Re: [Patch] Increase USBFS Bulk Transfer size

Alan Stern
2011-10-15T19:04:30Z

On Sat, 15 Oct 2011, Johannes Stezenbach wrote:

> What I meant to say is Markus' statement that the device only
> works at a certain transfer size cannot be true since
> this size is not visible to the device via the USB bus.

That's what I would expect, too. But did you take a look at the usbmon
traces Markus acquired?

http://marc.info/?l=linux-usb&m=131845614819045&w=2

They aren't completely definitive because the communications between
the computer and the device _before_ the bulk transfers started were
different. However they do clearly show the device working with
24064-byte transfers and not working with 12288+11776-byte transfers.

> If you queue two URBs, one 12288 and 11776 bytes, the device
> does not see any difference to one URB with 24064. It's just not
> in the USB wire protocol.

You can argue until you're blue in the face. It won't affect the
results that Markus got in real life.

> It would make a difference if the
> device violated the spec and sent 188 byte packets. However, the
> spec says a short packet terminates the transfer. But I wonder
> if this is really the case?

The device does not send short packets. If it did, the 24064-byte
transfers would end early.

Alan Stern