Re: [libusb] Question - USBFS limits

[Tim R. <ti...@pr...>]

Matthew Giassa wrote:
> The reason I ask is because I am working with a set of four high-end
> USB3 cameras, each of which puts out a 20 megapixel image at 24bits per
> pixel (ie: 60MB per image, and each camera has to queue up at least 10
> buffers in the driver to ensure constant performance).

No, it doesn't.  600MB is 2 full seconds worth of traffic.  That's just
silly.  Microsoft's usbvideo.sys, for example, circulates exactly 2
requests.  If you can't get one request turned around and resubmitted by
the time the second completes, then it really doesn't matter how many
you have.  Sooner or later you'll run dry.


> Interestingly enough, even when I set the USBFS_MEMORY_MB parameter to
> 2000, I can only use two of these cameras at once before getting a
> LIBUSB_ERROR_IO error when I try to start queuing up buffers with the
> third camera.

Are you spreading these over several host controllers?  Are you using an
isochronous pipe?  Remember that you can only reserve 90% of the
bandwidth for isochronous and interrupt endpoints.  That means you can
only fit one maximum-bandwidth isochronous device (384MB/s) on a single
host controller.

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.