Question about arm64 earlycon

[Duc Dang]

On Mon, Oct 24, 2016 at 3:17 AM, Marc Zyngier <marc.zyngier at arm.com> 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:
>>>
>>> <some garbage characters here>01c020000.serial: ttyS0 at MMIO
>>> 0x1c020000 (irq = 108, base_baud = 3125000) is a U6_16550A
>>> console [ttyS0] enabled, bootconsole disabled
>
> Well, you get two independent, unsynchronized bits of code writing to
> the same peripheral. Things are bound to go badly. Unless you're trying
> to debug things, do not use earlyprintk in production.

This is what I am testing for now: not passing earlyprintk parameter,
and I don't see any weird character.

Is it also true about earlycon? We should not use either earlyprintk
(already gone long time ago) or earlycon for production?

>
>>>
>>> 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.
>
>> If you can't use a modern kernel, try backporting all the
>> relevant earlycon changes.
>
> Agreed. That's the most sensible course of action.
>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny...
Regards,
Duc Dang.