Re: Report ID problem with HID-RAW interface usage

[Xiaofan Chen <xiaofanc@gmail.com>]

On Wed, Jun 30, 2010 at 12:30 AM, Alan Ott <alan@signal11.us> wrote:

> I thought I'd be able to just go check the HID specification and be able to
> say that the report number is not sent as part of the payload for Set_Report
> devices, since it would be redundant because the Report ID is sent over in
> the wValue field.
>
> Unfortunately, I was not able to see that clearly laid out in the HID
> specification in the section on Set_Report. I did however find language
> saying "if a Report ID tag was used in the Report descriptor, all reports
> include a single byte ID prefix," in section 8.1, Report Types.
>
> I did some tests with the Windows implementation, and indeed, when numbered
> reports are used in the HID descriptor, the Windows implementation will send
> the report ID as the first byte of the payload.
>
> Based on the language in the HID spec, and the behavior of Windows, I
> believe the buf+1, count-1 to be a bug.
>

Yes I think it is a bug.

And the HID specification states:
"If a Report ID tag was used in the Report descriptor, all reports
include a single byte ID prefix. If the Report ID tag was not used, all
values are returned in a single report and a prefix ID is not included
in that report."

My understanding is like the following and testing confirms it.
1) when HID device has explicit report IDs, then the report IDs will
be transmitted on the bus.
2) when the HID device does not use report ID at all (implicit report ID 0),
then this report ID 0 is not transmitted on the bus

Reference:
http://www.microchip.com/forums/tm.aspx?m=494711
http://libusb.6.n5.nabble.com/Missing-first-byte-when-reading-HID-Feature-Reports-with-ID-td9460.html
(we spent quite some efforts to get the libusb-1.0 Windows native
HID backend to deal with report ID correctly).

-- 
Xiaofan http://sourceforge.net/projects/libusb-win32/