Re: [Libusb-devel] Receiving interrupt transfer packets

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

On Tue, 20 Jul 2010, Nikolai Kondrashov wrote:

> 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?

Probably not.  In fact, it probably won't even work under Windows.

> 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?

That's right.

> 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?

Yes.

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

Good idea.

Alan Stern