linux-kernel - Re: console vs earlycon ?

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Date:	Wed, 21 Oct 2015 09:20:38 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	linux-serial@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Cc:	Rob Herring <robh@...nel.org>, Stefan Agner <stefan@...er.ch>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: console vs earlycon ?

On 10/21/2015 04:21 AM, 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].

For 8250 sub-drivers, the console_initcall() is only for legacy
platforms, and not for 8250 ports added via serial8250_register_8250_port().

In this contexts, legacy platforms are:
1. early_serial_setup()
2. platforms that define SERIAL_PORT_DFNS (eg., ISA ports)

At the point where console_initcall() occurs, the 8250 core has _not_
been initialized (ie., serial8250_init() has _not_ run).

> 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.

Since console_init() => console_initcall() occurs before almost everything,
it's not practical to use for probed drivers.

> 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?

PPC and x86 platforms still rely on console_init() so it will be some time
before this disappears.

>  - I am implementing  earlycon  for my own UART driver.
>    Is it meaningless to implement  console_initcall()  as well as earlycon?

Probed drivers should only implement earlycon and console. 8250 sub-drivers
should only implement earlycon and/or console if the base 8250 earlycon and/or
console are not operational.

Regards,
Peter Hurley
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