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

From: Nicolas Pitre <nico@fluxnic.net>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: "Pali Rohár" <pali.rohar@gmail.com>,
	"Rob Herring" <robherring2@gmail.com>,
	"Pavel Machek" <pavel@ucw.cz>,
	"Will Deacon" <will.deacon@arm.com>,
	"Ivaylo Dimitrov" <ivo.g.dimitrov.75@gmail.com>,
	"Sebastian Reichel" <sre@debian.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"Tony Lindgren" <tony@atomide.com>
Subject: Re: [PATCH] ARM: /proc/atags: Export also for DT
Date: Tue, 27 Jan 2015 15:03:23 -0500 (EST)	[thread overview]
Message-ID: <alpine.LFD.2.11.1501271446330.1322@knanqh.ubzr> (raw)
In-Reply-To: <20150127174818.GM26493@n2100.arm.linux.org.uk>

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On Tue, 27 Jan 2015, Russell King - ARM Linux wrote:

> On Tue, Jan 27, 2015 at 03:32:25PM +0100, Pali Rohár wrote:
> > On Tuesday 27 January 2015 15:16:45 Rob Herring wrote:
> > > What other devices? Where is bootreason in the list of ATAGS:
> > > 
> > > #define ATAG_MEM        0x54410002
> > > #define ATAG_VIDEOTEXT  0x54410003
> > > #define ATAG_RAMDISK    0x54410004
> > > #define ATAG_INITRD     0x54410005
> > > #define ATAG_INITRD2    0x54420005
> > > #define ATAG_SERIAL     0x54410006
> > > #define ATAG_REVISION   0x54410007
> > > #define ATAG_VIDEOLFB   0x54410008
> > > #define ATAG_CMDLINE    0x54410009
> > > #define ATAG_ACORN      0x41000101
> > > #define ATAG_MEMCLK     0x41000402
> > > 
> > > Rob
> > 
> > Each device is using own proprietary atag (or other information) 
> > to pass bootreason from bootloader to kernel. No standard way :-(

So that's what Pavel was alluding to.

> The reason that happens is because people refuse to discuss their
> requirements here - just like people refuse to report userspace API
> regressions to kernel people.  This rather pisses me off, because
> it creates all sorts of horrid per-platform yuck.
> 
> We _could_ (and have in the past) turned round and refused to support
> these kinds of hacks - which IMHO is quite a reasonable stance to
> take: the message we should be sending is "if you wish to design
> new methods without discussing it with us, we reserve the right not
> to support them in mainline kernels; please discuss with us your
> requirements."
> 
> Each time that we accept one of these hacks, we're sending a message
> that says "it's okay to work in this crappy way".
> 
> Yes, I realise that the N900 has little in the way of support, and we
> can't exert that kind of back pressure (since there's no one to direct
> that onto to effect any change) so I guess we just have to live with it.

If the method is: "let's pass non-standard ATAGs around and have ad-hoc 
user space code interpret it in some arbitrary way" then it's a complete 
abomination.

> > I think this kind of information (how was board/computer started) 
> > can be useful also for other architectures. E.g. on laptop you 
> > would like to know if if was started by RTC, power button, 
> > WakeOnLan, another ACPI event, rebooted machine, watchdog, etc... 
> > And scripts can act depending on this event (when by RTC, you 
> > need to run some planned job, when by watchdog reset you should 
> > check what caused that reason...).

Useful when properly designed and generic enough to be shared.

I'd suggest a DT property be proposed for that purpose if it doesn't 
already exist.  That at least has a chance to be generically useful.

Nicolas