有實際的usb通訊協議嗎？

我們大多數人從不考慮我們的USB裝置，我們只是把它們插進去，然後忘記它們。但是，如果你是一個好奇的型別誰想知道更多的USB如何工作？今天的超級使用者問答帖子回答了一位好奇的讀者的問題。...

我們大多數人從不考慮我們的USB裝置，我們只是把它們**去，然後忘記它們。但是，如果你是一個好奇的型別誰想知道更多的USB如何工作？今天的超級使用者問答帖子回答了一位好奇的讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

圖片由埃維爾卡·科瓦爾斯基（Flickr）提供。

問題

超級使用者讀者**eeb想知道是否真的有USB通訊協議：

According to Wikipedia, USB:

Defines the cables, connectors, and communicati*** protocols used in a bus for connection, communication, and power supply between computers and electronic devices.

But is there an actual USB Communication Protocol? My understanding is that:

You connect a USB device to a machine (Ubuntu or any kind of Linux for example).
Linux finds the device driver for that device somehow (bonus if you know!) and loads it.
The device is now connected under /dev/theDevice.
User space apps can now read and write to /dev/theDevice and the driver handles the low-level input and output to the underlying device/hardware.

As far as I can tell, a USB Communication Protocol does not appear anywhere in this flow. If my understanding is correct, the USB is just the cable and electrical connection between the PC and the device.

Am I wrong here? Does USB actually implement some kind of low-level protocol that underscores the flow above? If so, what is it and how does it work at a 30,000 foot view?

有實際的USB通訊協議嗎？

答案

超級使用者貢獻者RedGrittyBrick和projectdp為我們提供了答案。首先，紅磚：

Yes (see USB protocols).

As I understand it, the USB spec defines a complex set of layered protocols and device profiles.

For example, USB devices can conform to high-level templates like mass-storage, keyboard, human interface device, etc. and be managed by a generic device driver. Some USB devices can communicate at a lower level such that the operating system’s low level USB support can recognize that device-specific higher level drivers are needed.

接著是projectdp的回答：

Question

Is there a low-level USB communication protocol in action and what is it?

Answer

Yes, there is. The USB specification includes the USB protocol which defines the way the bus is used on a bit level. This would be the low-level protocol that underlies the higher level protocols (i.e. mass-storage, HID, etc.).

For specifics on how the USB protocol works, this OSDev wiki is helpful. Here is another interesting description using sequence diagrams to describe the various data transacti*** per the USB protocol.

Bonus Question

How does Linux find and load the device driver for that device?

Bonus Answer

On Linux when using a USB-enabled kernel, a working USB device will be detected via the hardware and kernel due to the USB specification. On the hardware side, the detection is done by the USB Host Controller. Then in the kernel the Host Controller Driver takes over and translates the low-level bits on the wire to USB protocol formatted information. This information then gets populated in the u**core driver in the kernel.

I paraphrased from this excellent Opensourceforu article, which has much more detail and clarity about your question in the Linux context.

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