[USB] Types of data transfers

OUT : Host computer --> USB device

IN : USB device --> Host computer

 

USB endpoint 의 종류

1. CONTROL : (Asynchronous) control endpoint 는 USB device에 access하기 위해서 사용됨
1. 예를 들어 configure device, retrieving information from device, sending command to the device, retrieving status reports about the device
2. 모든 USB device 는 CONTROL에 해당하는 "endpoint 0"를 가짐
1. USB device가 insert 될 때 사용됨
2. guaranteed by the USB protocol

2. INTERRUPT : (Periodic) transfer small amounts of data at a fixed rate every time
1. ex) keyboard, mouse
2. guaranteed by the USB protocol

3. BULK : (Asynchronous) Bulk endpoints transfer large amounts of data
1. Interrupt endpoint 보다는 많은 data를 가짐
2. data loss 없이 많은 data를 전달하고자 할 때 사용됨
3. data 양이 많을 때 multiple transfers로 나눠질 수도 있음
4. ex) printer, storage, network devices

4. ISOCHRONOUS : (Periodic) large amount of data 를 transfer 함, data 에 대한 guarantee가 없음
1. device can handle loss of data, real-time 성 장치에 유리
2. ex) audio, video 장치

Control Transaction

- This is initiated by the host via an appropriately coded token packet with information regarding the control command being issued.

- The recipient either replies with an ACK handshake, or ignores the token totally.

- This type of transfer also contains an optional unidirectional data transfer between the host and the device.

- Lastly, a status is returned to whatever end of the pipe was last to transmit.

Interrupt Transaction

- This function is queried by the host via a token packet,

- Return data if the has any interrupt related information to transfer, or NAK handshake if it doesn't

Bulk Transfers

- When the host wants to initiate a bulk transfer from a device, it sends a token packet specifying whether input or output is desired.

- (OUT) If the host wants input, the function in turn sends either a packet of data or a non-ACK handshake.

- (IN) If the host is sending a request to transmit, it will immediately send the data packet and wait for an appropriate handshake to be returned by the recipient.

Isochronous Transaction

- begin with an appropriate token packet being sent to the function, 

- and then data is transmitted as required.

- Due to the assumed error-tolerance of isochronous data, no handshake packets are used.

QEMU Implementation

- for CONTROL

void usb_host_handle_control(udev, p, request, value, index, length, *data) @ hw/usb/host-libusb.c

switch(request) {

     case  DeviceOutRequest | USB_REQ_SET_ADDRESS:

...

case DeviceOutRequest | USB_REQ_SET_CONFIGURATION:

...

case InterfaceOutRequest | USB_REQ_SET_INTERFACE:

...

case EndpointOutRequest | USB_REQ_CLEAR_FEATURE:

...

- for BULK, INTERRUPT, ISOCHRONOUS transaction

void usb_host_handle_data(udev, p) @ hw/usb/host-libusb.c

switch (usb_ep_get_type(udev, p->pid, p->ep->nr)) {

    case USB_ENDPOINT_XFER_BULK:

   …

    case USB_ENDPOINT_XFER_INT:

  …

    case USB_ENDPOINT_XFER_ISOC:

  …

reference: 

[1] Linux Device Drivers , Chapter 13. USB Drivers

[2] http://www.geoffknagge.com/uni/elec101/essay.shtml

[3] http://www.ftdichip.com/Support/Documents/TechnicalNotes/TN_116_USB%20Data%20Structure.pdf