On Thu, 22 Sep 2011, Matthias Dellweg wrote: > Hi! > Usb devio assumes that the wIndex in every control message apart from > those flagged as USB_TYPE_VENDOR holds the number of the Interface > being addressed. This is for example not true for the class specific > request GET_DEVICE_ID in the printer class: > > "The high-byte of the wIndex field is used to specify the zero-based > interface index. The low-byte of the wIndex field is used to specify > the zero-based alternate setting." [1] > > In this special case it misinterpretes the alternate setting 1 for the > interface and tries to claim a nonexisting one. Therefor you won't get > the printers name. > > The patch below is a minimal approach to fix this. Maybe it should be > extended to USB_TYPE_RESERVED. Maybe there should be an extended test > that knows something about specific classes. > > What do you think? > regards Matthias > > [1] http://www.usb.org/developers/devclass_docs/usbprint11.pdf In this case, it appears that the printer class specification contradicts the USB-2.0 specification. Section 9.3.1 says (referring to the low-order five bits of bmRequestType): Requests may be directed to the device, an interface on the device, or a specific endpoint on a device. This field also specifies the intended recipient of the request. When an interface or endpoint is specified, the wIndex field identifies the interface or endpoint. And Figure 9-3 shows that when wIndex is used to specify an interface, the interface number belongs in the low-order byte, not the high-order byte. I don't think it's safe to relax the test the way you have suggested. There are too many other class-specific requests that must be prevented. Maybe an exception could be added for this one particular case. Besides, you don't want to remove the test entirely -- you want to use the high-order byte of wIndex instead of the low-order byte. The printer spec really is spectacularly bad in this respect. What happens if the printer is a composite device, and the other interface uses the same bmRequestType and bRequest values for its own class-specific purpose, but uses the low-order byte of wIndex to indicate the interface number (as it should). Then the printer wouldn't know which interface was supposed to respond to the message! Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
