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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] chardev: add baud parameter for serial host device
Date: Sat, 15 Jun 2013 16:38:51 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

On 06/15/2013 04:25 PM, Peter Wu wrote:
> On Saturday 15 June 2013 16:14:23 Eric Blake wrote:
>> On 06/08/2013 10:49 PM, 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. Documentation is updated as well, so that
>>> users know about the new `baud` parameter for `-serial` and `-chardev
>>> serial` (and its alias `-chardev tty`).
>>> Note that the baud option is not implemented for Windows. QEMU does not
>>> change the default baud rate on Windows anyway. If somebody is going to
>>> implement it, do not forget to update the documentation on "COM" devices
>>> which is also of backend serial.

>>> -                                       'serial' : 'ChardevHostdev',
>>> +                                       'serial' : 'ChardevSerial',
>> without introspection, libvirt has no idea whether 'baud' is supported
>> in the qemu it is talking to, other than trying and failing when talking
>> to older qemu.  This patch forms yet another reason why libvirt wants to
>> learn when we add optional parameters to a pre-existing QMP command.
> I have do not use libvirt, do you want me to do further things for this patch 
> to get accepted? Or is it more a side-note?

I'm wondering if there is an alternative that we can come up with that
doesn't require the addition of an optional parameter, and therefore
something that libvirt could reliably use without needing Amos'
introspection support to go in first.  Besides, weren't some of the
other review comments in this thread questioning whether we should even
allow users to set baud rate at creation, or should we be fixing the
real underlying bug of not matching bare-metal hardware that always
initializes to a fixed rate of 9600?  That is, I think there are still
unanswered questions that must be resolved before we can even decide
whether an amended version of this patch is even worth including.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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