Re: USB 1.1 Full Speed OHCI slow/high latency

[ValdikSS <iam@valdikss.org.ru>]

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On 14.08.2025 7:40 PM, Alan Stern wrote:
> Yes, but I can't imagine why delays would cause a NOTREADY, OFFLINE, or
> UNDERFLOW error.

The laser printers roll the paper with the constant speed, they don't 
slow down when they have insufficient data.

If the host doesn't send the image data in time (and here it happens 
during the print of a single page just as it rolls, as it is too large 
to fit into the printer's RAM), the job is marked as failed due to 
underflow of print data.

It's similar to CnC device in that sense. Modern printers wait for the 
full page, but this printer is from the era when RAM was expensive and 
could not (reliably) fit the whole page.


> Either .pcap files or the usbmon text output for both of these tests
> would be good.  But set the number of iterations to something very
> small, like 10 or so.  No point posting a log containing thousands of
> repetitions of the same information...

Here you are, check the attachment. It's a 30 times loop.

Also tried the speedtest.py on an old HP T510 thin client (x86, VIA 
chipset, UHCI), and the speeds are fine.

Avg delta: 0.990 ms
Min: 0.870 ms
Max: 1.984 ms

My guess it's OHCI to blame.

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