linux-kernel - Re: console vs earlycon ?

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Message-ID: <5490682.zH2vAEO2id@wuerfel>
Date:	Wed, 21 Oct 2015 12:36:45 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	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>,
	Peter Hurley <peter@...leysoftware.com>
Subject: Re: console vs earlycon ?

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.

> 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.

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

	Arnd
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