On Tue, 20 Feb 2007, Guido Körber wrote:
> OK, I am starting to understand where that comes from.
>
> First of all USB does not allow to send two reports in a single
> transfer.
That's right. Section 8.4 of the USB HID specification explicitly says:
Only one report is allowed in a single USB transfer.
However vendors do not always obey the specification.
> The effect that was described in the forum is likely something that
> happens in the buffers of the host controller. Since he writes that
> he had to ignore interrupts for a while to see it this situation
> likely only arises if another transfer arrives before the last one
> has been serviced.
They would still be presented as two separate transfers. The host
controller drivers are very careful about keeping different transfers
apart.
> I would suspect that there is some problem in the host controller
> driver that concatenates reports under certain conditions.
No. In fact, the driver in question had requested a single packet
transfer.
Much more likely is that the delay caused by ignoring interrupts gave the
keyboard time to accumulate more events, and the keyboard combined the
reports into a single packet. Without seeing the actual data, though, we
can only speculate about the real cause.
Alan Stern
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