linux-kernel - Re: [PATCH v2] console: Don't prefer first registered if DT specifies stdout-path

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Message-Id: <20160809145726.1f9f5e5752c94b3113d77674@linux-foundation.org>
Date:	Tue, 9 Aug 2016 14:57:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Burton <paul.burton@...tec.com>
Cc:	<linux-mips@...ux-mips.org>, Ralf Baechle <ralf@...ux-mips.org>,
	Tejun Heo <tj@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Ivan Delalande <colona@...sta.com>,
	Thierry Reding <treding@...dia.com>,
	Borislav Petkov <bp@...e.de>, Jan Kara <jack@...e.com>,
	Petr Mladek <pmladek@...e.com>, <linux-kernel@...r.kernel.org>,
	Joe Perches <joe@...ches.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
	Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH v2] console: Don't prefer first registered if DT
 specifies stdout-path

On Tue, 9 Aug 2016 16:19:37 +0100 Paul Burton <paul.burton@...tec.com> wrote:

> If a device tree specifies a preferred device for kernel console output
> via the stdout-path or linux,stdout-path chosen node properties or the
> stdout alias then the kernel ought to honor it & output the kernel
> console to that device. As it stands, this isn't the case. Whilst we
> parse the stdout-path properties & set an of_stdout variable from
> of_alias_scan(), and use that from of_console_check() to determine
> whether to add a console device as a preferred console whilst
> registering it, we also prefer the first registered console if no other
> has been selected at the time of its registration.
> 
> This means that if a console other than the one the device tree selects
> via stdout-path is registered first, we will switch to using it & when
> the stdout-path console is later registered the call to
> add_preferred_console() via of_console_check() is too late to do
> anything useful. In practice this seems to mean that we switch to the
> dummy console device fairly early & see no further console output:
> 
>     Console: colour dummy device 80x25
>     console [tty0] enabled
>     bootconsole [ns16550a0] disabled
> 
> Fix this by not automatically preferring the first registered console if
> one is specified by the device tree. This allows consoles to be
> registered but not enabled, and once the driver for the console selected
> by stdout-path calls of_console_check() the driver will be added to the
> list of preferred consoles before any other console has been enabled.
> When that console is then registered via register_console() it will be
> enabled as expected.

Looks reasonable.  Could you please do `grep -r stdout-path
Documentation' and check that everything therein is complete, accurate
and necessary?