Question about arm64 earlycon

[Mark Rutland]

On Mon, Oct 24, 2016 at 11:17:36AM +0100, Marc Zyngier wrote:
> On 24/10/16 11:06, Arnd Bergmann wrote:
> > On Sunday, October 23, 2016 12:26:59 AM CEST Duc Dang wrote:
> >> Hi Catalin, Marc, Mark, Arnd,
> >>
> >> I am testing with 3.12 kernel with earlyprintk enabled and I see some
> >> garbage characters in the console log right before the message
> >> indicating that the real console device is initialized:

What exactly are you passing on the command line?

> >> <some garbage characters here>01c020000.serial: ttyS0 at MMIO
> >> 0x1c020000 (irq = 108, base_baud = 3125000) is a U6_16550A
> >> console [ttyS0] enabled, bootconsole disabled

Was the UART already configured by FW? Had the firmware output anything
at this point?

Did the firmware's UART rate match that of the kernel? If not, the issue
might just be that the rate doesn't match; earlycon/earlyprintk won't
configure that, while the real console will.

Or perhaps we race with some clock configuration...

> >> I looked through early_prink.c file and printk.c file and it looks
> >> like there is case that some early boot code can touch the UART
> >> hardware via ealy console driver while the 'real' console driver is
> >> setting up the same UART port? Please let me know if I missed some
> >> important piece of code that can prevent this.
> > 
> > I don't think we every supported earlyprintk on arm64, and
> > earlycon support may have been added later.
> 
> We did support some form of earlyprintk for a while (though not in the
> same way as 32bit ARM does), until Rob introduced earlycon.

Our earlyprintk up until that point was effectively a less general
earlycon (coming up at a similar time). In fact, in v3.12 we were
already using earlycon structures in arch/arm64/kernel/early_printk.c.

Thanks,
Mark.