usb-skeleton.c 之 13---skel_write

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)    **实际上 我们 有数据需要 写入
        goto exit;

    /* limit the number of URBs in flight to stop a user from using up all RAM */

                           **限制urb的数量，防止 一个用户用完所有的 RAM
    if (down_interruptible(&dev->limit_sem)) {
        retval = -ERESTARTSYS;
        goto exit;
    }

 

    spin_lock_irq(&dev->err_lock);
    if ((retval = dev->errors) < 0) {   ** errors  dev_skel定义  最后请求库 ？？
        /* any error is reported once */     任何错误 被报告一次
        dev->errors = 0;
        /* to preserve notifications about reset */  **保留没有 重启  注意项、通知
        retval = (retval == -EPIPE) ? retval : -EIO;
    }
    spin_unlock_irq(&dev->err_lock);
    if (retval < 0)
        goto error;

 

** 创建 urb、urb的缓冲区、将数据复制给urb

/* create a urb, and a buffer for it, and copy the data to the urb */
    urb = usb_alloc_urb(0, GFP_KERNEL);
    if (!urb) {
        retval = -ENOMEM;
        goto error;
    }

    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 */

***这个锁 使能确定  我们 不能  提交到已经运行的设备

    mutex_lock(&dev->io_mutex);
    if (!dev->interface) {        /* disconnect() was called */
        mutex_unlock(&dev->io_mutex);
        retval = -ENODEV;
        goto error;
    }

    /* initialize the urb properly */  **适当的 urb 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;   **状态
    usb_anchor_urb(urb, &dev->submitted);    **不知道 什么作用 ？？？

    /* send the data out the bulk port */  **发送数据到批量端口
    retval = usb_submit_urb(urb, GFP_KERNEL);  **urb 提交按给  usb core
    mutex_unlock(&dev->io_mutex);
    if (retval) {
        err("%s - failed submitting write urb, error %d", __func__, retval);
        goto error_unanchor;
    }

    /* release our reference to this urb, the USB core will eventually free it entirely */
    usb_free_urb(urb);         ** 释放我们对这个urb 的作用  usb core将最终完全的 释放他


    return writesize;

error_unanchor:
    usb_unanchor_urb(urb);
error:
    if (urb) {
        usb_buffer_free(dev->udev, writesize, buf, urb->transfer_dma);
        usb_free_urb(urb);
    }
    up(&dev->limit_sem);

exit:
    return retval;
}