Cómo crear un atributo de clase sysfs simple en Linux kernel v3.2

Aunque mi conocimiento todavía es bastante bajo sobre el tema, voy a publicar una respuesta solo por la antigüedad de esta pregunta. Si alguien más tiene una mejor respuesta, por favor publícala! :)

En primer lugar, supondré que ha leído todo el capítulo (específicamente sobre kobjects y ksets). Entonces, casi todas las estructuras en el modelo de controlador de dispositivo tienen estos lindamente incluidos. Si desea manipular el kobject para la clase en sí (no estoy seguro de si eso es inteligente o no), ese es su struct class dev_kobj de miembro.

Sin embargo, desea manipular los atributos de esa clase. Yo creo lo haces definiendo una matriz (generalmente estática) terminada en NULL de ellos de la siguiente manera y luego asignando su dirección al struct class de class_attrs miembro (tomado de 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,
};

Cuando no sé cómo usar algo, normalmente git grep el repositorio para otra persona que lo haya usado y trate de aprender de él de esa manera. Parecería que es por eso que tienden a decir kernel "hackers" y no "desarrolladores".

Hay un buen tutorial en el siguiente enlace

http://pete.akeo.ie/2011/08/escritura-linux-dispositivo-controlador-para-kernels.html

loro_conductor.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);

conductor_loro.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)

Ejemplo ejecutable mínimo

Uso:

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 ascendente.

Probado con el kernel de Linux 5.0.