Re: FW: other_speed_configuration string

[Xiaofan C. <xia...@gm...>]

On Thu, Dec 13, 2012 at 5:57 AM, Walker, Richard <Wal...@co...> wrote:
>> Are you fetching descriptor type 7, or are you fetching string (type 3)
>> descriptor number 7?  Those are two different things.  Rather than
>> subvert the meaning of the actual OTHER_SPEED_CONFIGURATION descriptor...
>
> I'm not sure.  I know that the value we need is an ascii-encoded string,
> so I suspect it will be a string type.  I've included the technical description
> from the data sheet, so that maybe the implementation will be obvious to
> someone more familiar with the USB protocol.
>
> Currently the debug info that I need to access is described in the data sheet as:
>
> 2.3.5.17: String Descriptor for Other Speed Configuration String
>
>    offset    field                  value in byte order
>    --------  -------                ------------------------------
>       0      bDescLength            0x4e
>       1      bDesciptorType         0x03
>             2            wLangID                               debug information encoded ascii string
>
> The ascii string uses a base 32 ascii encoding which uses the
> characters from '@'-'O' to encode
> the received optical power and some of the bit error rate registers.

I think you are confusing the "Other Speed Configuration descriptor"
(which is descriptor type 7) with the "String Descriptor for Other Speed
Configuration" (which is probably a device specific string descriptor,
descriptor type 3).
Ref: http://www.usbmadesimple.co.uk/ums_4.htm

> Is this enough info to determine if libusb can recover this information?
> The data sheet is quite detailed, so I can probably find any other required
> information that might be needed.
>
> I've got the device identified, with a valid handle and device structure.
> What functions would I use next to access this information?  If it helps,
> I can post the code I've written so far.

For "String Descriptor for Other Speed Configuration" (descriptor type 3), you
should be able to use libusb_get_string_descriptor_ascii(). What you need
to find out is the index of this string descriptor.

http://libusb.sourceforge.net/api-1.0/group__desc.html#gaf3f92d0a7465d49a5e61eb3f8689fae4
int libusb_get_string_descriptor_ascii	(libusb_device_handle *  dev,
                                                         uint8_t 	desc_index,
                                                         unsigned char * 	data,
                                                         int 	length
                                                        )		
Retrieve a string descriptor in C style ASCII.

Wrapper around libusb_get_string_descriptor(). Uses the first
language supported by the device.

Parameters:
    dev	a device handle
    desc_index	the index of the descriptor to retrieve
    data	output buffer for ASCII string descriptor
    length	size of data buffer
Returns:
    number of bytes returned in data, or LIBUSB_ERROR code on failure

-- 
Xiaofan