmp, lsl \irqnr
              bne 1001f
              @@ the number specified is not a valid irq, so try
              @@ and work it out for ourselves
              mov \irqnr, #0 @@ start here
              @@ work out which irq (if any) we got
              movs \tmp, \irqstat, lsl#16
              addeq \irqnr, \irqnr, #16
              moveq \irqstat, \irqstat, lsr#16
              tst \irqstat, #0xff
              addeq \irqnr, \irqnr, #8
              moveq \irqstat, \irqstat, lsr#8
              tst \irqstat, #0xf
              addeq \irqnr, \irqnr, #4
              moveq \irqstat, \irqstat, lsr#4
              tst \irqstat, #0x3
              addeq \irqnr, \irqnr, #2
              moveq \irqstat, \irqstat, lsr#2
              tst \irqstat, #0x1
              addeq \irqnr, \irqnr, #1
              @@ we have the value
      1001:

       adds \irqnr, \irqnr, #IRQ_EINT0 @加 上中断号的基准数值，得到最终的中断号，注意：此时没有考虑子中断的具体情况，（子中断的问题后面会有讲解）。IRQ_EINT0在 include/asm/arch-s3c2410/irqs.h中定义.从这里可以看出，中断号的具体值是有平台相关的代码决定的，和硬件中断挂起寄存 器中的中断号是不等的。

1002:
              @@ exit here, Z flag unset if IRQ
          .endm

3.6 asm_do_IRQ实现过程，arch/arm/kernel/irq.c

asmlinkage void __exception asm_do_IRQ(unsigned int irq, struct pt_regs *regs)
        {
              struct pt_regs *old_regs = set_irq_regs(regs);
              struct irq_desc *desc = irq_desc + irq;//根据中断号找到对应的irq_desc
              /*
              * Some hardware gives randomly wrong interrupts. Rather
              * than crashing, do something sensible.
              */
              if (irq >= NR_IRQS)
              desc = &bad_irq_desc;
              irq_enter();//没做什么特别的工作，可以跳过不看
              desc_handle_irq(irq, desc);// 根据中断号和desc进入中断处理
              /* AT91 specific workaround */
              irq_finish(irq);
              irq_exit();
              set_irq_regs(old_regs);
       }

static inline void desc_handle_irq(unsigned int irq, struct irq_desc *desc)
         {
                     desc->handle_irq(irq, desc);//中断处理
          }

上述asmlinkage void __exception asm_do_IRQ(unsigned int irq, struct pt_regs *regs)使用了asmlinkage标识。那么这个标识的含义如何理解呢？
该符号定义在kernel/include/linux/linkage.h中，如下所示：

#include <asm/linkage.h>//各个具体处理器在此文件中定义asmlinkage
        #ifdef __cplusplus
        #define CPP_ASMLINKAGE extern "C"
        #else
        #define CPP_ASMLINKAGE
        #endif

#ifndef asmlinkage//如果以前没有定义asmlinkage
        #define asmlinkage CPP_ASMLINKAGE
        #endif

对于ARM处理器的<asm/linkage.h>，没有定义asmlinkage，所以没有意义（不要以为参数是从堆栈传递的，对于ARM平台来说还是符合ATPCS过程调用标准，通过寄存器传递的）。

但对于X86处理器的<asm/linkage.h>中是这样定义的：

#define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))

表示函数的参数传递是通过堆栈完成的。

3.7 描述3.3节中的ret_to_user 中断返回过程，/arch/arm/kernel/entry-common.S

ENTRY(ret_to_user)
        ret_slow_syscall:
              disable_irq @ disable interrupts
              ldr r1, [tsk, #TI_FLAGS]
              tst r1, #_TIF_WORK_MASK
              bne work_pending
        no_work_pending:
              /* perform architecture specific actions before user return */
              arch_ret_to_user r1, lr

      @ slow_restore_user_regs
              ldr r1, [sp, #S_PSR] @ get calling cpsr
              ldr lr, [sp, #S_PC]! @ get pc
              msr spsr_cxsf, r1 @ save in spsr_svc
              ldmdb sp, {r0 - lr}^ @ get calling r0 - lr
              mov r0, r0
              add sp, sp, #S_FRAME_SIZE - S_PC
              movs pc, lr @ return & move spsr_svc into cpsr

第三章主要跟踪了从中断发生到调用到对应中断号的desc->handle_irq(irq, desc)中断函数的过程。后面的章节还会继续讲解后面的内容。