linux-kernel - Re: PROBLEM: USB keyboards works only 4 per PC host port

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Date:	Tue, 19 Aug 2008 09:40:57 -0700 (PDT)
From:	juanslayton@...extreme.com
To:	"Alan Stern" <stern@...land.harvard.edu>
Cc:	"Aivils Stoss" <aivils@...net.lv>, jkosina@...e.cz,
	"lud" <linux-usb@...r.kernel.org>, linux-kernel@...r.kernel.org,
	"Randy Dunlap" <rdunlap@...otime.net>,
	linuxconsole-dev@...ts.sourceforge.net, linux-input@...r.kernel.org
Subject: Re: PROBLEM: USB keyboards works only 4 per PC host port

I have to apologize for my limited knowledge, but mere ignorance never yet
kept a teacher quiet.  I am trying to follow this discussion because
Aivils and I are  basically trying to do the same thing.  In my case I am
presently running 20 keyboards on a 2.6.26 kernel, using an EHCI
controller, and they all work fine.

If I understand your analysis, Aivils' problem is because the UHCI
controllers are too slow.  (I just happened to get lucky with another
controller.)  I am left with two questions.

1.  What if Aivils needs more than 16 keyboards?  (Standard class sizes
here in California run from 20 to about 30.)  Is there any way he can do
this without a faster controller?

2.  What does the bandwidth limitation say about the maximum number of
boards with the EHCI controller?  I modified the kernel evdev.c and
input.c files to raise the maximum events from 32 to 64, but I hadn't
figured in the effects of bandwidth.

> Now if you plugged some of these keyboards into different UHCI
> controllers on the computer, then the problem wouldn't arise.  Each of
> your four UHCI controllers has two ports.  So without using any hubs at
> all, you can plug 8 keyboards into the computer and they will all work.
>
> If you use some extra hubs as well then you can plug four keyboards
> into each controller, allowing you to use 16 keyboards total.
>
> Alan Stern
>
>
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