Re: console vs earlycon ?

From: Masahiro Yamada
Date: Thu Oct 22 2015 - 00:47:41 EST


Hi Peter

2015-10-21 22:20 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/