Re: USB Interrupt Transfer limited to 1 packet?

On Thu, Feb 01, 2007 at 11:19:43AM -0500, Alan Stern wrote:
> >  My device has an interrupt endpoint that
> > delivers 122 bytes every 10 ms.  I have written a simple program that
> > uses libusb to read the data, and it produces two interrupts for each
> > 122-byte packet (as observed by looking at /proc/interrupts).
> 
> I don't know why there are two interrupts; there should only be one.  
> Still, an average of 200 interrupts per second isn't excessively high.

There is quite a bit of CPU work in analyzing that data in real-time
and displaying it, so we are trying to optimize all we can.

> > Looking at the code, it seems that the interrupt transfers are limited 
> > to 1 packet?  There is a comment to that effect at line 1470 in 
> > drivers/usb/usb-uhci.c (function is uhci_submit_int_urb):
> > 
> >    // INT allows only one packet
> >    if (urb->transfer_buffer_length > usb_maxpacket (urb->dev, pipe, usb_pipeout (pipe)))
> >       return -EINVAL;
> > 
> > The same text is present in linux-2.4.34 as well.
> > 
> > Code to the same effect is present also in uhci.c (function
> > uhci_submit_interrupt) but without the comment.
> > 
> > I have tried to find the same check in the linux-2.6.20-rcX but could
> > not find it.
> 
> Because it isn't there.  That limitation has been removed in 2.6.

Good!

> > I have read the Intel 82801E manual (this is the chipset that I am
> > using) and the USB 2.0 spec (section 8.5.4 in particular, in contrast
> > to 8.5.2) but I did not find a hard reference to this limitation of a
> > single data packet per interrupt transfer.  Do you have a hard
> > reference to this limitation and can you send me a link or a
> > document/section number?
> 
> It isn't a hardware limitation; it's a software driver limitation.  So you
> won't find it in any hardware manuals.

Does that mean that I can just remove the check and everything should
work?  Do you know if there is anything that assumes the check
upstream and might break if removed?  I will certainly try it on my
end and report my findings.

Thank you,
florin

-- 
Bruce Schneier expects the Spanish Inquisition.
      http://geekz.co.uk/schneierfacts/fact/163

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