hotplug/uevent机制分析

字符设备驱动中可以实现自动创建设备节点也就是以下函数

class_device_create()，为了让mdev(udev的简化版本)根据这些信息来创建设备节点，分析自动创建设备节点的过程，class_device_create函数最终会调用到kobject_uevent_env函数

struct class_device *class_device_create(struct class *cls,struct class_device *parent,dev_t devt,struct device *device,const char *fmt, ...)
{
...va_end(args);retval = class_device_register(class_dev);if (retval)goto error;return class_dev;error:kfree(class_dev);return ERR_PTR(retval);
}...
int class_device_register(struct class_device *class_dev)
{class_device_initialize(class_dev);return class_device_add(class_dev);
}...int class_device_add(struct class_device *class_dev)
{struct class *parent_class = NULL;struct class_device *parent_class_dev = NULL;struct class_interface *class_intf;int error = -EINVAL;...kobject_uevent(&class_dev->kobj, KOBJ_ADD);/* notify any interfaces this device is now here */down(&parent_class->sem);list_add_tail(&class_dev->node, &parent_class->children);list_for_each_entry(class_intf, &parent_class->interfaces, node) {if (class_intf->add)class_intf->add(class_dev, class_intf);}up(&parent_class->sem);...return error;
}...
int kobject_uevent(struct kobject *kobj, enum kobject_action action)
{return kobject_uevent_env(kobj, action, NULL);
}...int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,char *envp_ext[])
{char **envp;char *buffer;char *scratch;const char *action_string;const char *devpath = NULL;const char *subsystem;struct kobject *top_kobj;struct kset *kset;struct kset_uevent_ops *uevent_ops;u64 seq;char *seq_buff;int i = 0;int retval = 0;int j;pr_debug("%s\n", __FUNCTION__);action_string = action_to_string(action);if (!action_string) {pr_debug("kobject attempted to send uevent without action_string!\n");return -EINVAL;}.../* environment values */buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);if (!buffer) {retval = -ENOMEM;goto exit;}/* complete object path */devpath = kobject_get_path(kobj, GFP_KERNEL);if (!devpath) {retval = -ENOENT;goto exit;}/* event environemnt for helper process only */envp[i++] = "HOME=/";envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";/* default keys */scratch = buffer;envp [i++] = scratch;scratch += sprintf(scratch, "ACTION=%s", action_string) + 1;envp [i++] = scratch;scratch += sprintf (scratch, "DEVPATH=%s", devpath) + 1;envp [i++] = scratch;scratch += sprintf(scratch, "SUBSYSTEM=%s", subsystem) + 1;
.../* call uevent_helper, usually only enabled during early boot */if (uevent_helper[0]) {char *argv [3];argv [0] = uevent_helper;argv [1] = (char *)subsystem;argv [2] = NULL;call_usermodehelper (argv[0], argv, envp, 0);}exit:kfree(devpath);kfree(buffer);kfree(envp);return retval;
}

分析kobject_uevent_env函数，其中action_string为action_to_string返回值，而action传进的参数为KOBJ_ADD，因此action_string就等于add

static char *action_to_string(enum kobject_action action)
{switch (action) {case KOBJ_ADD:return "add";case KOBJ_REMOVE:return "remove";case KOBJ_CHANGE:return "change";case KOBJ_OFFLINE:return "offline";case KOBJ_ONLINE:return "online";case KOBJ_MOVE:return "move";default:return NULL;}
}

分配保存环境变量的内存，后面对环境变量进行设置，而环境变量是什么东西，在开发板中敲env命令，这些就是环境变量，而这些环境变了是sh程序的环境变量，每一个应用程序都有环境变量

buffer = kmalloc(BUFFER_SIZE, GFP_KERNEL);

# env
USER=root
HOME=/
TERM=vt102
PATH=/sbin:/usr/sbin:/bin:/usr/bin
SHELL=/bin/sh
PWD=/
#

在kobject_uevent_env中uevent_helper就是/sbin/mdev，证明uevent_helper就是/sbin/mdev，在其面前添加打印语句，对于call_usermodehelper，调用用户模式应用程序，创建一个进程，传入环境变量，应用程序就会根据这些环境变量来创建设备节点

     printk("xiaoma:uevent_helper = %s\n", uevent_helper);for(i = 0; i < envp[i]; i++){printk("envp[%d] = %s\n", i, envp[i]);}argv [0] = uevent_helper;  argv [1] = (char *)subsystem;argv [2] = NULL;call_usermodehelper (argv[0], argv, envp, 0);

编译新内核加载一个按键驱动程序

# insmod fifth_drv.ko
xiaoma :uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=add
envp[3] = DEVPATH=/module/fifth_drv
envp[4] = SUBSYSTEM=module
envp[5] = SEQNUM=711
xiaoma :uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=add
envp[3] = DEVPATH=/class/fifthdrv/buttons
envp[4] = SUBSYSTEM=fifthdrv
envp[5] = SEQNUM=712
envp[6] = MAJOR=252
envp[7] = MINOR=0

分析busybox的mdev.c，在mdev_main会判断-s选项，在开发板查看mdev信息，加-s选项是当系统启动的时候扫描sys目录(根据里面的信息创建各种设备节点)产生/dev目录，这是启动脚本中有加入mdev -s，而加载驱动应该跳过此选项，执行make_device函数，其path参数就是/sys/env_path，而env_path如上为/class/fifthdrv/buttons，对着相对应的文件，在make_device中执行strcat(path, "/dev")，查看/sys/class/fifthdrv/buttons/dev,存放这节点的信息，在后面device_name = bb_basename(path)确定设备文件名,类型,主次设备号，而type = path[5]=='c' ? S_IFCHR : S_IFBLK判断第五个字符是否为c，是的话则为字符设备驱动，接着执行fd = open("/etc/mdev.conf", O_RDONLY)如果有配置文件的话，会优先执行配置，最终调用到mknod(device_name, mode | type, makedev(major, minor)创建设备节点

# mdev
BusyBox v1.7.0 (2020-12-27 14:56:30 CST) multi-call binary

Usage: mdev [-s]

-s Scan /sys and populate /dev during system boot

Called with no options (via hotplug) it uses environment variables
to determine which device to add/remove.

# ls /sys/class/fifthdrv/buttons
dev subsystem uevent
# cat /sys/class/fifthdrv/buttons/dev
252:0

int mdev_main(int argc, char **argv)
{char *action;char *env_path;RESERVE_CONFIG_BUFFER(temp,PATH_MAX);xchdir("/dev");/* Scan */if (argc == 2 && !strcmp(argv[1],"-s")) {struct stat st;xstat("/", &st);root_major = major(st.st_dev);root_minor = minor(st.st_dev);recursive_action("/sys/block",ACTION_RECURSE | ACTION_FOLLOWLINKS,fileAction, dirAction, temp, 0);recursive_action("/sys/class",ACTION_RECURSE | ACTION_FOLLOWLINKS,fileAction, dirAction, temp, 0);/* Hotplug */} else {action = getenv("ACTION");env_path = getenv("DEVPATH");if (!action || !env_path)bb_show_usage();sprintf(temp, "/sys%s", env_path);if (!strcmp(action, "remove"))make_device(temp, 1);else if (!strcmp(action, "add")) {make_device(temp, 0);if (ENABLE_FEATURE_MDEV_LOAD_FIRMWARE)load_firmware(getenv("FIRMWARE"), temp);}}if (ENABLE_FEATURE_CLEAN_UP) RELEASE_CONFIG_BUFFER(temp);return 0;
}
..../* mknod in /dev based on a path like "/sys/block/hda/hda1" */
static void make_device(char *path, int delete)
{const char *device_name;int major, minor, type, len;int mode = 0660;uid_t uid = 0;gid_t gid = 0;char *temp = path + strlen(path);char *command = NULL;/* Try to read major/minor string.  Note that the kernel puts \n after* the data, so we don't need to worry about null terminating the string* because sscanf() will stop at the first nondigit, which \n is.  We* also depend on path having writeable space after it. */if (!delete) {strcat(path, "/dev");len = open_read_close(path, temp + 1, 64);*temp++ = 0;if (len < 1) return;}/* Determine device name, type, major and minor */device_name = bb_basename(path);type = path[5]=='c' ? S_IFCHR : S_IFBLK;/* If we have a config file, look up permissions for this device */if (ENABLE_FEATURE_MDEV_CONF) {char *conf, *pos, *end;int line, fd;/* mmap the config file */fd = open("/etc/mdev.conf", O_RDONLY);if (fd < 0)goto end_parse;len = xlseek(fd, 0, SEEK_END);conf = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);close(fd);if (!conf)goto end_parse;line = 0;/* Loop through lines in mmaped file*/for (pos=conf; pos-conf<len;) {int field;char *end2;line++;/* find end of this line */for (end=pos; end-conf<len && *end!='\n'; end++);/* Three fields: regex, uid:gid, mode */for (field=0; field < (3 + ENABLE_FEATURE_MDEV_EXEC);field++){/* Skip whitespace */while (pos<end && isspace(*pos)) pos++;if (pos==end || *pos=='#') break;for (end2=pos;end2<end && !isspace(*end2) && *end2!='#'; end2++);if (field == 0) {/* Regex to match this device */char *regex = xstrndup(pos, end2-pos);regex_t match;regmatch_t off;int result;/* Is this it? */xregcomp(&match,regex, REG_EXTENDED);result = regexec(&match, device_name, 1, &off, 0);regfree(&match);free(regex);/* If not this device, skip rest of line */if (result || off.rm_so|| off.rm_eo != strlen(device_name))break;}if (field == 1) {/* uid:gid */char *s, *s2;/* Find : */for (s=pos; s<end2 && *s!=':'; s++);if (s == end2) break;/* Parse UID */uid = strtoul(pos, &s2, 10);if (s != s2) {struct passwd *pass;char *_unam = xstrndup(pos, s-pos);pass = getpwnam(_unam);free(_unam);if (!pass) break;uid = pass->pw_uid;}s++;/* parse GID */gid = strtoul(s, &s2, 10);if (end2 != s2) {struct group *grp;char *_grnam = xstrndup(s, end2-s);grp = getgrnam(_grnam);free(_grnam);if (!grp) break;gid = grp->gr_gid;}}if (field == 2) {/* mode */mode = strtoul(pos, &pos, 8);if (pos != end2) break;}if (ENABLE_FEATURE_MDEV_EXEC && field == 3) {// Command to runconst char *s = "@$*";const char *s2;s2 = strchr(s, *pos++);if (!s2) {// Force errorfield = 1;break;}if ((s2-s+1) & (1<<delete))command = xstrndup(pos, end-pos);}pos = end2;}/* Did everything parse happily? */if (field > 2) break;if (field) bb_error_msg_and_die("bad line %d",line);/* Next line */pos = ++end;}munmap(conf, len);end_parse:  /* nothing */ ;}umask(0);if (!delete) {if (sscanf(temp, "%d:%d", &major, &minor) != 2) return;if (mknod(device_name, mode | type, makedev(major, minor)) && errno != EEXIST)bb_perror_msg_and_die("mknod %s", device_name);if (major == root_major && minor == root_minor)symlink(device_name, "root");if (ENABLE_FEATURE_MDEV_CONF) chown(device_name, uid, gid);}if (command) {/* setenv will leak memory, so use putenv */char *s = xasprintf("MDEV=%s", device_name);putenv(s);if (system(command) == -1)bb_perror_msg_and_die("cannot run %s", command);s[4] = '\0';unsetenv(s);free(s);free(command);}if (delete) unlink(device_name);
}

卸载的时候最终也会导致kobject_uevent_env调用call_usermodehelper这个应用程序被调用，以前是add，卸载的时候就变成了remove，mdev就会根据这些环境变量来删除设备节点

# rmmod fifth_drv
xiaoma :uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=remove
envp[3] = DEVPATH=/class/fifthdrv/buttons
envp[4] = SUBSYSTEM=fifthdrv
envp[5] = SEQNUM=713
envp[6] = MAJOR=252
envp[7] = MINOR=0
xiaoma :uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=remove
envp[3] = DEVPATH=/class/fifthdrv
envp[4] = SUBSYSTEM=class
envp[5] = SEQNUM=714
xiaoma :uevent_helper = /sbin/mdev
envp[0] = HOME=/
envp[1] = PATH=/sbin:/bin:/usr/sbin:/usr/bin
envp[2] = ACTION=remove
envp[3] = DEVPATH=/module/fifth_drv
envp[4] = SUBSYSTEM=module
envp[5] = SEQNUM=715

为什么uevent_helper就是/sbin/mdev，是因为调用应用程序: 比如mdev，启动脚本，在脚本中echo /sbin/mdev > /proc/sys/kernel/hotplug，设置了uevent_helper为"/sbin/mdev"

修改mdev配置支持U盘自动挂载

当我们插上U盘后，自动创建了设备节点，其中sda表示整个U盘，sda1表示第一个主分区，这样就可以手动来mount挂载，而我们想实现自动挂载

# usb 1-1: new full speed USB device using s3c2410-ohci and address 2
usb 1-1: configuration #1 chosen from 1 choice
scsi0 : SCSI emulation for USB Mass Storage devices

# scsi 0:0:0:0: Direct-Access SMI USB DISK 1100 PQ: 0 ANSI: 4
sd 0:0:0:0: [sda] 61440000 512-byte hardware sectors (31457 MB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Assuming drive cache: write through
sd 0:0:0:0: [sda] 61440000 512-byte hardware sectors (31457 MB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Assuming drive cache: write through
sda: sda1
sd 0:0:0:0: [sda] Attached SCSI removable disk

# ls -l /dev/sda*
brw-rw---- 1 0 0 8, 0 Mar 1 15:49 /dev/sda
brw-rw---- 1 0 0 8, 1 Mar 1 15:49 /dev/sda1

在分析uevent机制的时候，分析busybox的mdev.c中如果有配置文件的话，会优先执行配置，通过配置文件让mdev这个应用程序来执行我们的命令，在busybox中有个mdev.txt，告诉我们怎么用mdev

其中mdev.conf的格式如下，device regex：正则表达式，表示哪一个设备，uid：owner，gid主ID，octal permissions：以八进制表示的属性，@：创建设备节点之后执行命令，$：删除设备节点之前执行命令，*: 创建设备节点之后和删除设备节点之前执行命令，command：要执行的命令(例如写入自动挂载的命令)

<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]

其中表示默认属性为660，而我们可以修改其属性，mdev根据device regex判断哪一个设备，根据这个设备创建设备节点，再根据这个配置文件来创建属性

The file has the format:
   <device regex> <uid>:<gid> <octal permissions>
For example:
   hd[a-z][0-9]* 0:3 660

The config file parsing stops at the first matching line. If no line is
matched, then the default of 0:0 660 is used. To set your own default, simply
create your own total match like so:
   .* 1:1 777

而device regex正则表达式是什么意思,有一个30分钟入门教程，在windows下查看文件例如*.c，*表示通配符，任意字符，如果想更精确的话，就用正则表达式，举例一些简单的字符规则

.	表示任意字符(换行符除外)
*	重复0次或更多次
+	重复1次或更多次
？	重复0次或1次
[-]	中括号里一系列字符，表示这些字符里的某一个

例如，以下有一个led驱动程序，以次设备号分别创建，默认属性为660，写出mdev.conf来改变属性

# insmod myleds.ko
leds initialized
# ls -l /dev/led*
crw-rw---- 1 0 0 231, 1 Mar 1 16:25 /dev/led1
crw-rw---- 1 0 0 231, 2 Mar 1 16:25 /dev/led2
crw-rw---- 1 0 0 231, 3 Mar 1 16:25 /dev/led3
crw-rw---- 1 0 0 231, 0 Mar 1 16:25 /dev/leds

修改配置文件，重新加载驱动程序，查看驱动信息，属性为改为了777

# vi /etc/mdev.conf
leds 0:0 777
led1 0:0 777
led2 0:0 777
led3 0:0 777

# rmmod myleds
# insmod myleds.ko
leds initialized
# ls -l /dev/led*
crwxrwxrwx 1 0 0 231, 1 Mar 1 16:29 /dev/led1
crwxrwxrwx 1 0 0 231, 2 Mar 1 16:29 /dev/led2
crwxrwxrwx 1 0 0 231, 3 Mar 1 16:29 /dev/led3
crwxrwxrwx 1 0 0 231, 0 Mar 1 16:29 /dev/leds

这样写麻烦利用正则表达式进行修改，修改为leds?[123]?，?问号表示前面的字符重复0次或者1次，一样的效果

# vi /etc/mdev.conf
leds?[123]? 0:0 777
# rmmod myleds
# insmod myleds.ko
leds initialized
# ls -l /dev/led*
crwxrwxrwx 1 0 0 231, 1 Mar 1 16:29 /dev/led1
crwxrwxrwx 1 0 0 231, 2 Mar 1 16:29 /dev/led2
crwxrwxrwx 1 0 0 231, 3 Mar 1 16:29 /dev/led3
crwxrwxrwx 1 0 0 231, 0 Mar 1 16:29 /dev/leds

增加命令，打印增加某个设备，在文档中有说明用$MDEV环境变量来表示哪个设备节点

leds?[123]? 0:0 777 @ echo create /dev/$MDEV > /dev/console

For your convenience, the shell env var $MDEV is set to the device name. So if
the device 'hdc' was matched, MDEV would be set to "hdc".

# vi /etc/mdev.conf
leds?[123]? 0:0 777 @ echo create /dev/$MDEV > /dev/console

# rmmod myleds
# insmod myleds.ko
leds initialized
# create /dev/leds
create /dev/led1
create /dev/led2
create /dev/led3

用*符号来加载卸载前后都打印，而我们怎么分辩是加载还是卸载，以shell脚本命令利用环境变量来实现

# vi /etc/mdev.conf
leds?[123]? 0:0 777 * if [ $ACTION = "add" ]; then echo create /dev/$MDEV > /dev/console; else echo remove /dev/$MDEV > /dev/console; fi

# rmmod myleds
# remove /dev/led1
remove /dev/leds
remove /dev/led2
remove /dev/led3
remove /dev/leds
# insmod myleds.ko
leds initialized
# create /dev/leds
create /dev/led2
create /dev/led1
create /dev/led3

把命令写入一个脚本给脚本添加执行权限chmod +x /bin/add_remove_led.sh，一样的效果

leds?[123]? 0:0 777 * /bin/add_remove_led.sh

add_remove_led.sh：

#!/bin/sh
if [ $ACTION = "add" ];
then
echo create /dev/$MDEV > /dev/console;
else
echo remove /dev/$MDEV > /dev/console;
fi

因此实现U盘自动挂载就很容易了，也可以把命令写入一个脚本，过程如上，+表示重复1次或者更多次，至少1次

sda[1-9]+ 0:0 777 * if [ $ACTION = "add" ]; then mount /dev/$MDEV /mnt; else umount /mnt; fi

效果如下，插上U盘后自动挂载

# usb 1-1: USB disconnect, address 2
usb 1-1: new full speed USB device using s3c2410-ohci and address 3
usb 1-1: configuration #1 chosen from 1 choice
scsi1 : SCSI emulation for USB Mass Storage devices
scsi 1:0:0:0: Direct-Access SMI USB DISK 1100 PQ: 0 ANSI: 4
sd 1:0:0:0: [sda] 61440000 512-byte hardware sectors (31457 MB)
sd 1:0:0:0: [sda] Write Protect is off
sd 1:0:0:0: [sda] Assuming drive cache: write through
sd 1:0:0:0: [sda] 61440000 512-byte hardware sectors (31457 MB)
sd 1:0:0:0: [sda] Write Protect is off
sd 1:0:0:0: [sda] Assuming drive cache: write through
sda: sda1
sd 1:0:0:0: [sda] Attached SCSI removable disk

# ls /dev/sda*
/dev/sda /dev/sda1
# ls /dev/sda* -l
brw-rw---- 1 0 0 8, 0 Mar 1 17:13 /dev/sda
brwxrwxrwx 1 0 0 8, 1 Mar 1 17:13 /dev/sda1
# cd /mnt/
# ls
1.txt autorun.inf
System Volume Information eaget.ico

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