linux-kernel - Re: [PATCH -next 00/13] Extensible console matching & direct earlycon

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Message-ID: <CAL_JsqLcTHNpsXmG1TSPAvvEpdVYahnFm-LRLUUWJd3eotMAEQ@mail.gmail.com>
Date:	Thu, 26 Feb 2015 08:58:15 -0600
From:	Rob Herring <robh@...nel.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Jiri Slaby <jslaby@...e.cz>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 00/13] Extensible console matching & direct earlycon

On Thu, Feb 26, 2015 at 8:48 AM, Peter Hurley <peter@...leysoftware.com> wrote:
> Hi Rob,
>
> On 02/24/2015 03:20 PM, Rob Herring wrote:
>> On Tue, Feb 24, 2015 at 1:53 PM, Peter Hurley <peter@...leysoftware.com> wrote:
>
> [...]
>
>>>>> Direct earlycon
>>>>>
>>>>> This feature enables arches and proms to start an earlycon directly,
>>>>> rather than requiring an "earlycon=" command line parameter.
>>>>> Devicetree can already do this via the 'linux,stdout-path' property,
>>>>> but arch and prom code requires direct coupling to the serial driver.
>>>>>
>>>>> This support is implemented by judicious refactoring and the same
>>>>> construct that devicetree and early_param use: a link table containing
>>>>> the necessary information (name and setup() function) to find and
>>>>> bind the appropriate earlycon "driver".
>>>>
>>>> I've skimmed thru this and it looks like a great improvement.
>>>>
>>>> One problem we have currently with DT stdout-path and earlycon is a
>>>> preferred console does not get registered, so the console will get
>>>> switched to tty0 and you lose your console. The problem is DT does not
>>>> know the console name to register a preferred console. It looks like
>>>> this series may help this problem, but I'm not sure and wanted your
>>>> thoughts.
>>>
>>> I thought that of_alias_scan() + of_console_check() caused DT stdout-path
>>> to add_preferred_console() the driver console @ port registration time
>>> via uart_add_one_port() -> of_console_check().
>>>
>>> Is that not how that works?
>>
>> Yes, I believe that is how it works with earlycon not enabled. This
>> doesn't work when earlycon is enabled with just "earlycon" on the
>> command line. The fix I have is here[1], but I don't like putting DT
>> specifics into the console code.
>
> After much gnashing of teeth and pulling of hair yesterday, I managed
> to mock up the situation you describe, but I need to study it in more
> detail. Some things I did learn:
>
> 1. The serial console _does_ come back up when using stdout-path but the
>    line settings don't match, because the serial core sets them to the
>    default of 9600n8 if unspecified.

That may have been what I saw as I tested on QEMU which ignores the
baud rate. But it does stop between the time tty0 is enabled and the
"real" serial console which is a time period we really want the
console.

> 2. The line settings can now be set with stdout-path like,
>      stdout-path = "serial0:115200n8"
>    but this breaks DT earlycon (as I wrote in the other email you were
>    cc'd on).

Right. We should fix libfdt.

> 3. omap doesn't support ioremap() at early param parsing :(

ARM in general does not.

> 4. the ARM arch doesn't support fixmap hacking at early param parsing :(

There's a patch on the list to enable it. It's been slow as fixmap has
been a moving target.

Rob
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