linux-kernel - Re: console vs earlycon ?

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Message-ID: <5627AFFF.6060900@hurleysoftware.com>
Date:	Wed, 21 Oct 2015 11:32:15 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stefan Agner <stefan@...er.ch>, linux-serial@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: console vs earlycon ?

On 10/21/2015 10:13 AM, Arnd Bergmann wrote:
> On Wednesday 21 October 2015 09:53:47 Peter Hurley wrote:
>> On 10/21/2015 06:36 AM, Arnd Bergmann wrote:
>>> On Wednesday 21 October 2015 18:30:05 Masahiro Yamada wrote:
>>>>>
>>>>>> 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.
>>>>>
>>>>> It works with all those that use of_serial.c, right?
>>>>
>>>> I doubt it.
>>>>
>>>>
>>>> I have a board with a pure 8250-compat device working.
>>>> (  compatible = "ns16550a")
>>>> It uses of_serial.c, of course.
>>>>
>>>>
>>>> As far as I tested, it would never enable the console at console_init().
>>>
>>> Ok, that sounds like a bug. Peter and others have changed that code
>>> a lot in the last year. I wonder if this has never worked then or
>>> if it has regressed.
>>
>> No, not a bug. console_init() is only for legacy platforms, not for
>> probed drivers.
> 
> Ah, I had no idea we were moving in this direction.

I would not say this was a conscious design decision, but rather an
outcome of getting-something-working-without-breaking-existing-usage.

My main focus with earlycon/console has been to try to reduce and
generalize the existing code.

> So I guess the idea
> is not to add another for_each_compatible_node() loop when we already have
> two places (earlycon and tty) in the code that do this?

I wouldn't want to do that just because, but rather only to enable some
functionality that doesn't work now.

FWIW, console_init() is basically a hack. The dummy color console
registration is a good example of the gymnastics made necessary by
console_init().

OTOH, I don't like the reverse-linkage that exists now between
serial core and OF (ie., of_console_check()). And now ACPI wants to
recapitulate that design. :/


>>> * arch/powerpc/kernel/legacy_serial.c does everything we need, but
>>>   does not live in architecture independent code and does a few
>>>   things that we probably don't need or want there. It relies
>>>   on scanning the device tree for known UART device nodes before
>>>   the platform devices are added.
>>>
>>> * for console_initcall() to do the right thing, we want both the
>>>   ttyS devices to get added early for console=ttyS1 to work, as well
>>>   as having the preferred console work based on the stdout-property.
>>>
>>> * we parse the /chosen/stdout-path property in drivers/of/base and
>>>   store the device node pointer in the global 'of_stdout' variable,
>>>   but do not use it until the uart is added by the tty driver
>>>   and calls of_console_check() to add the default console device.
>>
>> I'm assuming the issue with trying to get console_init() working
>> is because the dummy color console causes the earlycon to be disabled?
> 
> I don't think so.
> 
> My line of thinking was more about usability: earlycon requires that
> you edit the kernel command line at the moment, while console_init()
> doesn't require any user interaction and just uses the stdout-path.
> 
> I guess we could enable earlycon using a Kconfig symbol if we want
> to, or make it a per-architecture decision whether it's enabled even
> in the absence of the command line flag.

Ah, I see. You want to start the stdout-path console at console_init()
time.

Regards,
Peter Hurley
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