Re: [PATCH 0/3] ARM: dts: stm32: lxa: change stdout-path baud rate from 9600 to 115200

[David Laight <david.laight.linux@gmail.com>]

On Mon, 15 Jun 2026 09:53:49 +0200
Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:

> Hello David,
> 
> On 6/12/26 8:53 AM, David Laight wrote:
> > On Thu, 11 Jun 2026 22:33:18 +0200
> > Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
> >   
> >> Hi David,
> >>
> >> On 6/11/26 21:43, David Laight wrote:  
> >>> On Thu, 11 Jun 2026 20:12:32 +0200
> >>> Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
> >>>     
> >>>> The LXA boards are the only STM32 boards that set stdout-path = &uart*
> >>>> instead of explicitly specifying a baud rate.
> >>>>
> >>>> This would mean the default of 9600 is used, but it goes unnoticed when
> >>>> booting normally as barebox fixes up a console= line that includes a
> >>>> baud rate.
> >>>>
> >>>> When EFI booting GRUB however, GRUB will not pass along the console=
> >>>> line and thus the board ends up with a 9600 baud Linux console,
> >>>> confusing users.    
> >>>
> >>> Is it possible to determine the current baud rate (by reading the hardware
> >>> register) and default to that value.
> >>> Then if grub has initialised the uart the kernel will use the same
> >>> baud rate.    
> >>
> >> I think so, yes. In addition to the register divider configuration, one
> >> would need the input clock rate as well, but that's not a problem.
> >>
> >> Do you know if any drivers already do this?  
> > 
> > I've seen it done somewhere, certainly x86, but possibly NetBSD.
> > That would have been preserving the baud rate set by the bios.
> > You don't want the baud rate changing half way through the boot sequence.  
> 
> I agree in general, but in this case here, the BIOS defaults to 115200:

That would certainly make 115200 a better default than 9600.

	David

> 
> https://github.com/linux-automation/meta-lxatac/blob/wrynose/meta-lxatac-bsp/recipes-bsp/barebox/files/lxatac/defconfig#L171
> https://elixir.bootlin.com/barebox/v2026.06.0/source/common/console.c#L349
> 
> Cheers,
> Ahmad
> 
> > 
> > 	David
> >   
> >>
> >> Nevertheless, I would like the LXA device trees changed, even if only
> >> to align them with all other existing STM32 device trees.
> >>
> >> Cheers,
> >> Ahmad
> >>
> >>  
> >>>
> >>> 	David
> >>>     
> >>>>
> >>>> This series fixes this. As the device trees were added at different
> >>>> times, they are fixed each in a separate commit with its own Fixes: tag.
> >>>>
> >>>> ---
> >>>> Ahmad Fatoum (3):
> >>>>       ARM: dts: stm32: lxa-mc1: change stdout-path baud rate from 9600 to 115200
> >>>>       ARM: dts: stm32: lxa-tac: change stdout-path baud rate from 9600 to 115200
> >>>>       ARM: dts: stm32: fairytux2: change stdout-path baud rate from 9600 to 115200
> >>>>
> >>>>  arch/arm/boot/dts/st/stm32mp153c-lxa-fairytux2.dtsi | 2 +-
> >>>>  arch/arm/boot/dts/st/stm32mp157c-lxa-mc1.dts        | 2 +-
> >>>>  arch/arm/boot/dts/st/stm32mp15xc-lxa-tac.dtsi       | 2 +-
> >>>>  3 files changed, 3 insertions(+), 3 deletions(-)
> >>>> ---
> >>>> base-commit: 4549871118cf616eecdd2d939f78e3b9e1dddc48
> >>>> change-id: 20260611-lxa-stdout-path-baudrate-7cf454cdae07
> >>>>
> >>>> Best regards,
> >>>> --  
> >>>> Ahmad Fatoum <a.fatoum@pengutronix.de>
> >>>>
> >>>>    
> >>>
> >>>     
> >>
> >>  
> > 
> >   
>