linux-kernel - Re: console vs earlycon ?

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Message-ID: <14202684.GgqlkWAZA0@wuerfel>
Date:	Wed, 21 Oct 2015 11:19:26 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux-serial@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Stefan Agner <stefan@...er.ch>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: console vs earlycon ?

On Wednesday 21 October 2015 18:09:33 Masahiro Yamada wrote:
> Hi Arnd,
> 
> 
> 2015-10-21 17:57 GMT+09:00 Arnd Bergmann <arnd@...db.de>:
> > On Wednesday 21 October 2015 17:21:07 Masahiro Yamada wrote:
> >> Hi.
> >>
> >>
> >>
> >>
> >> I think there are three places where console could be enabled.
> >>
> >> [1] earlycon
> >>
> >> Each driver entry is declared with
> >> EARLYCON_DECLARE()  or OF_EARLYCON_DECLARE()
> >>
> >>
> >>
> >> [2] console_init()
> >>
> >> Each entry is declared with  console_initcall()
> >>
> >>
> >>
> >> [3]  when driver is probed
> >> The console is usually enabled at this point
> >> unless some special treatment is done.
> >>
> >>
> >>
> >>
> >> My question is about [2].
> >>
> >> I am using 8250-ish UART device.
> >>
> >>
> >> I noticed univ8250_console_match() and univ8250_console_setup()
> >> always fail at the point of [2] unless early_serial_setup() has been
> >> called in advance;
> >> however, it looks like early_serial_setup() is only used for old platforms.
> >>
> >> So, console cannot be enabled at [2] for modern platforms.
> >>
> >>
> >> My questions are:
> >>
> >>  -  Given that earlycon can be now available for major architectures such ARM,
> >>      [2] will be deprecated at some point in the future?
> >>
> >>  - I am implementing  earlycon  for my own UART driver.
> >>    Is it meaningless to implement  console_initcall()  as well as earlycon?
> >
> > I would still do both. We don't enable earlycon by default at the moment,
> > and I'd say things should remain working with just console_initcall().
> >
> > How closely related to 8250 is your hardware? If it's not all that different,
> > you should probably reuse the existing driver.
> 
> 
> Very close.
> It is already in the mainline.
> 
> drivers/tty/serial/8250/8250_uniphier.c
> 
> The core parts are shared with 8250_core.c

Ok, but I guess no close enough that it makes sense to use drivers/serial/of_serial.c


> 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 think they are
the majority.

	Arnd
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