linux-kernel - Re: console vs earlycon ?

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Message-ID: <5490668.HV7UIzY6eu@wuerfel>
Date:	Wed, 21 Oct 2015 10:57:07 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	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 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.

	Arnd
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