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

[Peter Wu]

On Monday 10 June 2013 15:28:32 Peter Wu wrote:
> 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  sides.
> 
> 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.

Aha, I just checked that machine again and realised something. The serial 
cable has only four pins connected to a printer (DB9-DB25), namely RX, TX, DTR 
and RTS. The remaining cables are cut (originally it was a null modem cable, 
but that did not work with the printer).

So, what is the likely issue here? Having a printer instead of a serial 
console or the hardware (cables) missing some lines? FYI, when I set the speed 
and options manually after starting QEMU, the printer(s) work(s) as expected, 
otherwise I get garbage out.

Regards,
Peter