[libusb] less data length in async bulk transfer with 2 cameras

[zheng r. <zhe...@gm...>]

Hi all,

I am developing an application using libusb with a custom USB camera 
(using CY7C68013 USB chip).

I am using USB2.0 bulk (512Byte) asynchronous transfer mode.

On the device side, the image frame size = 360960=512*705, so in the 
firmware the device will send 705 packets ended with a zero-length 
packet (totally 706 pkts). The camera streams images frame at about 30fps.

On the host side, the application code works in async mode. It 
initialize a transfer queue (e.g. 16) and submit the transfer request. 
Each transfer will request 706 packets with 5 seconds timeout setting. 
In the xfer_callback, it will check the data length and re-submit the 
transfer. I run the libusb_handle_events in main loop. ( also tried in 
an independent thread)

When running one camera, the application code works fluently without any 
problem.

But when running 2 cameras in the same computer from another terminal 
(using the same executable binary file, but will request different 
camera serial number), these two cameras will lost data. Some transfers 
are completed, but the actual transfer length is less than the expected 
length(e.g. 360852 < 360960). It seems that the 2nd camera interfere the 
data transfer, which results in the incomplete data frame of 2 cameras.

Also I tried to implement the 2 cameras instances in my code. In the 
code I submit transfers in (A-B-A-B-A-B-A-B....) mode, and use one 
xfer_callback function. In the callback function I use the different  
device_handle to identify the different camera device. 
libusb_handle_event runs in a separate thread. But in the result I got 
the same problem.

Another try is that, I defined the camera device class and declare 2 
camera objects. For each camera, I start a new thread to configure, 
submit transfer and run libusb_handle_events. But got the same problem.


1. What may be the reason of this problem?

  a. Bandwidth? I don't think so. I tested off-the-shelf USB2.0 camera 
on the same PC, 2 cameras can works fluently at 60fps for each. (the 
camera interface is USB2.0, and PC interface is USB3.0).

  b. Multi-thread?

  c. Any other possible reasons?

2. By the way, I noticed the "context" concept in libusb, but I can't 
understand it clearly. When should I use different context rather than 
default context?

  a. If I define a camera class and declare 2 camera instance in my app 
code, is it necessary to use two different context?

  b. If only one camera is used in app code, but 2 app are executed in 
different linux terminal, in this case libusb works in different 
process, so the default context can be used (i.e. using 
libusb_init(NULL)), right?

  c. In my current code implementation, I am using libusb_init(NULL) 
i.e. default context for the USB transfer. Is this may be the reason 
that caused the data loss?

I have tried a lot, but didn't find any clue to solve this problem.
Any help will be greatly appreciated.

Thanks,
Zheng