linux-kernel - Re: console vs earlycon ?

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Date:	Wed, 21 Oct 2015 18:09:33 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Arnd Bergmann <arnd@...db.de>
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 ?

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


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.



-- 
Best Regards
Masahiro Yamada
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