linux-kernel - Re: questions about 8250 uart support for adhoc boards

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Message-ID: <20070227173844.GA10380@flint.arm.linux.org.uk>
Date:	Tue, 27 Feb 2007 17:38:45 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Rob Prowel <prowel@...hera.com>
Cc:	Stuart MacDonald <stuartm@...necttech.com>,
	linux-kernel@...r.kernel.org
Subject: Re: questions about 8250 uart support for adhoc boards

On Tue, Feb 27, 2007 at 09:24:32AM -0500, Rob Prowel wrote:
> Russell King wrote:
> >On Mon, Feb 26, 2007 at 12:02:57PM -0500, Rob Prowel wrote:
> >  
> >>See below for a (script) dump of that demonstrates the behavior I 
> >>describe.  Additional ports are not configurable until a driver such as 
> >>fourport is loaded.  This is in 2.6.20.1 and 2.6.17 (those versions I 
> >>have tested).
> >>    
> >
> >Check what /proc/tty/driver/serial contains, rather than relying on
> >setserial.  setserial distributed by distros hasn't kept up with the
> >changes in the serial layer.
> >
> >  
> The setserial being used is the latest one from sourceforge.  Its 
> relevant output matches the /proc entries.  The problem still remains 
> though.  Entries for ttyS4 and up seem to be read-only until I add a 
> supplemental driver such as fourport.

After experimenting here, it turns out the reason is you're trying to
configure a port with a zero base baud.  Unfortunately, it starts off
as zero.

However, when you try to change a port without specifying the base baud,
it is possible for the port to become "usable" and unless we enforce
the "must not have zero base baud" rule we have the possibility to oops
the kernel via a divide by zero.

So, the answer is to specify the base baud when using setserial to
configure new ports.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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