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SubjectRe: [PATCH] enable usb control message with class specific request
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



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