Re: USB 1.1 Full Speed OHCI slow/high latency

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, Aug 15, 2025 at 06:53:33PM +0300, ValdikSS wrote:
> Thanks Alan, this definitely makes things better
> 
> Before the patch:
> 
> Avg delta: 1.916 ms
> Min: 1.443 ms
> Max: 2.891 ms
> 
> After the patch:
> 
> Avg delta: 0.944 ms
> Min: 0.722 ms
> Max: 6.904 ms

I guess that 6.9 ms maximum occurred because the test process was 
preempted.  Not the sort of thing you can easily prevent.

> However the printout is still not in full. Now the same picture prints about
> 4/5 of the page, but still not 100% (previously it failed on 1/3).
> 
> Would the .pcap of the printing process be useful?

I rather doubt it.  You might be able to figure something out by looking 
at the timing of the various transfers, particularly near the end when 
the underflow occurred.  But I don't even know what the printer's timing 
requirements are (and maybe you don't either).

Overall this probably isn't a battle worth fighting.  Either just attach 
the printer to a high-speed hub or get a better printer.

Alan Stern