Char Device Driver
Major and Minor Numbers
Character device的access是透過file system上的name（special file、device file or node）。Device number是由major+minor所構成，可參考以下範例。
ls -l /dev/
total 0
major minor
crw------- 1 root wheel 17, 1 4 28 17:18 afsc_type5
crw------- 1 root wheel 10, 0 4 28 10:02 auditpipe
crw-r--r-- 1 root wheel 9, 3 4 28 10:02 auditsessions
crw------- 1 root wheel 21, 0 4 28 10:02 autofs
crw------- 1 root wheel 34, 0 4 28 10:02 autofs_control
crw-rw-rw- 1 root wheel 33, 0 4 28 10:02 autofs_homedirmounter
crw-rw-rw- 1 root wheel 31, 0 4 28 10:02 autofs_notrigger
crw-rw-rw- 1 root wheel 22, 6 4 28 10:02 autofs_nowait
Major在傳統上是表示device所配合的driver。現在kernel允許多個driver共用一個major number，不過大部分還是會諄造船同上的一對一的編號。然而minor number是讓kernel用來表示哪個device的referred to。
The Internal Representation of Device Numbers
Device number在kernel是以dev_t來表示，但我們不應該假設device number的tpye，因此應該要借用linux提供的macro，MAJOR()跟MINOR()，透過macro可避免未來kernel變動時造成移植上的問題。
若有需要將minor跟mijor合併，請使用MKDEV()這個macro。
#include <linux/types.h>
// major minor
// |--12-bit--|---20-bit---|
#include <linux/kdev_t.h>
MAJOR(dev_t dev);
MINOR(dev_t dev);
MKDEV(int major, int minor);
#include <linux/types.h>
// major minor
// |--12-bit--|---20-bit---|
#include <linux/kdev_t.h>
MAJOR(dev_t dev);
MINOR(dev_t dev);
MKDEV(int major, int minor);
Allocating and Freeing Device Numbers
Device driver第一件事是取得一個或是多個device number。使用register_chrdev_region這個function。
可在/proc/devices/跟sysfs下出現。
不過kernel開發上還是建議使用動態配置，這樣可以減少device跟number之間的固定關係。可改用alloc_chrdev_region這個function。最後不使用driver時，透過unregister_chrdev_region釋放device number。通常是在unload fuction裡面。
#include <linux/fs.h>
int register_chrdev_region(dev_t first, unsigned int count,
char *name);
first是你所想要配置device number的region的起始位子。count是想要申請的連續編號總數。name則是你最終會獲取的device name。
int alloc_chrdev_region(dev_t *dev, unsigned int firstminor,
unsigned int count, char *name);
dev是當配置成功時，dev會持有region的第一個device number。firstminor是想要申請的第一個minor number(通常是0)。count是想要申請的連續編號總數。name則是你最終會獲取的device name。
void unregister_chrdev_region(dev_t first, unsigned int count);
Dynamic Allocation of Major Numbers
目前常見的device都已經被記錄在Documentation/devices.txt當中。因此建議使用alloc_chrdev_region來分配mijor nember。但有一缺點是，無法事先建立/dev/node，這是因為動態配置無法保證每次的number都相同。
在翻譯版本中提到，使用devfs跟udev可以解決。目前udev是devfs的user-space的解決方法。是使用kernel發出hotplug出法user-space的process，此process會從sysfs取得新的device的資訊，並動態建立device node。最新狀況是udev已經被整理到systemd當中，有興趣的可以參考以下連結：http://man7.org/linux/man-pages/man7/udev.7.html。
旁邊可以看到scull是如何決定mijor number。
if (scull_major) {
dev = MKDEV(scull_major, scull_minor);
result = register_chrdev_region(dev, scull_nr_devs, "scull");
} else {
result = alloc_chrdev_region(&dev, scull_minor, scull_nr_devs, "scull");
scull_major = MAJOR(dev);
}
if (result < 0) {
printk(KERN_WARNING "scull: can't get major %d\n", scull_major);
return result;
}
The Design of char device
首先，在我們需要定義driver對於user-space' program提供哪些capabilities（mechanism）。記住，device其實只是一塊memory。
這邊提出scull這個device driver temperate，使用抽象及封裝的技巧來方便開發。包含以下類型：
scull#
- 使用memory所構成
- Global：在被開啟多次的情況下，fd(file descriptor)可共享device所擁有的data。
- Persistent ：就算closed或reopen，data也不會流
- 可使用cp、cat、I/O redirection操作
scullpipe#
- 使用FIFO的device
- 類似pipe。一個process寫入，可由另一個process讀取
- 這邊不使用hardware inperrutp來實作blocking/nonblocking的存取
scullsingle
- 一次只能有一個process執行open()
scullpriv
- 每個virtual console都可擁有私人的scullpriv
- 各個virtual console都會得到不同的memory area
scullid
- 若device已被佔用，試圖執行open則會發出Device Busy
scullwuid
- 若device已被佔用，試圖執行open則會blocking到device被釋放為止
這些driver在mechanism上都完全一樣(abstract)，只不過使用不同的policy。
Some Important Data Structure
Device主要會涉及三種data structure:
file
struct file { union { struct llist_node fu_llist; struct rcu_head fu_rcuhead; } f_u; struct path f_path; struct inode *f_inode; /* cached value */ const struct file_operations *f_op; /* * Protects f_ep_links, f_flags. * Must not be taken from IRQ context. */ spinlock_t f_lock; atomic_long_t f_count; unsigned int f_flags; fmode_t f_mode; struct mutex f_pos_lock; loff_t f_pos; struct fown_struct f_owner; const struct cred *f_cred; struct file_ra_state f_ra; u64 f_version; #ifdef CONFIG_SECURITY void *f_security; #endif /* needed for tty driver, and maybe others */ void *private_data; #ifdef CONFIG_EPOLL /* Used by fs/eventpoll.c to link all the hooks to this file */ struct list_head f_ep_links; struct list_head f_tfile_llink; #endif /* #ifdef CONFIG_EPOLL */ struct address_space *f_mapping; } __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */- file_operations
struct file_operations { struct module *owner; loff_t (*llseek) (struct file *, loff_t, int); ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); int (*iterate) (struct file *, struct dir_context *); int (*iterate_shared) (struct file *, struct dir_context *); unsigned int (*poll) (struct file *, struct poll_table_struct *); long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); long (*compat_ioctl) (struct file *, unsigned int, unsigned long); int (*mmap) (struct file *, struct vm_area_struct *); int (*open) (struct inode *, struct file *); int (*flush) (struct file *, fl_owner_t id); int (*release) (struct inode *, struct file *); int (*fsync) (struct file *, loff_t, loff_t, int datasync); int (*fasync) (int, struct file *, int); int (*lock) (struct file *, int, struct file_lock *); ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); int (*check_flags)(int); int (*flock) (struct file *, int, struct file_lock *); ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); int (*setlease)(struct file *, long, struct file_lock **, void **); long (*fallocate)(struct file *file, int mode, loff_t offset, loff_t len); void (*show_fdinfo)(struct seq_file *m, struct file *f); #ifndef CONFIG_MMU unsigned (*mmap_capabilities)(struct file *); #endif ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int); int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t, u64); ssize_t (*dedupe_file_range)(struct file *, u64, u64, struct file *, u64); }; inode
struct inode { umode_t i_mode; unsigned short i_opflags; kuid_t i_uid; kgid_t i_gid; unsigned int i_flags; #ifdef CONFIG_FS_POSIX_ACL struct posix_acl *i_acl; struct posix_acl *i_default_acl; #endif const struct inode_operations *i_op; struct super_block *i_sb; struct address_space *i_mapping; #ifdef CONFIG_SECURITY void *i_security; #endif /* Stat data, not accessed from path walking */ unsigned long i_ino; /* * Filesystems may only read i_nlink directly. They shall use the * following functions for modification: * * (set|clear|inc|drop)_nlink * inode_(inc|dec)_link_count */ union { const unsigned int i_nlink; unsigned int __i_nlink; }; dev_t i_rdev; loff_t i_size; struct timespec i_atime; struct timespec i_mtime; struct timespec i_ctime; spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */ unsigned short i_bytes; unsigned int i_blkbits; blkcnt_t i_blocks; #ifdef __NEED_I_SIZE_ORDERED seqcount_t i_size_seqcount; #endif /* Misc */ unsigned long i_state; struct rw_semaphore i_rwsem; unsigned long dirtied_when; /* jiffies of first dirtying */ unsigned long dirtied_time_when; struct hlist_node i_hash; struct list_head i_io_list; /* backing dev IO list */ #ifdef CONFIG_CGROUP_WRITEBACK struct bdi_writeback *i_wb; /* the associated cgroup wb */ /* foreign inode detection, see wbc_detach_inode() */ int i_wb_frn_winner; u16 i_wb_frn_avg_time; u16 i_wb_frn_history; #endif struct list_head i_lru; /* inode LRU list */ struct list_head i_sb_list; struct list_head i_wb_list; /* backing dev writeback list */ union { struct hlist_head i_dentry; struct rcu_head i_rcu; }; u64 i_version; atomic_t i_count; atomic_t i_dio_count; atomic_t i_writecount; #ifdef CONFIG_IMA atomic_t i_readcount; /* struct files open RO */ #endif const struct file_operations *i_fop; /* former ->i_op->default_file_ops */ struct file_lock_context *i_flctx; struct address_space i_data; struct list_head i_devices; union { struct pipe_inode_info *i_pipe; struct block_device *i_bdev; struct cdev *i_cdev; char *i_link; unsigned i_dir_seq; }; __u32 i_generation; #ifdef CONFIG_FSNOTIFY __u32 i_fsnotify_mask; /* all events this inode cares about */ struct hlist_head i_fsnotify_marks; #endif #if IS_ENABLED(CONFIG_FS_ENCRYPTION) struct fscrypt_info *i_crypt_info; #endif void *i_private; /* fs or device private pointer */ };
scull divice driver5只實作最重要的methods，file_operations初始化如下:
struct file_operations scull_fops = {
.owner = THIS_MODULE,
.llseek = scull_llseek,
.read = scull_read,
.write = scull_write,
.unlocked_ioctl = scull_ioctl,
.open = scull_open,
.release = scull_release,
};
Char Device Registration
Kernel主要是使用struct cdev代表char device。若要讓kernel操作device1，就必須註冊一個以上的cdev。並使用cdev_alloc()配置，並將cdev->ops指向f_ops。
#include <linux/cdev.h>
struct cdev *my_cdev=cdev_alloc();
my_cdev->ops=&my_fops;
Scull需要將cdev放入設計的特殊data structure，因此在這種特殊情況，需要使用cdev_init()來得到cdev。
#include <linux/cdev.h>
void cdev_init(struct cdev *cdev, struct file_operations *fops);
最後一個步驟是使用cdev_add()將cdev加入kernel。
#include <linux/cdev.h>
int cdev_add(struct cdev *dev, dev_t num, unsigned int count)
dev是設定好的cdev structure，num是第一個device number，count是device number的總數。