linux-kernel - Re: console vs earlycon ?

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Date:	Thu, 22 Oct 2015 13:47:21 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	linux-serial@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stefan Agner <stefan@...er.ch>
Subject: Re: console vs earlycon ?

Hi Peter

2015-10-21 22:20 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
> 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.

Sorry, let me to be sure a bit.

Which does "console" in this context mean
  - console_initcall
  - console enabled when the driver is probed

I am assuming the latter.


Thanks,



-- 
Best Regards
Masahiro Yamada
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