Re: [Patch] Increase USBFS Bulk Transfer size

Alan Stern
2011-10-14T20:19:37Z

On Fri, 14 Oct 2011, Johannes Stezenbach wrote:

> I don't really want to help Markus with his proprietary, binary-only
> userspace driver crap, but I wonder why nobody seems to remember
> how the USB protocol works on the wire?

I remember it perfectly well.

> The transfer size is
> never seen by the device, thus it cannot matter if two small URBs
> or one large URB are queued. What matters is the packet size.

That's what I have been saying. Markus's experience contradicts this,
however.

> Apparently the device can only handle fixed size packets
> of either 188 or 2*188 byte, thus it breaks with 12288 or 11776.

No. The device expects 512-byte packets because it uses a bulk
endpoint.

> The endpoint's wMaxPacketSize might reflect this.

For high-speed devices, a bulk endpoint's wMaxPacketSize must always be
512.

> I guess a transfer size of e.g. 188*60=11280 would work.
> See the first mail of this thread.

According to Markus, with this particular device nothing but 24064
works. The discussion is a little difficult to follow because he
talked about two different devices without always being clear about
which was which.

> See also Sergei's comment in
> http://lkml.org/lkml/2011/10/12/183

Alan Stern