linux-kernel - Re: [PATCH 5/5] arm: boot: store ATAGs structure into DT "/chosen/linux,atags" entry

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Date:	Tue, 15 Dec 2015 12:04:35 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Tony Lindgren <tony@...mide.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Sebastian Reichel <sre@...ian.org>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, Rob Herring <robherring2@...il.com>,
	Pavel Machek <pavel@....cz>,
	Grant Likely <grant.likely@...aro.org>,
	linux-omap@...r.kernel.org, Frank Rowand <frowand.list@...il.com>,
	Andreas Färber <afaerber@...e.de>
Subject: Re: [PATCH 5/5] arm: boot: store ATAGs structure into DT "/chosen/linux,atags" entry

On Tuesday 15 December 2015 10:33:25 Pali Rohár wrote:
> On Monday 30 November 2015 11:09:42 Nicolas Pitre wrote:
> > On Mon, 30 Nov 2015, Pali Rohár wrote:
> > > On Monday 30 November 2015 07:23:53 Tony Lindgren wrote:
> > > > * Pali Rohár <pali.rohar@...il.com> [151129 16:16]:
> > > > > On Monday 30 November 2015 01:09:17 Nicolas Pitre wrote:
> > > > > > On Sun, 29 Nov 2015, Russell King - ARM Linux wrote:
> > > In arch/arm/kernel/setup.c is function setup_arch() and it calls:
> > > 
> > >   mdesc = setup_machine_fdt(__atags_pointer);
> > >   if (!mdesc)
> > >       mdesc = setup_machine_tags(__atags_pointer, __machine_arch_type);
> > > 
> > > So it looks like that on atags address is stored either atags structure
> > > or DT structure... so it is truth kernel uncompress code put DT blob to
> > > same offset where is expected atags structure?
> > 
> > No.  It doesn't put it anywhere. Those functions read DT/ATAGs from the 
> > passed address.  But you know this address won't be the one you want for 
> > the legacy ATAGs.
> > 
> > What you should do is to add a init_early hook to your mdesc structure 
> > and retrieve your ATAGs from there directly at PAGE_OFFSET + 0x100.
> > 
> > Now I suspect paging_init() marks the point where the ATAGs will be 
> > overwritten.  To prevent this, you might have to add an additional tweak 
> > in arm_mm_memblock_reserve() similar to the one already present for 
> > CONFIG_SA1111. Something like:
> > 
> >       memblock_reserve(PHYS_OFFSET, PAGE_SIZE);
> > 
> > And later on you can return that page back to the system.
> > 
> 
> So am I understand correctly that solution would be to hack
> arch/arm/mm/mmu.c to not overwrite page at PHYS_OFFSET?

I would think we can just copy the data from PAGE_OFFSET + 0x100
to a some other page from your init_early hook. IIRC you can't use
kmalloc there, but memblock_alloc() should work.

	Arnd
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