了解一下Linux系统的节拍定时器吧（system tick timer）

thinkfunny · 发表于 2020-6-12 11:05

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内核版本2.6.30

CPU：S3C2440

本文主要说明节拍定时器是如何添加到内核中的，不对定时器实现进行说明。

ARM 平台的节拍定时器对象使用sys_timer来表示，该结构体位于arch/arm/include/asm/mach/time.h。

/*
* This is our kernel timer structure.
*
* - init
* Initialise the kernels jiffy timer source, claim interrupt
* using setup_irq.  This is called early on during initialisation
* while interrupts are still disabled on the local CPU.
* - suspend
* Suspend the kernel jiffy timer source, if necessary.  This
* is called with interrupts disabled, after all normal devices
* have been suspended.  If no action is required, set this to
* NULL.
* - resume
* Resume the kernel jiffy timer source, if necessary.  This
* is called with interrupts disabled before any normal devices
* are resumed.  If no action is required, set this to NULL.
* - offset
* Return the timer offset in microseconds since the last timer
* interrupt.  Note: this must take account of any unprocessed
* timer interrupt which may be pending.
*/
struct sys_timer {
      struct sys_device       dev;
      void                      (*init)(void);
      void                      (*suspend)(void);
      void                      (*resume)(void);
#ifndef CONFIG_GENERIC_TIME
      unsigned long             (*offset)(void);
#endif
};

S3C2440的sys_timer的定义在linux/arch/arm/plat-s3c24xx/time.c中，给出的三个方法也在该文件中定义。
struct sys_timer s3c24xx_timer = {
.init       = s3c2410_timer_init,
.offset       = s3c2410_gettimeoffset,
.resume       = s3c2410_timer_setup
};

如何将该定时器告知内核呢？
在linux/arch/arm/mach-s3c2440/mach-smdk2440.c的最后使用了宏

MACHINE_START(S3C2440, "SMDK2440")
      /* Maintainer: Ben Dooks <ben@fluff.org> */
      .phys_io       = S3C2410_PA_UART,
      .io_pg_offst       = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
      .boot_params       = S3C2410_SDRAM_PA + 0x100,

      .init_irq       = s3c24xx_init_irq,
      .map_io             = smdk2440_map_io,
      .init_machine       = smdk2440_machine_init,
      .timer             = &s3c24xx_timer,
MACHINE_END

该宏用于初始化结构体struct machine_desc，结构体成员如下：
struct machine_desc {
      /*
      * Note! The first four elements are used
      * by assembler code in head.S, head-common.S
      */
      unsigned int             nr;             /* architecture number       */
      unsigned int             phys_io;       /* start of physical io       */
      unsigned int             io_pg_offst;       /* byte offset for io
                                             * page tabe entry       */

      const char             *name;             /* architecture name       */
      unsigned long             boot_params;       /* tagged list             */

      unsigned int             video_start;       /* start of video RAM       */
      unsigned int             video_end;       /* end of video RAM       */

      unsigned int             reserve_lp0 :1;       /* never has lp0       */
      unsigned int             reserve_lp1 :1;       /* never has lp1       */
      unsigned int             reserve_lp2 :1;       /* never has lp2       */
      unsigned int             soft_reboot :1;       /* soft reboot             */
      void                      (*fixup)(struct machine_desc *,
                                       struct tag *, char **,
                                       struct meminfo *);
      void                      (*map_io)(void);/* IO mapping function       */
      void                      (*init_irq)(void);
      struct sys_timer       *timer;             /* system tick timer       */
      void                      (*init_machine)(void);
};

在这里我们看到了timer指针。通过该宏将timer指针指向了s3c24xx_timer。
接着，再看linux/arch/arm/kernel/setup.c中的setup_arch函数，该函数中执行了下面这话：

system_timer = mdesc->timer;

mdesc->timer即为上面宏所给出的s3c24xx_timer，而system_timer为全局变量。通过该全局变量就可使用节拍定时器。

最后，我们看下节拍定时器的init方法何时被调用，调用顺序如下：

start_kernel ()-->  time_init()  --> system_timer->init()

time_init  -函数位于linux/arch/arm/kernel/time.c。

younicp · 发表于 2020-6-12 13:13

节拍定时器

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了解一下Linux系统的节拍定时器吧（system tick timer）

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