Re: [Libusb-devel] r102 and linux

[Alan S. <st...@ro...>]

On Wed, 27 Jan 2010, Pete Batard wrote:

> /usr/src/libusb-pbatard# examples/xusb -k
> Opening device...
> 
> Reading device descriptor:
>              length: 18
>        device class: 0
>                 S/N: 3
>             VID:PID: 0BDA:0156
>           bcdDevice: 5195
>     iMan:iProd:iSer: 1:2:3
>            nb confs: 1
> 
> Reading configuration descriptors:
>               nb interfaces: 1
> interface[0].altsetting[0]: num endpoints = 2
>     Class.SubClass.Protocol: 08.06.50
>         endpoint[0].address: 01
>             max packet size: 0200
>            polling interval: 00
>         endpoint[1].address: 82
>             max packet size: 0200
>            polling interval: 00
> 
> Claiming interface 0...
>     Failed. Trying to detach driver...
>     Claiming interface again...
> 
> Reading string descriptors:
>     String (1/3): "Generic"
>     String (2/3): "USB2.0-CRW"
>     String (3/3): "20060413092100000"
> Reading Max LUN:
>     Max LUN = 0
> Sending Inquiry:
>     sent 6 CDB bytes
>     Pipe error
>     Input/output error

Okay, there are two problems.

First, I was wrong to say that the transfer length should be 
sizeof(cbw).  I had forgotten that the structure would be padded to a 
4-byte boundary.  The transfer length must always be exactly 31 bytes.

Second, when reading the INQUIRY data you have to be prepared to handle
a STALL.  (In fact, you should be prepared for this during any IN data
transfer, but it's more likely to occur during an INQUIRY than during
other commands.)  The device may present a Unit Attention, telling you 
that it was just reset, for example.  When this happens you have to 
clear the STALL and go straight ahead to the status phase of the 
transfer.  In the case of INQUIRY you might then want to try repeating 
the command.

Alan Stern