freeBSD usbdriver

BSD usbconfig source code:

and lib code:

sys code:

so we may get all BSD related code in this repository:

Repository Root:

关于freeBSD下的USB驱动，
在/dev/usb/storage/目录下就有文件：
umass.c， urio.c, ustorage_fs.c
尤其是umass.c和ustorage_fs.c的区别：
分别有如下代码：

static int
umass_probe(device_t dev)
{
    struct usb_attach_arg *uaa = device_get_ivars(dev);
    struct umass_probe_proto temp;

    if (uaa->usb_mode != USB_MODE_HOST) {
        return (ENXIO);
    }
    temp = umass_probe_proto(dev, uaa);

    return (temp.error);
}

和

static int
ustorage_fs_probe(device_t dev)
{
    struct usb_attach_arg *uaa = device_get_ivars(dev);
    struct usb_interface_descriptor *id;

    if (uaa->usb_mode != USB_MODE_DEVICE) {
        return (ENXIO);
    }
    /* Check for a standards compliant device */
    id = usbd_get_interface_descriptor(uaa->iface);
    if ((id == NULL) ||
     (id->bInterfaceClass != UICLASS_MASS) ||
     (id->bInterfaceSubClass != UISUBCLASS_SCSI) ||
     (id->bInterfaceProtocol != UIPROTO_MASS_BBB)) {
        return (ENXIO);
    }
    return (BUS_PROBE_GENERIC);
}

同样都是device的驱动，为什么mode不相同？求高人指点。

answer from HPS:
umass.c is providing a class HOST driver for memory sticks.
ustorage_fs.c is providing a class DEVICE driver for memory sticks, I.E. emulating a memory stick. Of course you need a host controller which supports this, typically called OTG or USB device controllers. --HPS

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