How to create a simple sysfs class attribute in Linux kernel v3.2
Even though my knowledge is still fairly low on the topic, I'm going to post an answer just because of the age of this question. If somebody else has a better answer, please post! :)
First off, I'm going to assume that you've read that whole chapter (specifically about kobjects & ksets). So just about every struct in the device driver model has these cutely included in them. If you want to manipulate the kobject for the class its self (not sure if that's wise or not), that's your struct class's dev_kobj member.
However, you want to manipulate the attributes of that class. I believe you do this by defining a (usually static), NULL-terminated array of them as follows and then assigning its address to the struct class's class_attrs member (taken from drivers/uwb/driver.c):
static struct class_attribute uwb_class_attrs[] = {
__ATTR(beacon_timeout_ms, S_IWUSR | S_IRUGO,
beacon_timeout_ms_show, beacon_timeout_ms_store),
__ATTR_NULL,
};
/** Device model classes */
struct class uwb_rc_class = {
.name = "uwb_rc",
.class_attrs = uwb_class_attrs,
};
When I don't know how to use something, I usually git grep the repository for somebody else who has used it and try to learn from it that way. It would seem that this is why they tend to say kernel "hackers" and not "developers".
There is a good tutorial in the link below
http://pete.akeo.ie/2011/08/writing-linux-device-driver-for-kernels.html
parrot_driver.c:
/*
* Linux 2.6 and 3.0 'parrot' sample device driver
*
* Copyright (c) 2011, Pete Batard <[email protected]>
*
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/types.h>
#include <linux/mutex.h>
#include <linux/kfifo.h>
#include "parrot_driver.h"
/* Module information */
MODULE_AUTHOR(AUTHOR);
MODULE_DESCRIPTION(DESCRIPTION);
MODULE_VERSION(VERSION);
MODULE_LICENSE("GPL");
/* Device variables */
static struct class* parrot_class = NULL;
static struct device* parrot_device = NULL;
static int parrot_major;
/* Flag used with the one_shot mode */
static bool message_read;
/* A mutex will ensure that only one process accesses our device */
static DEFINE_MUTEX(parrot_device_mutex);
/* Use a Kernel FIFO for read operations */
static DECLARE_KFIFO(parrot_msg_fifo, char, PARROT_MSG_FIFO_SIZE);
/* This table keeps track of each message length in the FIFO */
static unsigned int parrot_msg_len[PARROT_MSG_FIFO_MAX];
/* Read and write index for the table above */
static int parrot_msg_idx_rd, parrot_msg_idx_wr;
/* Module parameters that can be provided on insmod */
static bool debug = false; /* print extra debug info */
module_param(debug, bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(debug, "enable debug info (default: false)");
static bool one_shot = true; /* only read a single message after open() */
module_param(one_shot, bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(debug, "disable the readout of multiple messages at once (default: true)");
static int parrot_device_open(struct inode* inode, struct file* filp)
{
dbg("");
/* Our sample device does not allow write access */
if ( ((filp->f_flags & O_ACCMODE) == O_WRONLY)
|| ((filp->f_flags & O_ACCMODE) == O_RDWR) ) {
warn("write access is prohibited\n");
return -EACCES;
}
/* Ensure that only one process has access to our device at any one time
* For more info on concurrent accesses, see http://lwn.net/images/pdf/LDD3/ch05.pdf */
if (!mutex_trylock(&parrot_device_mutex)) {
warn("another process is accessing the device\n");
return -EBUSY;
}
message_read = false;
return 0;
}
static int parrot_device_close(struct inode* inode, struct file* filp)
{
dbg("");
mutex_unlock(&parrot_device_mutex);
return 0;
}
static ssize_t parrot_device_read(struct file* filp, char __user *buffer, size_t length, loff_t* offset)
{
int retval;
unsigned int copied;
/* The default from 'cat' is to issue multiple reads until the FIFO is depleted
* one_shot avoids that */
if (one_shot && message_read) return 0;
dbg("");
if (kfifo_is_empty(&parrot_msg_fifo)) {
dbg("no message in fifo\n");
return 0;
}
retval = kfifo_to_user(&parrot_msg_fifo, buffer, parrot_msg_len[parrot_msg_idx_rd], &copied);
/* Ignore short reads (but warn about them) */
if (parrot_msg_len[parrot_msg_idx_rd] != copied) {
warn("short read detected\n");
}
/* loop into the message length table */
parrot_msg_idx_rd = (parrot_msg_idx_rd+1)%PARROT_MSG_FIFO_MAX;
message_read = true;
return retval ? retval : copied;
}
/* The file_operation scructure tells the kernel which device operations are handled.
* For a list of available file operations, see http://lwn.net/images/pdf/LDD3/ch03.pdf */
static struct file_operations fops = {
.read = parrot_device_read,
.open = parrot_device_open,
.release = parrot_device_close
};
/* Placing data into the read FIFO is done through sysfs */
static ssize_t sys_add_to_fifo(struct device* dev, struct device_attribute* attr, const char* buf, size_t count)
{
unsigned int copied;
dbg("");
if (kfifo_avail(&parrot_msg_fifo) < count) {
warn("not enough space left on fifo\n");
return -ENOSPC;
}
if ((parrot_msg_idx_wr+1)%PARROT_MSG_FIFO_MAX == parrot_msg_idx_rd) {
/* We've looped into our message length table */
warn("message length table is full\n");
return -ENOSPC;
}
/* The buffer is already in kernel space, so no need for ..._from_user() */
copied = kfifo_in(&parrot_msg_fifo, buf, count);
parrot_msg_len[parrot_msg_idx_wr] = copied;
if (copied != count) {
warn("short write detected\n");
}
parrot_msg_idx_wr = (parrot_msg_idx_wr+1)%PARROT_MSG_FIFO_MAX;
return copied;
}
/* This sysfs entry resets the FIFO */
static ssize_t sys_reset(struct device* dev, struct device_attribute* attr, const char* buf, size_t count)
{
dbg("");
/* Ideally, we would have a mutex around the FIFO, to ensure that we don't reset while in use.
* To keep this sample simple, and because this is a sysfs operation, we don't do that */
kfifo_reset(&parrot_msg_fifo);
parrot_msg_idx_rd = parrot_msg_idx_wr = 0;
return count;
}
/* Declare the sysfs entries. The macros create instances of dev_attr_fifo and dev_attr_reset */
static DEVICE_ATTR(fifo, S_IWUSR, NULL, sys_add_to_fifo);
static DEVICE_ATTR(reset, S_IWUSR, NULL, sys_reset);
/* Module initialization and release */
static int __init parrot_module_init(void)
{
int retval;
dbg("");
/* First, see if we can dynamically allocate a major for our device */
parrot_major = register_chrdev(0, DEVICE_NAME, &fops);
if (parrot_major < 0) {
err("failed to register device: error %d\n", parrot_major);
retval = parrot_major;
goto failed_chrdevreg;
}
/* We can either tie our device to a bus (existing, or one that we create)
* or use a "virtual" device class. For this example, we choose the latter */
parrot_class = class_create(THIS_MODULE, CLASS_NAME);
if (IS_ERR(parrot_class)) {
err("failed to register device class '%s'\n", CLASS_NAME);
retval = PTR_ERR(parrot_class);
goto failed_classreg;
}
/* With a class, the easiest way to instantiate a device is to call device_create() */
parrot_device = device_create(parrot_class, NULL, MKDEV(parrot_major, 0), NULL, CLASS_NAME "_" DEVICE_NAME);
if (IS_ERR(parrot_device)) {
err("failed to create device '%s_%s'\n", CLASS_NAME, DEVICE_NAME);
retval = PTR_ERR(parrot_device);
goto failed_devreg;
}
/* Now we can create the sysfs endpoints (don't care about errors).
* dev_attr_fifo and dev_attr_reset come from the DEVICE_ATTR(...) earlier */
retval = device_create_file(parrot_device, &dev_attr_fifo);
if (retval < 0) {
warn("failed to create write /sys endpoint - continuing without\n");
}
retval = device_create_file(parrot_device, &dev_attr_reset);
if (retval < 0) {
warn("failed to create reset /sys endpoint - continuing without\n");
}
mutex_init(&parrot_device_mutex);
/* This device uses a Kernel FIFO for its read operation */
INIT_KFIFO(parrot_msg_fifo);
parrot_msg_idx_rd = parrot_msg_idx_wr = 0;
return 0;
failed_devreg:
class_unregister(parrot_class);
class_destroy(parrot_class);
failed_classreg:
unregister_chrdev(parrot_major, DEVICE_NAME);
failed_chrdevreg:
return -1;
}
static void __exit parrot_module_exit(void)
{
dbg("");
device_remove_file(parrot_device, &dev_attr_fifo);
device_remove_file(parrot_device, &dev_attr_reset);
device_destroy(parrot_class, MKDEV(parrot_major, 0));
class_unregister(parrot_class);
class_destroy(parrot_class);
unregister_chrdev(parrot_major, DEVICE_NAME);
}
/* Let the kernel know the calls for module init and exit */
module_init(parrot_module_init);
module_exit(parrot_module_exit);
parrot_driver.h:
/*
* Linux 2.6 and 3.0 'parrot' sample device driver
*
* Copyright (c) 2011, Pete Batard <[email protected]>
*
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#define DEVICE_NAME "device"
#define CLASS_NAME "parrot"
#define PARROT_MSG_FIFO_SIZE 1024
#define PARROT_MSG_FIFO_MAX 128
#define AUTHOR "Pete Batard <[email protected]>"
#define DESCRIPTION "'parrot' sample device driver"
#define VERSION "0.3"
/* We'll use our own macros for printk */
#define dbg(format, arg...) do { if (debug) pr_info(CLASS_NAME ": %s: " format , __FUNCTION__ , ## arg); } while (0)
#define err(format, arg...) pr_err(CLASS_NAME ": " format, ## arg)
#define info(format, arg...) pr_info(CLASS_NAME ": " format, ## arg)
#define warn(format, arg...) pr_warn(CLASS_NAME ": " format, ## arg)
Minimal runnable example
Usage:
insmod /sysfs.ko
cd /sys/kernel/lkmc_sysfs
printf 12345 >foo
cat foo
# => 1234
dd if=foo bs=1 count=2 skip=1 status=none
# => 23
sysfs.c
#include <linux/init.h>
#include <linux/kobject.h>
#include <linux/module.h>
#include <linux/stat.h>
#include <linux/string.h>
#include <linux/sysfs.h>
#include <uapi/linux/stat.h> /* S_IRUSR, S_IWUSR */
enum { FOO_SIZE_MAX = 4 };
static int foo_size;
static char foo_tmp[FOO_SIZE_MAX];
static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
char *buff)
{
strncpy(buff, foo_tmp, foo_size);
return foo_size;
}
static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buff, size_t count)
{
foo_size = min(count, (size_t)FOO_SIZE_MAX);
strncpy(foo_tmp, buff, foo_size);
return count;
}
static struct kobj_attribute foo_attribute =
__ATTR(foo, S_IRUGO | S_IWUSR, foo_show, foo_store);
static struct attribute *attrs[] = {
&foo_attribute.attr,
NULL,
};
static struct attribute_group attr_group = {
.attrs = attrs,
};
static struct kobject *kobj;
static int myinit(void)
{
int ret;
kobj = kobject_create_and_add("lkmc_sysfs", kernel_kobj);
if (!kobj)
return -ENOMEM;
ret = sysfs_create_group(kobj, &attr_group);
if (ret)
kobject_put(kobj);
return ret;
}
static void myexit(void)
{
kobject_put(kobj);
}
module_init(myinit);
module_exit(myexit);
MODULE_LICENSE("GPL");
GitHub upstream.
Tested with Linux kernel 5.0.