Character drivers

@ 2021-21 Embitude Trainings <info@embitude.in>
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Character Driver

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What to Expect
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What is Character driver
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Major & Minor Number
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Registering the character driver
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Exchanging data with user space
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Udev & netlink socket
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Dynamic device file creation
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IOCTL

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What is character driver?
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The drivers which allow byte oriented
transaction.
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Character device special files
– ls -l /dev | grep ^c
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Two attributes for files
– Name (For application space)
– Number (For Kernel specific)

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Device file number
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Represented by type dev_t. Is split it into 2
parts
– MSB 12 bits define the category of driver
– LSB 20 bits defines the functionality
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Macros
– MAJOR(dev)
– MINOR(dev)
– MKDEV(major, minor)

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Character Driver Flow
VFS
/dev/abc
Applicaton
Driver
HW
Kernel
Space
User
Space
Hardware
Space
open()
open()

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Registering/Deregistering the driver
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Registration
– register_char_dev_region(dev_t dev, unsigned int
count, char *name)
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register_char_dev_region(MKDEV(250, 0), 3, “abc”);
– alloc_char_dev_region(dev_t *dev, unsigned int
first, unsigned int count, char *name)
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alloc_char_dev_region(&dev, 0, 3, “abc);
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Deregistration
– unregister_char_dev_region(dev_t dev, unsinged int
count);

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File operations
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Callback handlers to be invoked by VFS
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struct file_operations
– struct module_owner
– int (*open)(struct inode *, struct file *)
– int (*release)(struct inode , struct file *)
– int ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
– ssize_t (*write)(struct file *, const char __user *, size_t, loff_t
*);
– loff_t (*llseek)(struct file *, loff_t, int);
– int (*unlocked_ioctl)(struct file *, unsigned int, unsigned
long);

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Declare & initialize the struct file_operations
– struct file_operations fops
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Declare & initialize the variable of type struct cdev
– struct cdev c_dev
– cdev_init(&c_dev, &fops)
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The registration
– int cdev_add(struct dev *cdev, dev_t num, unsigned int
cound)
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The Unregistration
– Int cdev_del(struct cdev *cdev)
(Un)Registering the file operations

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Read & write callbacks
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ssize_t my_read(struct file *f, char __user *buf, size_t cnt, loff_t *off)
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{
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...
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return read_cnt;
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}
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ssize_t my_write(struct file *f, char __user *buf, size_t cnt, loff_t *off)
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{
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...
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return wrote_cnt;
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}

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Exchanging the Data with User space
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A single value
– get_user(v, p)
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The kernel variable v gets the value pointed by user space pointer p
– put_user(v, p)
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The value pointed by p is set to the contents of kernel variable v
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A buffer
– unsigned long copy_to_user(void __user *to, const void
*from, unsigned long n)
– Unsigned logn copy_from_user(void *to, const void __user
*from, unsigned long n)
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Returns 0 on success, non-zero on failure

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udev
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Udev (user space dev) – a device manager
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Runs in user space
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Automatically creates/removes device enteries
in dev according to inserted/removed drivers
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Listens on the netlink socket for uvents
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Kernel shares the Major/Minor numbers through
/sys interface

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udev files
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Udev configuration file
– /etc/udev/udev.conf
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Standard udev event matching rules
– /lib/udev/rules.d
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Custom udev event matching rules
– /etc/udev/rules.d/*.rules
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dev/*
– Device files creation

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Class create & destroy
– struct class *class_create(struct module *owner, char
*name);
– void class_destroy(struct class *cl);
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Device create & destroy
– struct class_device *device_create(struct class *cl, NULL,
dev_t devnum, NULL, const char *fmt, ...);
– void device_destroy(struct class *cl, dev_t devnum);

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IOCTL
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All the miscellenious IO operations
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long unlocked_ioctl(struct file *f, unsigned int cmd,
unsigned long arg)
– Associated to the ioctl system call
– Extends the driver capabilities beyond the limited read/
write API
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Changing the speed of the serial port, setting video format,
quering the device serial number
– cmd is the number identifying the operation to perform
– arg is the argument passed to the command. Can be an
integer, an address

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IOCTL ...
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Command is split as follows
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Macros
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_IO, _IOW, _IOR, _IOWR
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Parameters
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type (character) [15:8]
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number (index) [7:0]
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size (param type) [29:16]
size[29:16] type[15:8] num[7:0]
dir[31:30]

Character drivers

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