On Fri, 29 Jun 2007, jidong xiao wrote:
> Sorry I cannot find this in usb 2.0 spec.
It isn't in the USB 2.0 spec. That's why you can't find it. :-)
> But inside the code,drivers/usb/core/hub.c:
> 103 /* USB 2.0 spec Section 11.24.4.5 */
> 104 static int get_hub_descriptor(struct usb_device *hdev, void
> *data, int size)
> 105 {
> 106 int i, ret;
> 107
> 108 for (i = 0; i < 3; i++) {
> 109 ret = usb_control_msg(hdev, usb_rcvctrlpipe(hdev, 0),
> 110 USB_REQ_GET_DESCRIPTOR, USB_DIR_IN |
> USB_RT_HUB,
> 111 USB_DT_HUB << 8, 0, data, size,
> 112 HZ * USB_CTRL_GET_TIMEOUT);
> 113 if (ret >= (USB_DT_HUB_NONVAR_SIZE + 2))
> 114 return ret;
> 115 }
> 116 return -EINVAL;
> 117 }
> what does this macro mean? And why here we loop three times?
The macro is defined in drivers/usb/core/hub.h. It refers to the
length of the non-variable part of the hub descriptor. The parts of
the descriptor following the first 7 bytes will vary according to the
number of ports.
We loop 3 times in case a communication occurs. Lots of code in the
hub driver does this; many devices have problems sending their
descriptors.
Alan Stern
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