1. Advanced Char Driver Operations
Sarah Diesburg
CIS 4930

2. Resources LDD Chapter 3 LDD module source code for 3.2.x
Red font in slides where up-to-date code diverges from book
LDD module source code for 3.2.x

3. Resources LXR – Cross-referenced Linux Go to http://lxr.linux.no/
Click on Linux and later
Select your kernel version from drop-down menu

4. Topics Managing ioctl command numbers Block/unblocking a process
Seeking on a device
Access control

5. ioctl For operations beyond simple data transfers Alternatives
Eject the media
Report error information
Change hardware settings
Self destruct
Alternatives
Embedded commands in the data stream
Driver-specific file systems

6. ioctl User-level interface int ioctl(int fd, int request, ...); ...
Variable number of arguments
Problematic for the system call interface
In this context, it is meant to pass a single optional argument
Traditionally a char *argp
Just a way to bypass the type checking
For more information, look at man page

7. ioctl Driver-level interface Ioctl has changed from the LDD3 era
int (*unlocked_ioctl) (struct file *filp,
unsigned int cmd,
unsigned long arg);
cmd is passed from the user unchanged
arg can be an integer or a pointer
Compiler does not type check
Ioctl has changed from the LDD3 era
Modified to remove the big kernel lock (BKL)

8. Choosing the ioctl Commands
Need a numbering scheme to avoid mistakes
E.g., issuing a command to the wrong device (changing the baud rate of an audio device)
Check include/linux/ioctl.h and directory Documentation/ioctl/

9. Choosing the ioctl Commands
A command number uses four bitfields
Defined in <linux/ioctl.h>
< direction, type, number, size>
direction: direction of data transfer
_IOC_NONE
_IOC_READ
_IOC_WRITE
_IOC_READ | WRITE

10. Choosing the ioctl Commands
type (ioctl device type)
8-bit (_IOC_TYPEBITS) magic number
Associated with the device
number
8-bit (_IOC_NRBITS) sequential number
Unique within device
size: size of user data involved
The width is either 13 or 14 bits (_IOC_SIZEBITS)

11. Choosing the ioctl Commands
Useful macros to create ioctl command numbers
_IO(type, nr)
_IOR(type, nr, datatype)
_IOW(type, nr, datatype)
_IOWR(type, nr, datatype)
size = sizeof(datatype)

12. Choosing the ioctl Commands
Useful macros to decode ioctl command numbers
_IOC_DIR(nr)
_IOC_TYPE(nr)
_IOC_NR(nr)
_IOC_SIZE(nr)

13. Choosing the ioctl Commands
The scull example
/* Use 'k' as magic number */
#define SCULL_IOC_MAGIC 'k‘
/* Please use a different 8-bit number in your code */
#define SCULL_IOCRESET _IO(SCULL_IOC_MAGIC, 0)

14. Choosing the ioctl Commands
The scull example /*
* S means "Set" through a ptr,
* T means "Tell" directly with the argument value
* G means "Get": reply by setting through a pointer
* Q means "Query": response is on the return value
* X means "eXchange": switch G and S atomically
* H means "sHift": switch T and Q atomically */
#define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC, 1, int)
#define SCULL_IOCSQSET _IOW(SCULL_IOC_MAGIC, 2, int)
#define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC, 3)
#define SCULL_IOCTQSET _IO(SCULL_IOC_MAGIC, 4)
#define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC, 5, int)
Set new value and return the old value

15. Choosing the ioctl Commands
The scull example
#define SCULL_IOCGQSET _IOR(SCULL_IOC_MAGIC, 6, int)
#define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC, 7)
#define SCULL_IOCQQSET _IO(SCULL_IOC_MAGIC, 8)
#define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC, 9, int)
#define SCULL_IOCXQSET _IOWR(SCULL_IOC_MAGIC,10, int)
#define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC, 11)
#define SCULL_IOCHQSET _IO(SCULL_IOC_MAGIC, 12)
#define SCULL_IOC_MAXNR 14

16. The Return Value When the command number is not supported
Return –EINVAL
Or –ENOTTY (according to the POSIX standard)

17. The Predefined Commands
Handled by the kernel first
Will not be passed down to device drivers
Three groups
For any file (regular, device, FIFO, socket)
Magic number: “T.”
For regular files only
Specific to the file system type

18. Using the ioctl Argument
If it is an integer, just use it directly
If it is a pointer
Need to check for valid user address
int access_ok(int type, const void *addr,
unsigned long size);
type: either VERIFY_READ or VERIFY_WRITE
Returns 1 for success, 0 for failure
Driver then results –EFAULT to the caller
Defined in <linux/uaccess.h>
Mostly called by memory-access routines

19. Using the ioctl Argument
The scull example
int scull_ioctl(struct file *filp,
unsigned int cmd, unsigned long arg) {
int err = 0, tmp;
int retval = 0;
/* check the magic number and whether the command is defined */
if (_IOC_TYPE(cmd) != SCULL_IOC_MAGIC) {
return -ENOTTY; }
if (_IOC_NR(cmd) > SCULL_IOC_MAXNR) { …

20. Using the ioctl Argument
The scull example …
/* the concept of "read" and "write" is reversed here */
if (_IOC_DIR(cmd) & _IOC_READ) {
err = !access_ok(VERIFY_WRITE, (void __user *) arg,
_IOC_SIZE(cmd));
} else if (_IOC_DIR(cmd) & _IOC_WRITE) {
err = !access_ok(VERIFY_READ, (void __user *) arg, }
if (err) return -EFAULT;

21. Using the ioctl Argument
Data transfer functions optimized for most used data sizes (1, 2, 4, and 8 bytes)
If the size mismatches
Cryptic compiler error message:
Conversion to non-scalar type requested
Use copy_to_user and copy_from_user
#include <linux/uaccess.h>
put_user(datum, ptr)
Writes to a user-space address
Calls access_ok()
Returns 0 on success, -EFAULT on error

22. Using the ioctl Argument
__put_user(datum, ptr)
Does not check access_ok()
Can still fail if the user-space memory is not writable
get_user(local, ptr)
Reads from a user-space address
Calls access_ok()
Stores the retrieved value in local
Returns 0 on success, -EFAULT on error
__get_user(local, ptr)
Can still fail if the user-space memory is not readable

23. Capabilities and Restricted Operations
Limit certain ioctl operations to privileged users
See <linux/capability.h> for the full set of capabilities
To check a certain capability call
int capable(int capability);
In the scull example
if (!capable(CAP_SYS_ADMIN)) {
return –EPERM; }
A catch-all capability for many system administration operations

24. The Implementation of the ioctl Commands
A giant switch statement …
switch(cmd) {
case SCULL_IOCRESET:
scull_quantum = SCULL_QUANTUM;
scull_qset = SCULL_QSET;
break;
case SCULL_IOCSQUANTUM: /* Set: arg points to the value */
if (!capable(CAP_SYS_ADMIN)) {
return -EPERM; }
retval = __get_user(scull_quantum, (int __user *)arg);

25. The Implementation of the ioctl Commands…
case SCULL_IOCTQUANTUM: /* Tell: arg is the value */
if (!capable(CAP_SYS_ADMIN)) {
return -EPERM; }
scull_quantum = arg;
break;
case SCULL_IOCGQUANTUM: /* Get: arg is pointer to result */
retval = __put_user(scull_quantum, (int __user *) arg);
case SCULL_IOCQQUANTUM: /* Query: return it (> 0) */
return scull_quantum;

26. The Implementation of the ioctl Commands…
case SCULL_IOCXQUANTUM: /* eXchange: use arg as pointer */
if (!capable(CAP_SYS_ADMIN)) {
return -EPERM; }
tmp = scull_quantum;
retval = __get_user(scull_quantum, (int __user *) arg);
if (retval == 0) {
retval = __put_user(tmp, (int __user *) arg);
break;

27. The Implementation of the ioctl Commands…
case SCULL_IOCHQUANTUM: /* sHift: like Tell + Query */
if (!capable(CAP_SYS_ADMIN)) {
return -EPERM; }
tmp = scull_quantum;
scull_quantum = arg;
return tmp;
default: /* redundant, as cmd was checked against MAXNR */
return -ENOTTY;
} /* switch */
return retval;
} /* scull_ioctl */

28. The Implementation of the ioctl Commands
Six ways to pass and receive arguments from the user space
Need to know command number
int quantum;
ioctl(fd,SCULL_IOCSQUANTUM, &quantum); /* Set by pointer */
ioctl(fd,SCULL_IOCTQUANTUM, quantum); /* Set by value */
ioctl(fd,SCULL_IOCGQUANTUM, &quantum); /* Get by pointer */
quantum = ioctl(fd,SCULL_IOCQQUANTUM); /* Get by return value */
ioctl(fd,SCULL_IOCXQUANTUM, &quantum); /* Exchange by pointer */
/* Exchange by value */
quantum = ioctl(fd,SCULL_IOCHQUANTUM, quantum);

29. Device Control Without ioctl
Writing control sequences into the data stream itself
Example: console escape sequences
Advantages:
No need to implement ioctl methods
Disadvantages:
Need to make sure that escape sequences do not appear in the normal data stream (e.g., cat a binary file)
Need to parse the data stream

30. Blocking I/O Needed when no data is available for reads
When the device is not ready to accept data
Output buffer is full

31. Introduction to Sleeping

32. Introduction to Sleeping
A process is removed from the scheduler’s run queue
Certain rules
Never sleep when running in an atomic context
Multiple steps must be performed without concurrent accesses
Not while holding a spinlock, seqlock, or RCU lock
Not while disabling interrupts

33. Introduction to Sleeping
Okay to sleep while holding a semaphore
Other threads waiting for the semaphore will also sleep
Need to keep it short
Make sure that it is not blocking the process that will wake it up
After waking up
Make no assumptions about the state of the system
The resource one is waiting for might be gone again
Must check the wait condition again

34. Introduction to Sleeping
Wait queue: contains a list of processes waiting for a specific event
#include <linux/wait.h>
To initialize statically, call
DECLARE_WAIT_QUEUE_HEAD(my_queue);
To initialize dynamically, call
wait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);

35. Simple Sleeping Call variants of wait_event macros
wait_event(queue, condition)
queue = wait queue head
Passed by value
Waits until the boolean condition becomes true
Puts into an uninterruptible sleep
Usually is not what you want
wait_event_interruptible(queue, condition)
Can be interrupted by signals
Returns nonzero if sleep was interrupted
Your driver should return -ERESTARTSYS

36. Simple Sleeping wait_event_timeout(queue, condition, timeout)
Wait for a limited time (in jiffies)
Returns 0 regardless of condition evaluations
wait_event_interruptible_timeout(queue,
condition,
timeout)

37. Simple Sleeping To wake up, call variants of wake_up functions
void wake_up(wait_queue_head_t *queue);
Wakes up all processes waiting on the queue
void wake_up_interruptible(wait_queue_head_t *queue);
Wakes up processes that perform an interruptible sleep

38. Multiple threads can wake up at this point
Simple Sleeping
Example module: sleepy
static DECLARE_WAIT_QUEUE_HEAD(wq);
static int flag = 0;
ssize_t sleepy_read(struct file *filp, char __user *buf,
size_t count, loff_t *pos) {
printk(KERN_DEBUG "process %i (%s) going to sleep\n",
current->pid, current->comm);
wait_event_interruptible(wq, flag != 0);
flag = 0;
printk(KERN_DEBUG "awoken %i (%s)\n", current->pid,
current->comm);
return 0; /* EOF */ }
Multiple threads can wake up at this point

39. Simple Sleeping Example module: sleepy
ssize_t sleepy_write(struct file *filp, const char __user *buf,
size_t count, loff_t *pos) {
printk(KERN_DEBUG "process %i (%s) awakening the readers...\n",
current->pid, current->comm);
flag = 1;
wake_up_interruptible(&wq);
return count; /* succeed, to avoid retrial */ }

40. Blocking and Nonblocking Operations
By default, operations block
If no data is available for reads
If no space is available for writes
Non-blocking I/O is indicated by the O_NONBLOCK flag in filp->f_flags
Defined in <linux/fcntl.h>
Only open, read, and write calls are affected
Returns –EAGAIN immediately instead of block
Applications need to distinguish non-blocking returns vs. EOFs

41. A Blocking I/O Example scullpipe A read process A write process
Blocks when no data is available
Wakes a blocking write when buffer space becomes available
A write process
Blocks when no buffer space is available
Wakes a blocking read process when data arrives

42. A Blocking I/O Example scullpipe data structure struct scull_pipe {
wait_queue_head_t inq, outq; /* read and write queues */
char *buffer, *end; /* begin of buf, end of buf */
int buffersize; /* used in pointer arithmetic */
char *rp, *wp; /* where to read, where to write */
int nreaders, nwriters; /* number of openings for r/w */
struct fasync_struct *async_queue; /* asynchronous readers */
struct semaphore sem; /* mutual exclusion semaphore */
struct cdev cdev; /* Char device structure */ };

43. A Blocking I/O Example
static ssize_t scull_p_read(struct file *filp, char __user *buf,
size_t count, loff_t *f_pos) {
struct scull_pipe *dev = filp->private_data;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
while (dev->rp == dev->wp) { /* nothing to read */
up(&dev->sem); /* release the lock */
if (filp->f_flags & O_NONBLOCK)
return -EAGAIN;
if (wait_event_interruptible(dev->inq, (dev->rp != dev->wp)))
return -ERESTARTSYS; }

44. A Blocking I/O Example if (dev->wp > dev->rp)
count = min(count, (size_t)(dev->wp - dev->rp));
else /* the write pointer has wrapped */
count = min(count, (size_t)(dev->end - dev->rp));
if (copy_to_user(buf, dev->rp, count)) {
up (&dev->sem);
return -EFAULT; }
dev->rp += count;
if (dev->rp == dev->end) dev->rp = dev->buffer; /* wrapped */
/* finally, awake any writers and return */
wake_up_interruptible(&dev->outq);
return count;

45. Advanced Sleeping

46. Advanced Sleeping Uses low-level functions to affect a sleep
How a process sleeps
1. Allocate and initialize a wait_queue_t structure
DEFINE_WAIT(my_wait); Or
wait_queue_t my_wait;
init_wait(&my_wait);
Queue element

47. Advanced Sleeping
2. Add to the proper wait queue and mark a process as being asleep
TASK_RUNNING TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE
Call
void prepare_to_wait(wait_queue_head_t *queue,
wait_queue_t *wait,
int state);

48. Advanced Sleeping 3. Give up the processor
Double check the sleeping condition before going to sleep
The wakeup thread might have changed the condition between steps 1 and 2
if (/* sleeping condition */) {
schedule(); /* yield the CPU */ }

49. Advanced Sleeping 4. Return from sleep
Remove the process from the wait queue if schedule() was not called
void finish_wait(wait_queue_head_t *queue,
wait_queue_t *wait);

50. Advanced Sleeping scullpipe write method /* How much space is free? */
static int spacefree(struct scull_pipe *dev) {
if (dev->rp == dev->wp)
return dev->buffersize - 1;
return ((dev->rp + dev->buffersize - dev->wp)
% dev->buffersize) - 1; }

51. Advanced Sleeping static ssize_t
scull_p_write(struct file *filp, const char __user *buf,
size_t count, loff_t *f_pos) {
struct scull_pipe *dev = filp->private_data;
int result;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
/* Wait for space for writing */
result = scull_getwritespace(dev, filp);
if (result)
return result; /* scull_getwritespace called up(&dev->sem) */
/* ok, space is there, accept something */
count = min(count, (size_t)spacefree(dev));

52. Advanced Sleeping if (dev->wp >= dev->rp)
count = min(count, (size_t)(dev->end - dev->wp));
else /* the write pointer has wrapped, fill up to rp - 1 */
count = min(count, (size_t)(dev->rp - dev->wp - 1));
if (copy_from_user(dev->wp, buf, count)) {
up (&dev->sem); return -EFAULT; }
dev->wp += count;
if (dev->wp == dev->end) dev->wp = dev->buffer; /* wrapped */
up(&dev->sem);
wake_up_interruptible(&dev->inq);
if (dev->async_queue)
kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
return count;

53. Advanced Sleeping Queue: full Task state: RUNNING
/* Wait for space for writing; caller must hold device semaphore.
* On error the semaphore will be released before returning. */
static int scull_getwritespace(struct scull_pipe *dev,
struct file *filp) {
while (spacefree(dev) == 0) { /* full */
DEFINE_WAIT(wait);
up(&dev->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS; }
return 0;
Queue: full
Task state: RUNNING

54. Advanced Sleeping Queue: full Task state: RUNNING  INTERRUPTIBLE
/* Wait for space for writing; caller must hold device semaphore.
* On error the semaphore will be released before returning. */
static int scull_getwritespace(struct scull_pipe *dev,
struct file *filp) {
while (spacefree(dev) == 0) { /* full */
DEFINE_WAIT(wait);
up(&dev->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS; }
return 0;
Queue: full
Task state: RUNNING  INTERRUPTIBLE

55. Advanced Sleeping Queue: full Task state: INTERRUPTIBLE /* sleep */
/* Wait for space for writing; caller must hold device semaphore.
* On error the semaphore will be released before returning. */
static int scull_getwritespace(struct scull_pipe *dev,
struct file *filp) {
while (spacefree(dev) == 0) { /* full */
DEFINE_WAIT(wait);
up(&dev->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS; }
return 0;
Queue: full
Task state: INTERRUPTIBLE /* sleep */

56. Exclusive Waits Avoid waking up all processes waiting on a queue Call
Wakes up only one process
Call
void prepare_to_wait_exclusive(wait_queue_heat_t *queue,
wait_queue_t *wait, int state);
Set the WQ_FLAG_EXCLUSIVE flag
Add the queue entry to the end of the wait queue
wake_up stops after waking the first process with the flag set

57. The Details of Waking Up
/* wakes up all processes waiting on the queue */
void wake_up(wait_queue_head_t *queue);
/* wakes up processes that perform an interruptible sleep */
void wake_up_interruptible(wait_queue_head_t *queue);
/* wake up to nr exclusive waiters */
void wake_up_nr(wait_queue_head_t *queue, int nr);
void wake_up_interruptible_nr(wait_queue_head_t *queue, int nr);
/* wake up all exclusive waiters */
void wake_up_all(wait_queue_head_t *queue);
void wake_up_interruptible_all(wait_queue_head_t *queue);
/* do not lose the CPU during this call */
void wake_up_interruptible_sync(wait_queue_head_t *queue);

58. Testing the scullpipe Driver
Window 1
% cat /dev/scullpipe
Window2 %

59. Testing the scullpipe Driver
Window 1
% cat /dev/scullpipe
Window2
% ls –aF > /dev/scullpipe

60. Testing the scullpipe Driver
Window 1
% cat /dev/scullpipe ./
../
file1
file2
Window2
% ls –aF > /dev/scullpipe

61. poll and select
Nonblocking I/Os often involve the use of poll, select, and epoll system calls
Allow a process to determine whether it can read or write one or more open files without blocking
Can block a process until any of a set of file descriptors becomes available for reading and writing
select introduced in BSD Linux
poll introduced in System V
epoll added in for better scaling

62. poll and select All three calls supported through the poll method
unsigned int (*poll) (struct file *filp,
poll_table *wait);
1. Call poll_wait on one or more wait queues that could indicate a change in the poll status
If no file descriptors are available, wait
2. Return a bit mask describing the operations that could be immediately performed without blocking

63. poll and select poll_table defined in <linux/poll.h>
To add a wait queue into the poll_table, call
void poll_wait(struct file *,
wait_queue_head_t *,
poll_table *);
Bit mask flags defined in <linux/poll.h>
POLLIN
Set if the device can be read without blocking

64. poll and select POLLOUT POLLRDNORM POLLWRNORM POLLPRI
Set if the device can be written without blocking
POLLRDNORM
Set if “normal” data is available for reading
A readable device returns (POLLIN | POLLRDNORM)
POLLWRNORM
Same meaning as POLLOUT
A writable device returns (POLLOUT | POLLWRNORM)
POLLPRI
High-priority data can be read without blocking

65. poll and select POLLHUP POLLERR POLLRDBAND POLLWRBAND
Returns when a process reads the end-of-file
POLLERR
An error condition has occurred
POLLRDBAND
Out-of-band data is available for reading
Associated with sockets
POLLWRBAND
Data with nonzero priority can be written to the device

66. poll and select Example
static unsigned int scull_p_poll(struct file *filp,
poll_table *wait) {
struct scull_pipe *dev = filp->private_data;
unsigned int mask = 0;
down(&dev->sem);
poll_wait(filp, &dev->inq, wait);
poll_wait(filp, &dev->outq, wait);
if (dev->rp != dev->wp) /* circular buffer not empty */
mask |= POLLIN | POLLRDNORM; /* readable */
if (spacefree(dev)) /* circular buffer not full */
mask |= POLLOUT | POLLWRNORM; /* writable */
up(&dev->sem);
return mask; }

67. poll and select No end-of-file support
Scull pipe does not implement this
If it did…
The reader could see an end-of-file when all writers close the file
Check dev->nwriters in read and poll
Problem when a reader opens the scullpipe before the writer
Need blocking within open

68. Interaction with read and write
Reading from the device
If there is data in the input buffer, return at least one byte
poll returns POLLIN | POLLRDNORM
If no data is available
If O_NONBLOCK is set, return –EAGAIN
poll must report the device unreadable until one byte arrives
At the end-of-file, read returns 0, poll returns POLLHUP

69. Interaction with read and write
Writing to the device
If there is space in the output buffer, accept at least one byte
poll reports that the devices is writable by returning POLLOUT | POLLWRNORM
If the output buffer is full, write blocks
If O_NONBLOCK is set, write returns –EAGAIN
poll reports that the file is not writable
If the device is full, write returns -ENOSPC

70. Interaction with read and write
In write, never wait for data transmission before returning
Or, select may block
To make sure the output buffer is actually transmitted, use fsync call

71. Interaction with read and write
To flush pending output, call fsync
int (*fsync) (struct file *file, loff_t, loff_t, int datasync);
Should return only when the device has been completely flushed
datasync:
Used by file systems, ignored by drivers

72. The Underlying Data Structure

73. The Underlying Data Structure
When the poll call completes, poll_table is deallocated with all wait queue entries removed
epoll reduces this overhead of setting up and tearing down the data structure between every I/O

74. Asynchronous Notification
Polling
Inefficient for rare events
A solution: asynchronous notification
Application receives a signal whenever data becomes available
Two steps
Specify a process as the owner of the file (so that the kernel knows whom to notify)
Set the FASYNC flag in the device via fcntl command

75. Asynchronous Notification
Example (user space)
/* create a signal handler */
signal(SIGIO, &input_handler);
/* set current pid the owner of the stdin */
fcntl(STDIN_FILENO, F_SETOWN, getpid());
/* obtain the current file control flags */
oflags = fcntl(STDIN_FILENO, F_GETFL);
/* set the asynchronous flag */
fcntl(STDIN_FILENO, F_SETFL, oflags | FASYNC);

76. Asynchronous Notification
Some catches
Not all devices support asynchronous notification
Usually available for sockets and ttys
Need to know which input file to process
Still need to use poll or select

77. The Driver’s Point of View
1. When F_SETOWN is invoked, a value is assigned to filp->f_owner
2. When F_SETFL is executed to change the status of FASYNC
The driver’s fasync method is called
static int
scull_p_fasync(int fd, struct file *filp, int mode) {
struct scull_pipe *dev = filp->private_data;
return fasync_helper(fd, filp, mode, &dev->async_queue); }

78. The Driver’s Point of View
fasync_helper adds or removes processes from the asynchronous list
void fasync_helper(int fd, struct file *filp, int mode,
struct fasync_struct **fa);
3. When data arrives, send a SIGNO signal to all processes registered for asynchronous notification
Near the end of write, notify blocked readers
if (dev->async_queue)
kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
Similarly for read, as needed

79. The Driver’s Point of View
4. When the file is closed, remove the file from the list of asynchronous readers in the release method
scull_p_fasync(-1, filp, 0);

80. The llseek Implementation
Implements lseek and llseek system calls
Modifies filp->f_pos
loff_t scull_llseek(struct file *filp, loff_t off, int whence) {
struct scull_dev *dev = filp->private_data;
loff_t newpos;
switch(whence) {
case 0: /* SEEK_SET */
newpos = off;
break;
case 1: /* SEEK_CUR, relative to the current position */
newpos = filp->f_pos + off;

81. The llseek Implementation
case 2: /* SEEK_END, relative to the end of the file */
newpos = dev->size + off;
break;
default: /* can't happen */
return -EINVAL; }
if (newpos < 0) return -EINVAL;
filp->f_pos = newpos;
return newpos;

82. The llseek Implementation
Does not make sense for serial ports and keyboard inputs
Need to inform the kernel via calling nonseekable_open in the open method
int nonseekable_open(struct inode *inode, struct file *filp);
Replace llseek method with no_llseek (defined in <linux/fs.h> in your file_operations structure

83. Access Control on a Device File
Prevents unauthorized users from using the device
Sometimes permits only one authorized user to open the device at a time

84. Returns true, if the tested value is 0
Single-Open Devices
Example: scullsingle
static atomic_t scull_s_available = ATOMIC_INIT(1);
static int scull_s_open(struct inode *inode, struct file *filp) {
struct scull_dev *dev = &scull_s_device;
if (!atomic_dec_and_test(&scull_s_available)) {
atomic_inc(&scull_s_available);
return -EBUSY; /* already open */ }
/* then, everything else is the same as before */
if ((filp->f_flags & O_ACCMODE) == O_WRONLY) scull_trim(dev);
filp->private_data = dev;
return 0; /* success */
Returns true, if the tested value is 0

85. Single-Open Devices In the release call, marks the device idle
static int
scull_s_release(struct inode *inode, struct file *filp) {
atomic_inc(&scull_s_available); /* release the device */
return 0; }

86. Restricting Access to a Single User (with multiple processes) at a Time
Example: sculluid
Includes the following in the open call
spin_lock(&scull_u_lock);
if (scull_u_count && /* someone is using the device */
(scull_u_owner != current->uid) && /* not the same user */
(scull_u_owner != current->euid) && /* not the same effective uid (for su) */
!capable(CAP_DAC_OVERRIDE)) { /* not root override */
spin_unlock(&scull_u_lock);
return -EBUSY; /* -EPERM would confuse the user */ }
if (scull_u_count == 0) scull_u_owner = current->uid;
scull_u_count++;

87. Restricting Access to a Single User (with Multiple Processes) at a Time
Includes the following in the release call
static int scull_u_release(struct inode *inode,
struct file *filp) {
spin_lock(&scull_u_lock);
scull_u_count--; /* nothing else */
spin_unlock(&scull_u_lock);
return 0; }

88. Blocking open as an Alternative to EBUSY (scullwuid)
A user might prefer to wait over getting errors
E.g., data communication channel
spin_lock(&scull_w_lock);
while (!scull_w_available()) {
spin_unlock(&scull_w_lock);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
if (wait_event_interruptible(scull_w_wait,
scull_w_available()))
return -ERESTARTSYS; /* tell the fs layer to handle it */ }
if (scull_w_count == 0) scull_w_owner = current->uid;
scull_w_count++;

89. Blocking open as an Alternative to EBUSY (scullwuid)
The release method wakes pending processes
static int scull_w_release(struct inode *inode,
struct file *filp) {
int temp;
spin_lock(&scull_w_lock);
scull_w_count--;
temp = scull_w_count;
spin_unlock(&scull_w_lock);
if (temp == 0)
wake_up_interruptible_sync(&scull_w_wait);
return 0; }

90. Blocking open as an Alternative to EBUSY
Might not be the right semantics for interactive users
Blocking on cp vs. getting a return value –EBUSY or -EPERM
Incompatible policies for the same device
One solution: one device node per policy

91. Cloning the Device on open
Allows the creation of private, virtual devices
E.g., One virtual scull device for each process with different tty device number
Example: scullpriv

92. Cloning the Device on open
static int scull_c_open(struct inode *inode, struct file *filp) {
struct scull_dev *dev;
dev_t key;
if (!current->signal->tty) {
PDEBUG("Process \"%s\" has no ctl tty\n", current->comm);
return -EINVAL; }
key = tty_devnum(current->signal->tty);
spin_lock(&scull_c_lock);
dev = scull_c_lookfor_device(key);
spin_unlock(&scull_c_lock);
if (!dev) return -ENOMEM;
.../* then, everything else is the same as before */

93. Cloning the Device on open
/* The clone-specific data structure includes a key field */
struct scull_listitem {
struct scull_dev device;
dev_t key;
struct list_head list; };
/* The list of devices, and a lock to protect it */
static LIST_HEAD(scull_c_list);
static spinlock_t scull_c_lock = SPIN_LOCK_UNLOCKED;

94. Cloning the Device on open
/* Look for a device or create one if missing */
static struct scull_dev *scull_c_lookfor_device(dev_t key) {
struct scull_listitem *lptr;
list_for_each_entry(lptr, &scull_c_list, list) {
if (lptr->key == key)
return &(lptr->device); }
/* not found */
lptr = kmalloc(sizeof(struct scull_listitem), GFP_KERNEL);
if (!lptr) return NULL;

95. Cloning the Device on open
/* initialize the device */
memset(lptr, 0, sizeof(struct scull_listitem));
lptr->key = key;
scull_trim(&(lptr->device)); /* initialize it */
init_MUTEX(&(lptr->device.sem));
/* place it in the list */
list_add(&lptr->list, &scull_c_list);
return &(lptr->device); }

96. What’s going on? scull_listitem struct scull_dev device; dev_t key;
scull_c_list
struct list_head {
struct list_head *next;
struct list_head *prev; };
struct list_head {
struct list_head *next;
struct list_head *prev;
} list;