Re: USB 1.1 Full Speed OHCI slow/high latency

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

On Fri, Aug 15, 2025 at 09:13:09PM +0300, ValdikSS wrote:
> I *buy* these legacy devices to improve them in Linux :)
> 
> I'm making an open-source print server hardware for older printers, that's
> why for me this issue is worth researching, at least to document such
> hardware heisenbugs.
> There've been different USB behavior and incompatibilities with scanners
> (fixed it in SANE) and printers (fixed by CUPS quirks file), but this is the
> first time I'm going down to USB HC level.
> 
> This printer (Canon LBP1120) does not have drivers for 64-bit Windows or
> recent macOS, that's why for many it's a paperweight device you can't use or
> sell, unless fixed by other means (usually a VM with 32-bit Windows, and now
> my print server as well).
> 
> You gave me directions where I should look for, because I didn't know how to
> start debugging without the logic analyzer.
> Usbmon captures only USB communication packets and some of USB HUB
> controlling packets (such as TT related), but does not include
> device-controlling MMIO registers, which was the case here.
> 
> I'll add debug prints to everything related to MMIO and take a look what
> could be improved. Thanks!

Good luck!

> If you have other ideas I could try right away, please share them!

You might compare the usbmon traces for your patched OHCI against a UHCI 
system.  They should run at approximately the same speed, so it's not 
clear why the printer would work with one but not the other.

Alan Stern