Android架构分析之LOG模块

Android的LOG模块分为内核驱动部分和用户空间接口部分。

一、内核LOG模块分析

我们先来看内核驱动部分，其代码位于drivers/staging/android/logger.c文件中。按照分析Linux内核驱动程序的惯例，我们从模块初始化函数开始分析：

[cpp] 588static int __init logger_init(void)
589{
590    int ret;
591
592    ret =init_log(&log_main);
593    if (unlikely(ret))
594        goto out;
595
596    ret =init_log(&log_events);
597    if (unlikely(ret))
598        goto out;
599
600    ret =init_log(&log_radio);
601    if (unlikely(ret))
602        goto out;
603
604out:
605    return ret;
606}
607device_initcall(logger_init);

588static int __init logger_init(void)
589{
590    int ret;
591
592    ret =init_log(&log_main);
593    if (unlikely(ret))
594        goto out;
595
596    ret =init_log(&log_events);
597    if (unlikely(ret))
598        goto out;
599
600    ret =init_log(&log_radio);
601    if (unlikely(ret))
602        goto out;
603
604out:
605    return ret;
606}
607device_initcall(logger_init);
logger_init函数即是LOG模块初始化函数，其调用了3次init_log函数，注册了log_main，log_events，log_radio三个LOG设备，init_log函数定义如下：

[cpp] 571static int __init init_log(struct logger_log *log)
572{
573    int ret;
574
575    ret =misc_register(&log->misc);
576    if (unlikely(ret)) {
577        printk(KERN_ERR"logger: failed to register misc "
578               "device forlog '%s'!\n", log->misc.name);
579        return ret;
580    }
581
582    printk(KERN_INFO"logger: created %luK log '%s'\n",
583           (unsigned long)log->size >> 10, log->misc.name);
584
585    return 0;
586}

571static int __init init_log(struct logger_log *log)
572{
573    int ret;
574
575    ret =misc_register(&log->misc);
576    if (unlikely(ret)) {
577        printk(KERN_ERR"logger: failed to register misc "
578               "device forlog '%s'!\n", log->misc.name);
579        return ret;
580    }
581
582    printk(KERN_INFO"logger: created %luK log '%s'\n",
583           (unsigned long)log->size >> 10, log->misc.name);
584
585    return 0;
586}
575行，调用misc_register函数，注册misc设备。

init_log函数的参数是logger_log结构体类型，该类型代表一个LOG设备，其定义如下：

[cpp] 30/*
31 * struct logger_log -represents a specific log, such as 'main' or 'radio'
32 *
33 * This structure lives frommodule insertion until module removal, so it does
34 * not need additionalreference counting. The structure is protected by the
35 * mutex 'mutex'.
36 */
37struct logger_log {
38    unsigned char *     buffer; /* the ring buffer itself */
39    struct miscdevice   misc; /* misc device representing the log */
40    wait_queue_head_t   wq; /* wait queue for readers */
41    struct list_head    readers; /* this log's readers */
42    struct mutex        mutex; /* mutex protecting buffer */
43    size_t          w_off; /* current write head offset */
44    size_t          head;   /* new readers start here */
45    size_t          size;   /* size of the log */
46};

30/*
31 * struct logger_log -represents a specific log, such as 'main' or 'radio'
32 *
33 * This structure lives frommodule insertion until module removal, so it does
34 * not need additionalreference counting. The structure is protected by the
35 * mutex 'mutex'.
36 */
37struct logger_log {
38    unsigned char *     buffer; /* the ring buffer itself */
39    struct miscdevice   misc; /* misc device representing the log */
40    wait_queue_head_t   wq; /* wait queue for readers */
41    struct list_head    readers; /* this log's readers */
42    struct mutex        mutex; /* mutex protecting buffer */
43    size_t          w_off; /* current write head offset */
44    size_t          head;   /* new readers start here */
45    size_t          size;   /* size of the log */
46};
log_main，log_events，log_radio三个LOG设备都是logger_log结构体类型的变量，其定义如下：

[cpp] 533/*
534 * Defines a log structure with name 'NAME' and a size of 'SIZE'bytes, which
535 * must be a power of two, greater than LOGGER_ENTRY_MAX_LEN, andless than
536 * LONG_MAX minus LOGGER_ENTRY_MAX_LEN.
537 */
538#define DEFINE_LOGGER_DEVICE(VAR, NAME, SIZE)

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