linux-kernel - Re: [PATCH] ARM: /proc/atags: Export also for DT

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Date:	Wed, 28 Jan 2015 09:57:18 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Nicolas Pitre <nico@...xnic.net>
Cc:	Pali Rohár <pali.rohar@...il.com>,
	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Sebastian Reichel <sre@...ian.org>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: /proc/atags: Export also for DT

On Wed, Jan 28, 2015 at 8:33 AM, Nicolas Pitre <nico@...xnic.net> wrote:
> On Wed, 28 Jan 2015, Pali Rohár wrote:
>
>> On Wednesday 28 January 2015 01:50:33 Tony Lindgren wrote:
>> > On omaps, the bootrom passes the bootreason in r1 to the
>> > bootloader that can do whatever it wants with it. We could
>> > maybe pass it in the kernel cmdline to the watchdog driver
>> > for user space?
>> >
>>
>> Not truth for N900. Bootreason depends on PRM_RSTST omap
>> register, state of vbat charger pins, time how long was power key
>> pressed, R&D data stored in CAL partition and other undocumented
>> registers for omap HS devices. I already tried to implement at
>> least some subset of it in userspace (or kernel), but it is
>> impossible because NOLO bootloader clear status of PRM_RSTST
>> register.
>>
>> There is also copy of PRM_RSTST register stored at address
>> 0x4020FFB8 (tracing data) but that address is rewritten (probably
>> by kernel), so we really cannot implement reading bootreason in
>> kernel.
>>
>> But in early stage in uboot it is possible to read 0x4020FFB8
>> address and get some part of bootreason. But still PRM_RSTST is
>> not enough!
>>
>> I would be happy if DT kernel can export /proc/atags file with
>> ATAGs passed by bootloader. It would be enough for me. In
>> userspace I can parse content and do what is needed.
>
> What about defining a DT boot reason property instead?
> Maybe it already exists?  If not, it's something that could certainly be
> generically used on other platforms too.

I'm fine with that, but we just need to have a standard kernel
userspace interface in addition to something like
/proc/device-tree/bootreason. Perhaps this can be the default
implementation for the watchdog dev. Someday when we decide DT is crap
and have a new boot interface, we'll have people relying on
/proc/device-tree. I hope to be retired when that happens...

Rob
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