Linux中斷實現方法(二):中斷處理過程

 

Linux中斷實現方法(二):中斷處理過程

三、中斷處理過程
這一節將以S3C2410為例，描述linux-2.6.26核心中，從中斷開始，中斷是如何一步一步執行到我們註冊函數的。
3.1 中斷向量表 archarmkernelentry-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:

中斷髮生後，跳轉到b vector_irq + stubs_offset的位置執行。注意現在的向量表的初始位置是0xffff0000。

3.2 中斷跳轉的入口位置 archarmkernelentry-armv.S
      .globl   __stubs_start
    __stubs_start:
     /*
     * Interrupt dispatcher
     */
             vector_stub    irq, IRQ_MODE, 4 @IRQ_MODE在includeasmptrace.h中定義：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

上面代碼中vector_stub宏的定義為：
.macro vector_stub, name, mode, correction=0
             .align 5
        vector_ ame:
             .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 @為後面進入svc模式做準備
@
             @ the branch table must immediately follow this code
              @
             and lr, lr, #0x0f @進入中斷前的mode的後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] @如果進入中斷前是usr，則取出PC+4*0的內容，即__irq_usr @如果進入中斷前是svc，則取出PC+4*3的內容，即__irq_svc
             movs pc, lr @ 當指令的目標寄存器是PC，且指令以S結束，則它會把@ spsr的值恢複給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”，找到了我們中斷處理的入口位置為vector_irq（宏裡面的vector_ ame）。
從上面代碼中的注釋可以看出，根據進入中斷前的工作模式不同，程式下一步將跳轉到_irq_usr 、或__irq_svc等位置。我們先選擇__irq_usr作為下一步跟蹤的目標。

3.3 __irq_usr的實現 archarmkernelentry-armv.S
__irq_usr:
             usr_entry @後面有解釋
             kuser_cmpxchg_check
       #ifdef CONFIG_TRACE_IRQFLAGS
             bl trace_hardirqs_off
       #endif
             get_thread_info tsk @擷取當前進程的進程描述符中的成員變數thread_info的地址，並將該地址儲存到寄存器tsk等於r9（在entry-header.S中定義）
       #ifdef CONFIG_PREEMPT//如果定義了搶佔，增加搶佔數值
             ldr r8, [tsk, #TI_PREEMPT] @ get preempt count
             add r7, r8, #1 @ increment it
             str r7, [tsk, #TI_PREEMPT]
       #endif

irq_handler @中斷處理，我們最關心的地方，3.4節有實現過程。
      #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 @中斷處理完成，返回中斷產生的位置，3.7節有實現過程

上面代碼中的usr_entry是一個宏，主要實現了將usr模式下的寄存器、中斷返回地址儲存到堆棧中。

.macro usr_entry
       sub sp, sp, #S_FRAME_SIZE @ S_FRAME_SIZE的值在archarmkernelasm-offsets.c
       @ 中定義 DEFINE(S_FRAME_SIZE, sizeof(struct pt_regs));實際上等於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

上面的這段代碼主要在填充結構體pt_regs ，這裡提到的struct pt_regs，在include/asm/ptrace.h中定義。此時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的實現過程，archarmkernelentry-armv.S

  .macro irq_handler
              get_irqnr_preamble r5, lr
              @在include/asm/arch-s3c2410/entry-macro.s中定義了宏get_irqnr_preamble為空白操作，什麼都不做
              1: get_irqnr_and_base r0, r6, r5, lr @判斷中斷號，通過R0返回，3.5節有實現過程
              movne r1, sp
              @
              @ routine called with r0 = irq number, r1 = struct pt_regs *
              @
              adrne lr, 1b
              bne asm_do_IRQ @進入中斷處理。
       ……
              .endm

3.5 get_irqnr_and_base中斷號判斷過程，include/asm/arch-s3c2410/entry-macro.s

.macro get_irqnr_and_base, irqnr, irqstat, base, tmp
              mov ase, #S3C24XX_VA_IRQ
              @@ try the interrupt offset register, since it is there
              ldr irqstat, [ ase, #INTPND ]
              teq irqstat, #0
              beq 1002f
              ldr irqnr, [ ase, #INTOFFSET ] @通過判斷INTOFFSET寄存器得到中斷位置
              mov mp, #1
              tst irqstat, mp, 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 mp, 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 @加上中斷號的基準數值，得到最終的中斷號，注意：此時沒有考慮子中斷的具體情況，（子中斷的問題後面會有講解）。IRQ_EINT0在include/asm/arch-s3c2410/irqs.h中定義.從這裡可以看出，中斷號的具體值是有平台相關的代碼決定的，和硬體中斷掛起寄存器中的中斷號是不等的。

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

3.6 asm_do_IRQ實現過程，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;//根據中斷號找到對應的irq_desc
              /*
              * Some hardware gives randomly wrong interrupts. Rather
              * than crashing, do something sensible.
              */
              if (irq >= NR_IRQS)
              desc = &bad_irq_desc;
              irq_enter();//沒做什麼特別的工作，可以跳過不看
              desc_handle_irq(irq, desc);// 根據中斷號和desc進入中斷處理
              /* 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);//中斷處理
          }

上述asmlinkage void __exception asm_do_IRQ(unsigned int irq, struct pt_regs *regs)使用了asmlinkage標識。那麼這個標識的含義如何理解呢？
該符號定義在kernel/include/linux/linkage.h中，如下所示：

#include <asm/linkage.h>//各個具體處理器在此檔案中定義asmlinkage
        #ifdef __cplusplus
        #define CPP_ASMLINKAGE extern "C"
        #else
        #define CPP_ASMLINKAGE
        #endif

#ifndef asmlinkage//如果以前沒有定義asmlinkage
        #define asmlinkage CPP_ASMLINKAGE
        #endif

對於ARM處理器的<asm/linkage.h>，沒有定義asmlinkage，所以沒有意義（不要以為參數是從堆棧傳遞的，對於ARM平台來說還是符合ATPCS程序呼叫標準，通過寄存器傳遞的）。
但對於X86處理器的<asm/linkage.h>中是這樣定義的：
#define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
表示函數的參數傳遞是通過堆棧完成的。

3.7 描述3.3節中的ret_to_user 中斷返回過程，/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

第三章主要跟蹤了從中斷髮生到調用到對應中斷號的desc->handle_irq(irq, desc)中斷函數的過程。後面的章節還會繼續講解後面的內容。

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