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

From: Peter Wu
Subject: Re: [Qemu-devel] [PATCH] chardev: add baud parameter for serial host device
Date: Mon, 10 Jun 2013 15:28:32 +0200
User-agent: KMail/4.10.4 (Linux/3.9.1-1-custom; KDE/4.10.4; x86_64; ; )

On Monday 10 June 2013 14:58:07 Gerd Hoffmann wrote:
> 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?

I can confirm that a Linux guest can correctly control the speed. At home I 
only have a USB serial, but that shouldn't matter.
1. stty -F /dev/ttyUSB0 reports 9600
2. Start QEMU using:
qemu-system-x86_64 -enable-kvm -m 1G -serial /dev/ttyUSB0 \
-cdrom ubuntu-12.04.1-desktop-amd64.iso
3. stty -F /dev/ttyUSB0 reports 115200 before booting the live CD
4. After boot, both the guest and host report 9600 again
5. Changing it using stty -F /dev/ttyS0 from the guest is also visible on both 

On ms-dos I observe with 1.4.1 and 1.5.0 that the baud rate *is* changed 
correctly with my USB serial converter. I will have to check with the serial 
printer again whether I made a mistake before. I did actually have to use 
`stty -F /dev/ttyS0 raw 9600` to avoid characters being eaten which caused the 
serial printer to spit out empty lines or hang.


> > 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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