Linux kernel 笔记 （34）——模块参数

module_param和module_param_named定义在<linux/moduleparam.h>文件：

/**
 * module_param - typesafe helper for a module/cmdline parameter
 * @value: the variable to alter, and exposed parameter name.
 * @type: the type of the parameter
 * @perm: visibility in sysfs.
 *
 * @value becomes the module parameter, or (prefixed by KBUILD_MODNAME and a
 * ".") the kernel commandline parameter.  Note that - is changed to _, so
 * the user can use "foo-bar=1" even for variable "foo_bar".
 *
 * @perm is 0 if the the variable is not to appear in sysfs, or 0444
 * for world-readable, 0644 for root-writable, etc.  Note that if it
 * is writable, you may need to use kparam_block_sysfs_write() around
 * accesses (esp. charp, which can be kfreed when it changes).
 *
 * The @type is simply pasted to refer to a param_ops_##type and a
 * param_check_##type: for convenience many standard types are provided but
 * you can create your own by defining those variables.
 *
 * Standard types are:
 *  byte, short, ushort, int, uint, long, ulong
 *  charp: a character pointer
 *  bool: a bool, values 0/1, y/n, Y/N.
 *  invbool: the above, only sense-reversed (N = true).
 */
#define module_param(name, type, perm)              \
    module_param_named(name, name, type, perm)

/**
 * module_param_named - typesafe helper for a renamed module/cmdline parameter
 * @name: a valid C identifier which is the parameter name.
 * @value: the actual lvalue to alter.
 * @type: the type of the parameter
 * @perm: visibility in sysfs.
 *
 * Usually it's a good idea to have variable names and user-exposed names the
 * same, but that's harder if the variable must be non-static or is inside a
 * structure.  This allows exposure under a different name.
 */
#define module_param_named(name, value, type, perm)            \
    param_check_##type(name, &(value));                \
    module_param_cb(name, &param_ops_##type, &value, perm);        \
    __MODULE_PARM_TYPE(name, #type)

module_param用来定义一个模块参数，type指定类型（int，bool等等），perm指定用户访问权限，取值如下（<linux/stat.h>）：

#define S_IRWXU 00700
#define S_IRUSR 00400
#define S_IWUSR 00200
#define S_IXUSR 00100

#define S_IRWXG 00070
#define S_IRGRP 00040
#define S_IWGRP 00020
#define S_IXGRP 00010

#define S_IRWXO 00007
#define S_IROTH 00004
#define S_IWOTH 00002
#define S_IXOTH 00001

#define S_IRWXUGO   (S_IRWXU|S_IRWXG|S_IRWXO)
#define S_IALLUGO   (S_ISUID|S_ISGID|S_ISVTX|S_IRWXUGO)
#define S_IRUGO     (S_IRUSR|S_IRGRP|S_IROTH)
#define S_IWUGO     (S_IWUSR|S_IWGRP|S_IWOTH)
#define S_IXUGO     (S_IXUSR|S_IXGRP|S_IXOTH)

module_param_named则是为变量取一个可读性更好的名字。

以ktap源码为例：

int kp_max_loop_count = 100000;
module_param_named(max_loop_count, kp_max_loop_count, int, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(max_loop_count, "max loop execution count");

加载ktapvm模块，读取kp_max_loop_count的值：

[root@Linux ~]# cat /sys/module/ktapvm/parameters/max_loop_count
100000
[root@Linux ~]# ls -lt /sys/module/ktapvm/parameters/max_loop_count
-rw-r--r--. 1 root root 4096 Oct 22 22:51 /sys/module/ktapvm/parameters/max_loop_count

可以看到kp_max_loop_count变量在/sys/module/ktapvm/parameters文件夹下的名字是max_loop_count，值是100000，只有root用户拥有写权限。可以通过修改这个文件达到改变kp_max_loop_count变量的目的：

[root@Linux ~]# echo 200000 > /sys/module/ktapvm/parameters/max_loop_count
[root@Linux ~]# cat /sys/module/ktapvm/parameters/max_loop_count
200000

MODULE_PARM_DESC用来定义参数的描述信息，使用modinfo命令可以查看：

[root@Linux ~]# modinfo ktapvm.ko
.....
parm:           max_loop_count:max loop execution count (int)