[Libusb-devel] Asynchronous reads with libusb1.0-windows backend

[Domien N. <dom...@gm...>]

Hello all,

I'm using the windows backend of libusb1.0, with the WinUSB driver, and 
I'm trying to get better performance by submitting multiple bulk 
transfers at once.
My USB device is currently setup to send 4x512-byte packets and a single 
256-byte packet at once (and in that order) to the host, followed by a 
delay of a second, and then followed by a single 512-byte packet.

On the host, I have submitted 16 asynchronous transfers at once, each 
with a buffer capacity of 256K. Now my observations are as follows:
I get one transfer callback with an actual_length of 2304 bytes, 
followed by... nothing. My app just keeps idling and waiting for more 
packets since it keeps printing a "Tick" message to the screen every 
100ms after handling libusb events. No errors are reported by libusb 
whatsoever.

My USB device also reports it has succesfully sent 2816 bytes, and for 
those still listening it means that last 512-byte packet has gone into 
limbo on the host as far as libusb (or WinUSB) is concerned.
And, when I repeat the same test with a *synchronous* read, then libusb 
returns successfully with *all* 2816 bytes. Pretty weird.

So my question, is this the proper behavior of libusb?
I was under the impression that the buffer length passed in the libusb 
transfer was a maximum buffer size, and anything received smaller than 
that will get passed back to the application too.
I'd like to know how I can get that last 512-byte packet out with libusb 
through asychronous transfers.

Thanks in advance.
Domien Nowicki.