linux-kernel - 如何使用 linux 驱动程序读取/写入 USB 存储设备？

Question

在尝试为 USB 闪存驱动器编写我自己的简单 USB 驱动程序时，我在读取写入设备的数据时遇到了困难。所以，我的第一个问题是:

设备上的传输和存储是如何进行的？(详细)

我知道我必须执行以下步骤:

• 创建一个 urb(USB 请求 block )
• 分配一个 DMA 缓冲区
• 将数据从用户空间传输到 DMA 缓冲区
• 通过管道将数据发送到设备

我找不到任何关于设备如何处理这些数据的文档。

是否有可能编写这样的驱动程序，或者是否有必要拆卸 usb 设备以发送特殊命令？

我编写的代码类似于以下内容，来自 ldd3 和“http://lxr.free-electrons.com/source/drivers/usb/usb-skeleton.c”。它仅显示重要功能的简化版本。

将驱动程序加载到内核后，我可以毫无错误地写入设备，但如果我读取，则会出现EPIPE 错误。 Ldd3 提到 usb_clear_halt() 可以解决这个问题，但事实并非如此。

// This function is called when the device is plugged in
static int my_driver_probe(struct usb_interface* interface, const struct usb_device_id* id)
{
    struct usb_skel* dev = NULL;
    struct usb_device* udev = interface_to_usbdev(interface);
    struct usb_host_interface* iface_desc;
    struct usb_endpoint_descriptor* endpoint;
    int retval = -ENODEV;
    int i = 0;
    size_t buffer_size;

    dev = kzalloc(sizeof(struct usb_skel), GFP_KERNEL);

    // Check vendor and product id
    // … 

    dev->udev = udev;
    dev->interface = interface;

    // Set up the endpoint information
    iface_desc = interface->cur_altsetting;
    for(i=0; i < iface_desc->desc.bNumEndpoints; ++i) {
        endpoint = &iface_desc->endpoint[i].desc;

        if(!dev->bulk_in_endpointAddr && usb_endpoint_is_bulk_in(endpoint)) {
            buffer_size = endpoint->wMaxPacketSize;
            dev->bulk_in_size = buffer_size;
            dev->bulk_in_endpointAddr = endpoint->bEndpointAddress;
            dev->bulk_in_buffer = kmalloc(buffer_size, GFP_KERNEL);
            if(!dev->bulk_in_buffer) {
                printk("Could not allocate bulk_in_buffer\n");
                goto error;
            }
            dev->bulk_in_urb = usb_alloc_urb(0, GFP_KERNEL);
        }
        if(!dev->bulk_out_endpointAddr && usb_endpoint_is_bulk_out(endpoint))
            dev->bulk_out_endpointAddr = endpoint->bEndpointAddress;
    }
    // Check that the endpoints are set
    // … 

    // Save our data pointer in this interface device
    usb_set_intfdata(interface, dev);

    // Register the device
    retval = usb_register_dev(interface, &class_descr);
    return retval;
}

// Is called when another program writes into /dev/my_usb_driver
static ssize_t my_driver_write( struct file* file, const char __user* user_buffer, size_t count, loff_t* offs)
{
    struct usb_skel* dev = file->private_data;
    struct urb* urb = NULL;
    char* buf = NULL;
    int retval = 0;
    size_t writesize = min(count, (size_t)MAX_TRANSFER);

    // Create a urb, and a buffer for it, and copy the data to the urb
    urb = usb_alloc_urb(0, GFP_KERNEL);

    // Creates a DMA buffer
    buf = usb_alloc_coherent(dev->udev, writesize, GFP_KERNEL, &urb->transfer_dma);

    // The data that is passed to the driver should be copied into the DMA buffer
    copy_from_user(buf, user_buffer, writesize;

    // Initialize the urb proberly
    usb_fill_bulk_urb(urb, dev->udev, 
                  usb_sndbulkpipe(dev->udev, dev->bulk_out_endpointAddr),
                  buf, writesize, (void*)my_write_bulk_callback, dev);

    // Send the data out the bulk port
    urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;

    usb_submit_urb(urb, GFP_KERNEL);

    return writesize; 
}

// Is called when another program reads from /dev/my_usb_driver
static ssize_t my_driver_read( struct file *file, char* buffer, size_t count, loff_t* offs) 
{
    struct usb_skel* dev = file->private_data;
    int retval = 0;

    // Check that we have data to read
    // … 

    usb_fill_bulk_urb(dev->bulk_in_urb, dev->udev, 
                  usb_rcvbulkpipe(dev->udev, dev->bulk_in_endpointAddr),
                  dev->bulk_in_buffer, 
                  min(dev->bulk_in_size, count), read_bulk_callback, dev);  

    retval = usb_submit_urb(dev->bulk_in_urb, GFP_KERNEL);

    // If the read was succesful, copy the data to user space
    copy_to_user(buffer, dev->bulk_in_buffer, count);

    return retval;
}

Answer 1

USB 只是一个传输层。存储设备一般都实现SCSI协议(protocol)。创建一个 SCSI 命令来读取或写入用户空间发送的数据。然后为 SCSI 命令创建 URB 并将其发送到 USB 设备。

SCSI 是一个庞大的协议(protocol)，对于学习 USB 设备驱动程序开发，最好从简单的设备开始，例如 USB 转串口设备。