linux-kernel - Re: console vs earlycon ?

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Message-ID: <562798EB.7060202@hurleysoftware.com>
Date:	Wed, 21 Oct 2015 09:53:47 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stefan Agner <stefan@...er.ch>, linux-serial@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: console vs earlycon ?

On 10/21/2015 06:36 AM, Arnd Bergmann wrote:
> On Wednesday 21 October 2015 18:30:05 Masahiro Yamada wrote:
>>>
>>>> I am trying to implement OF_EARLYCON_DECLARE() for that.
>>>>
>>>> I was just wondering if console_initcall() should work as well.
>>>>
>>>>
>>>> As I said, I noticed the console_initcall() in 8250_core.c
>>>> only works on very limited platforms.
>>>
>>> It works with all those that use of_serial.c, right?
>>
>> I doubt it.
>>
>>
>> I have a board with a pure 8250-compat device working.
>> (  compatible = "ns16550a")
>> It uses of_serial.c, of course.
>>
>>
>> As far as I tested, it would never enable the console at console_init().
> 
> Ok, that sounds like a bug. Peter and others have changed that code
> a lot in the last year. I wonder if this has never worked then or
> if it has regressed.

No, not a bug. console_init() is only for legacy platforms, not for
probed drivers.

>> The console is enabled when the UART driver is probed.
>> (or earlycon if "earlycon" is specified in the kernel-parameter.)
> 
> Got it. Let me know if you think this interpretation is correct:
> 
> * univ8250_console_match() looks for the earlycon argument format like
>   "console=uart[8250],io|mmio|mmio32,<addr>[,<options>]", and that
>   would work, but we don't want to have to pass all that data when
>   the information should already be there.

This only works at console_init() time for legacy platforms that
have already initialized the 8250 port table.


> * univ8250_console_setup() looks for ports that have been added by
>   one of several other methods (including earlycon) already

8250 earlycon doesn't add ports.

>   , but we don't do that on ARM without earlycon being enabled.


> * arch/powerpc/kernel/legacy_serial.c does everything we need, but
>   does not live in architecture independent code and does a few
>   things that we probably don't need or want there. It relies
>   on scanning the device tree for known UART device nodes before
>   the platform devices are added.
> 
> * for console_initcall() to do the right thing, we want both the
>   ttyS devices to get added early for console=ttyS1 to work, as well
>   as having the preferred console work based on the stdout-property.
> 
> * we parse the /chosen/stdout-path property in drivers/of/base and
>   store the device node pointer in the global 'of_stdout' variable,
>   but do not use it until the uart is added by the tty driver
>   and calls of_console_check() to add the default console device.

I'm assuming the issue with trying to get console_init() working
is because the dummy color console causes the earlycon to be disabled?

I fixed that once before for stdout-path but there were some regressions
on PPC platforms; maybe I can revisit that.

Regards,
Peter Hurley
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