Re: [Libusb-devel] Receiving interrupt transfer packets

[Nikolai K. <sp...@gm...>]

On 07/20/2010 11:05 PM, Alan Stern wrote:
> Don't use a _large_ buffer -- use a buffer that is the maxpacket size.
Aha. I will. Although it seems I already did, but I did try a lot and may be
wrong :)

> Of course, the problem is that you generally won't know when a report
> is finished.  Fortunately it looks like you don't really care, so long
> as all the data can be dumped.
Well, I'm not sure if I care or not. I should consider the kernel behavior.
If it uses only the report descriptor to determine the size of the reports
then it should be alright. OTOH, if the device breaks the HID protocol and
terminates the reports incorrectly will it ever work?

Also, the HID specification says the following among other report
constraints:
     ...
     All reports except the longest which exceed wMaxPacketSize for the
     endpoint must terminate with a short packet. The longest report 
does not
     require a short packet terminator.
     ...
     If there are multiple reports in a top level collection then all
     reports, except the longest, must terminate with a short packet.
     ...

     - http://www.usb.org/developers/devclass_docs/HID1_11.pdf
       HID spec, 8.4 Report constraints, page 67 (57).

Does it mean that if there is only one report (i.e. no report IDs in the
descriptor) or the report being sent is the longest one the terminating
short packet is not necessary?

So if the device sends only one report which is exactly wMaxPacketSize
(which, I guess, is typically the case for mice) there will be no short
packets. Am I right?

I should just capture a trace and check it, I guess :)

Sincerely,
Nick