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Date: 九月 25, 2015

Linux kernel 笔记 (21)——per-CPU变量

per-CPU变量顾名思义,即当你声明一个per-CPU变量时,当前系统上的每个CPU都会有一份当前变量的copy。使用per-CPU变量好处是访问它几乎不需要加锁,因为每个CPU都有一份copy。此外,CPU可以把这个变量放在自己的cache里,访问起来会特别快。定义per-CPU变量方法如下:

DEFINE_PER_CPU(type, name);  

如果per-CPU变量是数组,则定义方式如下:

DEFINE_PER_CPU(type[length], array); 

per-CPU变量可以导出,供其它模块使用:

EXPORT_PER_CPU_SYMBOL(per_cpu_var);
EXPORT_PER_CPU_SYMBOL_GPL(per_cpu_var);

要在其它模块使用per-CPU变量,则需要声明:

DECLARE_PER_CPU(type, name);

访问per-CPU变量可以使用get_cpu_var(var)set_cpu_var(var)这两个macro

/* <linux/percpu.h>*/

/*
 * Must be an lvalue. Since @var must be a simple identifier,
 * we force a syntax error here if it isn't.
 */
#define get_cpu_var(var) (*({               \
    preempt_disable();              \
    &__get_cpu_var(var); }))

/*
 * The weird & is necessary because sparse considers (void)(var) to be
 * a direct dereference of percpu variable (var).
 */
#define put_cpu_var(var) do {               \
    (void)&(var);                   \
    preempt_enable();               \
} while (0)

因为kernel线程是允许preemption的,所以在get_cpu_var中需要调用preempt_disable,并且要和put_cpu_var配对使用。

访问另一个CPUper-CPU变量:

per_cpu(variable, int cpu_id);

参考资料:
Driver porting: per-CPU variables;
Per-CPU Variables

 

Linux kernel 笔记 (20)——设备的major和minor号

/dev目录下执行ls -lt命令:

 

Capture 上面红框框起来的部分就是设备号,前面是major,后面是minor major号表示设备所使用的驱动,而minor号则表示具体的设备。在上图中,tty的驱动都是driver 4,而利用minor号区别不同的tty设备。 另外,通过/proc/devices文件也可以看到设备所使用的驱动,即major号:

linux-a21w:/dev # cat /proc/devices
Character devices:
  1 mem
  4 /dev/vc/0
  4 tty
  4 ttyS
  5 /dev/tty
  5 /dev/console
  5 /dev/ptmx
  7 vcs
......

关于dev_tmajorminor号定义如下(kernel版本是4.0):

/* <linux/types.h>: */
typedef __u32 __kernel_dev_t;
typedef __kernel_dev_t      dev_t;

/* <linux/kdev_t.h> */
#define MINORBITS   20
#define MINORMASK   ((1U << MINORBITS) - 1)

#define MAJOR(dev)  ((unsigned int) ((dev) >> MINORBITS))
#define MINOR(dev)  ((unsigned int) ((dev) & MINORMASK))
#define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))

dev_t32 bit长,其中高12位是major,低20位是minor

获取设备号的两种方法:

(1)预先指定设备号:

int register_chrdev_region(dev_t from, unsigned count, const char *name) 

from包含majorminor,通常情况下minor指定为0count指定连续设备号的数量,name指定设备的名字。register_chrdev_region实现如下:

/**
 * register_chrdev_region() - register a range of device numbers
 * @from: the first in the desired range of device numbers; must include
 *        the major number.
 * @count: the number of consecutive device numbers required
 * @name: the name of the device or driver.
 *
 * Return value is zero on success, a negative error code on failure.
 */
int register_chrdev_region(dev_t from, unsigned count, const char *name)
{
    struct char_device_struct *cd;
    dev_t to = from + count;
    dev_t n, next;

    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        if (next > to)
            next = to;
        cd = __register_chrdev_region(MAJOR(n), MINOR(n),
                   next - n, name);
        if (IS_ERR(cd))
            goto fail;
    }
    return 0;
fail:
    to = n;
    for (n = from; n < to; n = next) {
        next = MKDEV(MAJOR(n)+1, 0);
        kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
    }
    return PTR_ERR(cd);
}

可以看到register_chrdev_region即是把from开始连续count个设备号(dev_t类型,包含majorminor)都注册。

举个例子(/drivers/tty/tty_io.c):

register_chrdev_region(MKDEV(TTYAUX_MAJOR, 1), 1, "/dev/console")

(2)动态分配设备号(推荐使用):

int alloc_chrdev_region(dev_t *dev, unsigned int firstminor, unsigned int count, char *name);

dev是传出参数,为动态获得的设备号;firstminor指定第一个minorcountnameregister_chrdev_region的参数定义。alloc_chrdev_region实现如下:

/**
 * alloc_chrdev_region() - register a range of char device numbers
 * @dev: output parameter for first assigned number
 * @baseminor: first of the requested range of minor numbers
 * @count: the number of minor numbers required
 * @name: the name of the associated device or driver
 *
 * Allocates a range of char device numbers.  The major number will be
 * chosen dynamically, and returned (along with the first minor number)
 * in @dev.  Returns zero or a negative error code.
 */
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
            const char *name)
{
    struct char_device_struct *cd;
    cd = __register_chrdev_region(0, baseminor, count, name);
    if (IS_ERR(cd))
        return PTR_ERR(cd);
    *dev = MKDEV(cd->major, cd->baseminor);
    return 0;
}

举个例子(/drivers/watchdog/watchdog_dev.c):

alloc_chrdev_region(&watchdog_devt, 0, MAX_DOGS, "watchdog");

释放设备号:

void unregister_chrdev_region(dev_t first, unsigned int count);

 

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