Linux USB驅(qū)動(dòng)框架分析（四）

????probe是usb子系統(tǒng)自動(dòng)調(diào)用的一個(gè)函數(shù)，有USB設(shè)備接到硬件集線器時(shí)，usb子系統(tǒng)會(huì)根據(jù)production ID和vendor ID的組合或者設(shè)備的class、subclass跟protocol的組合來識(shí)別設(shè)備調(diào)用相應(yīng)驅(qū)動(dòng)程序的probe（探測）函數(shù)，對(duì)于skeleton來說，就是skel_probe。系統(tǒng)會(huì)傳遞給探測函數(shù)一個(gè)usb_interface *跟一個(gè)struct usb_device_id *作為參數(shù)。他們分別是該USB設(shè)備的接口描述（一般會(huì)是該設(shè)備的第0號(hào)接口，該接口的默認(rèn)設(shè)置也是第0號(hào)設(shè)置）跟它的設(shè)備ID描述（包括Vendor ID、Production ID等）。probe函數(shù)比較長，我們分段來分析這個(gè)函數(shù)：

dev->udev = usb_get_dev(interface_to_usbdev(interface));

dev->interface = interface;

????在初始化了一些資源之后，可以看到第一個(gè)關(guān)鍵的函數(shù)調(diào)用——interface_to_usbdev。他同uo一個(gè)usb_interface來得到該接口所在設(shè)備的設(shè)備描述結(jié)構(gòu)。本來，要得到一個(gè)usb_device只要用interface_to_usbdev就夠了，但因?yàn)橐黾訉?duì)該usb_device的引用計(jì)數(shù)，我們應(yīng)該在做一個(gè)usb_get_dev的操作，來增加引用計(jì)數(shù)，并在釋放設(shè)備時(shí)用usb_put_dev來減少引用計(jì)數(shù)。這里要解釋的是，該引用計(jì)數(shù)值是對(duì)該usb_device的計(jì)數(shù)，并不是對(duì)本模塊的計(jì)數(shù)，本模塊的計(jì)數(shù)要由kref來維護(hù)。所以，probe一開始就有初始化kref。事實(shí)上，kref_init操作不單只初始化kref，還將其置設(shè)成1。所以在出錯(cuò)處理代碼中有kref_put，它把kref的計(jì)數(shù)減1，如果kref計(jì)數(shù)已經(jīng)為0，那么kref會(huì)被釋放。kref_put的第二個(gè)參數(shù)是一個(gè)函數(shù)指針，指向一個(gè)清理函數(shù)。注意，該指針不能為空，或者kfree。該函數(shù)會(huì)在最后一個(gè)對(duì)kref的引用釋放時(shí)被調(diào)用（如果我的理解不準(zhǔn)確，請(qǐng)指正）。下面是內(nèi)核源碼中的一段注釋及代碼：

/**

?* kref_put - decrement refcount for object.

* @kref: object.

?* @release: pointer to the function that will clean up the object when the

?*?? ???? last reference to the object is released.

?*?? ???? This pointer is required, and it is not acceptable to pass kfree

?*?? ???? in as this function.

?*

?* Decrement the refcount, and if 0, call release().

?* Return 1 if the object was removed, otherwise return 0.? Beware, if this

?* function returns 0, you still can not count on the kref from remaining in

?* memory.? Only use the return value if you want to see if the kref is now

?* gone, not present.

?*/

int kref_put(struct kref *kref, void (*release)(struct kref *kref))

{

???? WARN_ON(release == NULL);

???? WARN_ON(release == (void (*)(struct kref *))kfree);

? ????/*

???? ?* if current count is one, we are the last user and can release object

???? ?* right now, avoiding an atomic operation on 'refcount'

???? ?*/

???? if ((atomic_read(&kref->refcount) == 1) ||

???? ??? (atomic_dec_and_test(&kref->refcount))) {

???????? release(kref);

???????? return 1;

???? }

???? return 0;

}

????當(dāng)我們執(zhí)行打開操作時(shí)，我們要增加kref的計(jì)數(shù)，我們可以用kref_get，來完成。所有對(duì)struct kref的操作都有內(nèi)核代碼確保其原子性。

????得到了該usb_device之后，我們要對(duì)我們自定義的usb_skel各個(gè)狀態(tài)跟資源作初始化。這部分工作的任務(wù)主要是向usb_skel注冊(cè)該usb設(shè)備的端點(diǎn)。這里可能要補(bǔ)充以下一些關(guān)于usb_interface_descriptor的知識(shí)，但因?yàn)閮?nèi)核源碼對(duì)該結(jié)構(gòu)體的注釋不多，所以只能靠個(gè)人猜測。在一個(gè)usb_host_interface結(jié)構(gòu)里面有一個(gè)usb_interface_descriptor叫做desc的成員，他應(yīng)該是用于描述該interface的一些屬性，其中bNumEndpoints是一個(gè)8位（b for byte）的數(shù)字，他代表了該接口的端點(diǎn)數(shù)。probe然后遍歷所有的端點(diǎn)，檢查他們的類型跟方向，注冊(cè)到usb_skel中。

???? /* set up the endpoint information */

???? /* use only the first bulk-in and bulk-out endpoints */

???? 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 &&

?????? ?????? ((endpoint->bEndpointAddress & USB_ENDPOINT_DIR_MASK) = = USB_DIR_IN) &&

???????? ????((endpoint->bmAttributes & USB_ENDPOINT_XFERTYPE_MASK) = = USB_ENDPOINT_XFER_BULK)) {

????????????? /* we found a bulk in endpoint */

????????????? buffer_size = le16_to_cpu(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) {

?????????????????? err("Could not allocate bulk_in_buffer");

?????????????????? goto error;

????????????? }

?????? }

???????? if (!dev->bulk_out_endpointAddr &&

???? ??? ?? ((endpoint->bEndpointAddress & USB_ENDPOINT_DIR_MASK)= =USB_DIR_OUT) &&

???????? ????? ((endpoint->bmAttributes & USB_ENDPOINT_XFERTYPE_MASK)= = USB_ENDPOINT_XFER_BULK)) {

????????????? /* we found a bulk out endpoint */

????????????? dev->bulk_out_endpointAddr = endpoint->bEndpointAddress;

???????? }

???? }

???? if (!(dev->bulk_in_endpointAddr && dev->bulk_out_endpointAddr)) {

???????? err("Could not find both bulk-in and bulk-out endpoints");

???????? goto error;

???? }