linux-kernel - Re: [PATCH 0/8] Generic serial earlycon

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Message-Id: <201403232049.41866.arnd@arndb.de>
Date:	Sun, 23 Mar 2014 20:49:41 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Herring <robherring2@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Catalin Marinas <catalin.marinas@....com>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 0/8] Generic serial earlycon

On Sunday 23 March 2014, Rob Herring wrote:
> IIRC, the unflattening cannot be done before memblock is up which is
> in arm_memblock_init. I believe that will get done earlier similar to
> PPC with Laura's meminfo removal series. However, in interest of
> getting output enabled as early as possible, the earliest point is
> always going to be with a flat tree. So I think we do want to be able
> to parse the flat tree for this. There's code in u-boot to do the
> address translation on FDT we can steal. Really, i'd like to put that
> code in libfdt, but there's a licensing problem with the u-boot code
> being GPL and libfdt being BSD.

I suppose it depends on what the goal is. If you are trying to get as
close as possible to the debug_ll functionality without losing the
ability to turn it on unconditionally, I agree that it needs to be done
on the flat tree.

However, the goal that interests me more at the moment is to have
reliable debug output done in a simple way earlier than what we have
today with the normal console support. For this, it seems a better
tradeoff to initialize the early console slightly later. This
can actually be done independent of the the command line option,
I don't see any downsides from having a compile-time option that
would initialize the early console from setup_arch (after
unflatten_device_tree) whenever there is a linux,stdout-path
property that points to a usable device.

	Arnd
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