在学习了这么些天的驱动之后，个人觉得驱动就是个架构的问题，只要把架构弄清楚了
然后往里面添砖加瓦就可以了，所以似乎看起来不是太困难，但也许是是我经验不足吧，这只能算是个人浅见了
这两天在学习USB驱动开发，奇怪的是老师居然不讲USB的代码，让人不理解，后来在网上找资料才发现原来内核已经给我们准备了一个usb_skel的代码向我们介绍几本的USB驱动的架构，于是自己分析了一下代码，画了一个我认为的代码架构(比较难看)，写了一些注释
相关阅读：
废话不多说啦，直接上图上代码，请高手们批评指正！
/*
* USB Skeleton driver - 2.2
*
* Copyright (C) 2001-2004 Greg Kroah-Hartman (greg@kroah.com)
*
* 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, version 2.
*
* This driver is based on the 2.6.3 version of drivers/usb/usb-skeleton.c
* but has been rewritten to be easier to read and use.
*
*/
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/kref.h>
#include <asm/uaccess.h>
#include <linux/usb.h>
#include <linux/mutex.h>
/* Define these values to match your devices */
#define USB_SKEL_VENDOR_ID 0xfff0 //厂商ID
#define USB_SKEL_PRODUCT_ID 0xfff0 //产品ID
/* table of devices that work with this driver */
//驱动支持的USB设备列表
static struct usb_device_id skel_table [] = {
{ USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
{ }, /* Terminating entry */
};
MODULE_DEVICE_TABLE(usb, skel_table);
/* to prevent a race between open and disconnect */
static DEFINE_MUTEX(skel_open_lock);
/* Get a minor range for your devices from the usb maintainer */
#define USB_SKEL_MINOR_BASE 192 //USB主设备号
/* our private defines. if this grows any larger, use your own .h file */
#define MAX_TRANSFER (PAGE_SIZE - 512)
/* MAX_TRANSFER is chosen so that the VM is not stressed by allocations > PAGE_SIZE and the number of packets in a page is an integer 512 is the largest possible packet on EHCI */
#define WRITES_IN_FLIGHT 8
/* arbitrarily chosen */
//8、usb_skel结构体可以被看作一个私有数据结构体，应该根据具体的设备量身定制
struct usb_skel {
struct usb_device *udev; //该设备的usb_device指针
struct usb_interface *interface; //该设备的usb_interface指针
struct semaphore limit_sem; //限制进程写的数据量
unsigned char *bulk_in_buffer; //接收数据的缓冲区
size_t bulk_in_size; //接收缓冲区大小
__u8 bulk_in_endpointAddr; //批量IN端点的地址
__u8 bulk_out_endpointAddr; //批量OUT端点的地址
struct kref kref; //sturct kref作为内核中最基本的引用计数而存在
struct mutex io_mutex; //同步的IO互斥锁，保证
};
//5、声明一个USB骨架驱动结构体对象
static struct usb_driver skel_driver;
static void skel_delete(struct kref *kref)
{
struct usb_skel *dev = to_skel_dev(kref);
usb_put_dev(dev->udev);
kfree(dev->bulk_in_buffer);
kfree(dev);
}
static int skel_open(struct inode *inode, struct file *file)
{
struct usb_skel *dev;
struct usb_interface *interface;
int subminor;
int retval = 0;
subminor = iminor(inode); //获取次设备号
mutex_lock(&skel_open_lock); //上锁
interface = usb_find_interface(&skel_driver, subminor); //获得接口数据
if (!interface) {
mutex_unlock(&skel_open_lock); //解锁
err ("%s - error, can't find device for minor %d", __FUNCTION__, subminor);
retval = -ENODEV;
goto exit;
}
dev = usb_get_intfdata(interface);
if (!dev) {
mutex_unlock(&skel_open_lock);
retval = -ENODEV;
goto exit;
}
/* increment our usage count for the device */
kref_get(&dev->kref);
/* now we can drop the lock */
mutex_unlock(&skel_open_lock);
/* prevent the device from being autosuspended */
retval = usb_autopm_get_interface(interface);
if (retval) {
kref_put(&dev->kref, skel_delete);
goto exit;
}
/* save our object in the file's private structure */
file->private_data = dev;
exit:
return retval;
}
static int skel_release(struct inode *inode, struct file *file)
{
struct usb_skel *dev;
dev = (struct usb_skel *)file->private_data;
if (dev == NULL)
return -ENODEV;
/* allow the device to be autosuspended */
mutex_lock(&dev->io_mutex);
if (dev->interface)
usb_autopm_put_interface(dev->interface);
mutex_unlock(&dev->io_mutex);
/* decrement the count on our device */
kref_put(&dev->kref, skel_delete);
return 0;
}
static ssize_t skel_read(struct file *file, char *buffer, size_t count, loff_t *ppos)
{
struct usb_skel *dev;
int retval;
int bytes_read;
dev = (struct usb_skel *)file->private_data;
mutex_lock(&dev->io_mutex);
if (!dev->interface) { /* disconnect() was called */
retval = -ENODEV;
goto exit;
}
/* 进行阻塞的批量读以从设备获取数据 */
retval = usb_bulk_msg(dev->udev, usb_rcvbulkpipe(dev->udev, dev->bulk_in_endpointAddr), dev->bulk_in_buffer, min(dev->bulk_in_size, count), &bytes_read, 10000);
/* 如果读成功，复制到用户空间 */
if (!retval) {
if (copy_to_user(buffer, dev->bulk_in_buffer, bytes_read))
retval = -EFAULT;
else
retval = bytes_read;
}
exit:
mutex_unlock(&dev->io_mutex);
return retval;
}
/*当urb被成功传输到USB设备之后，urb回调函数将被USB核心调用，在我们的例子中，我们初始化urb，使它指向skel_write_bulk_callback函数*/
static void skel_write_bulk_callback(struct urb *urb)
{
struct usb_skel *dev;
dev = (struct usb_skel *)urb->context;
/* sync/async unlink faults aren't errors */
if (urb->status && !(urb->status == -ENOENT || urb->status == -ECONNRESET || urb->status == -ESHUTDOWN)) {
err("%s - nonzero write bulk status received: %d",
__FUNCTION__, urb->status);
}
/ * free up our allocated buffer */
usb_buffer_free(urb->dev, urb->transfer_buffer_length,
urb->transfer_buffer, urb->transfer_dma);
up(&dev->limit_sem);
}
static ssize_t skel_write(struct file *file, const char *user_buffer, size_t count, loff_t *ppos)
{
struct usb_skel *dev;
int retval = 0;
struct urb *urb = NULL;
char *buf = NULL;
size_t writesize = min(count, (size_t)MAX_TRANSFER);
dev = (struct usb_skel *)file->private_data;
/* verify that we actually have some data to write */
if (count == 0)
goto exit;
/* limit the number of URBs in flight to stop a user from using up all RAM */
if (down_interruptible(&dev->limit_sem)) {
retval = -ERESTARTSYS;
goto exit;
}
/* 创建一个urb,并且给它分配一个缓存*/
urb = usb_alloc_urb(0, GFP_KERNEL);
if (!urb) {
retval = -ENOMEM;
goto error;
}
/*当urb被成功分配后，还要创建一个DMA缓冲区来以高效的方式发送数据到设备，传递给驱动程序的数据要复制到这块缓冲中去*/
buf = usb_buffer_alloc(dev->udev, writesize, GFP_KERNEL, &urb->transfer_dma);
if (!buf) {
retval = -ENOMEM;
goto error;
}
if (copy_from_user(buf, user_buffer, writesize)) {
retval = -EFAULT;
goto error;
}
/* this lock makes sure we don't submit URBs to gone devices */
mutex_lock(&dev->io_mutex);
if (!dev->interface) {
/* disconnect() was called */
mutex_unlock(&dev->io_mutex);
retval = -ENODEV;
goto error;
}
/*当数据从用户空间正确复制到局部缓冲区后，urb必须在可以被提交给USB核心之前被正确初始化*/
usb_fill_bulk_urb(urb, dev->udev, usb_sndbulkpipe(dev->udev, dev->bulk_out_endpointAddr), buf, writesize, skel_write_bulk_callback, dev);
urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
/* 把数据从批量OUT端口发出 */
retval = usb_submit_urb(urb, GFP_KERNEL);
mutex_unlock(&dev->io_mutex);
if (retval) {
err("%s - failed submitting write urb, error %d", __FUNCTION__, retval);
goto error;
}
/* release our reference to this urb, the USB core will eventually free it entirely */
usb_free_urb(urb);
return writesize;
error:
if (urb) {
usb_buffer_free(dev->udev, writesize, buf, urb->transfer_dma);
usb_free_urb(urb);
}
up(&dev->limit_sem);
exit:
return retval;
}
//字符设备的file_operations结构体，这个结构体中的成员实现
static const struct file_operations skel_fops = {
.owner = THIS_MODULE,
.read = skel_read,
.write = skel_write,
.open = skel_open,
.release = skel_release,
};
/*
* usb class driver info in order to get a minor number from the usb core,
* and to have the device registered with the driver core
*/
static struct usb_class_driver skel_class = {
.name = "skel%d",
.fops = &skel_fops,
.minor_base = USB_SKEL_MINOR_BASE,
};
//7、探测函数skel_probe