Re: [PATCH] MIPS: Boston: Fix earlycon baud rate selection

[Matt Redfearn]

Hi Paul,


On 25/10/17 23:09, Paul Burton wrote:
> Hi Matt,
>
> On Tuesday, October 17, 2017 at 6:28 AM Matt Redfearn wrote:
> > During set up of the early console, the earlycon driver will attempt to
> > configure a baud rate, if one is set in the earlycon structure.
> > Previously, of_setup_earlycon left this field as 0, ignoring any baud
> > rate selected by the DT. Commit 31cb9a8575ca ("earlycon: initialise baud
> > field of earlycon device structure") changed this behaviour such that
> > any selected baud rate is now set. The earlycon driver must deduce the
> > divisor from the configured uartclk, which of_setup_earlycon sets to
> > BASE_BAUD. MIPS generic kernels do not set BASE_BAUD (there is no
> > practical way to set this generically for all supported platforms), so
> > when the early console is configured an incorrect divisor is calculated
> > for the selected baud rate, and garbage is printed to the console during
> > boot.
> >
> > Fix this by removing the configured baud rate from the device tree.
> > This causes the early console to inherit the baud rate settings from the
> > bootloader. By the time the real console is probed, the clock drivers
> > necessary to calculate the divisor are enabled and the kernel can
> > correctly configure the baud rate.
> Sadly I think this breaks the proper console - my current understanding
> is that we end up with it set to 9600 baud due to the defaults in
> serial8250_console_setup(). So with your patch I see correct output from
> the early console, then nothing when the proper console registers until
> my userland starts a getty on ttyS0 which reconfigures it to 115200 baud.

As long as you specify "console=ttyS0,115200" on the command line, then 
the correct baud rate gets applied. I know that's not ideal...

(CC'ing Eugene & Greg KH for other ideas to fix this)

Thanks,
Matt

> Thanks,
>      Paul
>
> > Fixes: 31cb9a8575ca ("earlycon: initialise baud field of earlycon device
> > structure")
> > Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxx>
> >
> > ---
> >
> >   arch/mips/boot/dts/img/boston.dts | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/boot/dts/img/boston.dts
> > b/arch/mips/boot/dts/img/boston.dts
> > index 53bfa29a7093..179691aae7d7 100644
> > --- a/arch/mips/boot/dts/img/boston.dts
> > +++ b/arch/mips/boot/dts/img/boston.dts
> > @@ -11,7 +11,7 @@
> >   	compatible = "img,boston";
> >
> >   	chosen {
> > -		stdout-path = "uart0:115200";
> > +		stdout-path = "uart0";
> >   	};
> >
> >   	aliases {
> > --
> > 2.7.4