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Subject: Re: What is wrong with FreeBSD and USB Support
To: Markus Rechberger <mrechberger@gmail.com>, freebsd-multimedia@freebsd.org
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From: Hans Petter Selasky <hps@selasky.org>
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Date: Mon, 6 Mar 2017 11:30:26 +0100
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On 03/06/17 09:53, Markus Rechberger wrote:
> Just to clarify this issue it's hardware and software related.
>
> In the linux libusb20 wrapper there's some code in it in the bulk
> callback function
>
> libusb20_tr_submit(xfer)
> if (xfer == xfer0)
>    xfer = xfer1
> else
>    xfer = xfer0
>
> libusb20_tr_start(xfer)
>
> why should someone toggle the other transfer and not the same one that
> is returning or required to set up?

Hi,

This is to be able to setup double-buffering of USB transfers.

> I would like to set up multiple requests at the same time and not only
> 2 of them (since 2 of them are not fast enough in our case).

Do do this correctly for a BULK endpoint, allocate two USB transfers, 
and then subdivide these two using the FreeBSD USB frames feature.

Divide 1x USB transfers into 32x frames which then equals 32 transfers 
in one completion event. Linux/MacOS/Windows doesn't have this feature 
and instead they allow setting up a bunch of wMaxPacketSize sized jobs 
which then in turn generate an enormous completion event traffic, which 
is not very intelligent.

> This is about resolving an active bug and misconception of the freebsd
> userspace usb API usage.
> Some hardware just need a lower latency.

--HPS