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Re: [Qemu-devel] [PATCH] chardev: add baud parameter for serial host dev

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH] chardev: add baud parameter for serial host device
Date: Mon, 10 Jun 2013 14:58:07 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130513 Thunderbird/17.0.6

On 06/10/13 10:42, Peter Wu wrote:
> On Monday 10 June 2013 07:56:01 Gerd Hoffmann wrote:
>> On 06/08/13 23:49, Peter Wu wrote:
>>> When QEMU starts, it always changes the serial port parameters including
>>> baud rate. This confused my guest which thought it was outputting at 9600
>>> baud while it was in fact changed to 115200.
>>> After this patch, I can use `-serial /dev/ttyS0,baud=9600` to override the
>>> default baud rate of 115200.
>> I think we should just flip the default to 9600.  IIRC this is the
>> power-on default baud rate of the 8250 uart family, so this should be
>> the qemu default too.  If a guest wants to use a higher baudrate it has
>> to reprogram the uart anyway (and qemu will apply the guest changes to
>> the host uart).
> FWIW, when I tried MODE.COM in ms-dos to change the baud rate, `stty -F 
> /dev/ttyS0 -a` still reported 115200 baud. This is on Linux 3.9 if that
> matters.

Hmm, with a linux guest changing the baudrate works just fine.  Any
chance mode.com takes a shortcut in case it thinks the rate didn't
change?  Does setting the speed first to 4800, then to 9600 work?

> Besides this comment, any other feedback on the patch itself?

Style is fine.  But it appears to paper over some bug, and I'd prefer to
find+fix the bug instead of allowing/requiring the user to set the baud
rate manually to the correct value.


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