From: Arnd Bergmann <arnd@arndb.de>
To: Pali =?ISO-8859-1?Q?Roh=E1r?= <pali.rohar@gmail.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>, Tony Lindgren <tony@atomide.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        linux-arm-kernel@lists.infradead.org,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>,
        Laura Abbott <lauraa@codeaurora.org>,
        Sebastian Reichel <sre@debian.org>, Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, Rob Herring <robherring2@gmail.com>,
        Pavel Machek <pavel@ucw.cz>, Grant Likely <grant.likely@linaro.org>,
        linux-omap@vger.kernel.org, Frank Rowand <frowand.list@gmail.com>,
        Andreas =?ISO-8859-1?Q?F=E4rber?= <afaerber@suse.de>
Subject: Re: [PATCH 5/5] arm: boot: store ATAGs structure into DT "/chosen/linux,atags" entry
Date: Tue, 15 Dec 2015 12:04:35 +0100
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References: <20150713131902.GH26485@atomide.com> <alpine.LFD.2.20.1511301041090.22569@knanqh.ubzr> <20151215093325.GK13531@pali>
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On Tuesday 15 December 2015 10:33:25 Pali Roh?r wrote:
> On Monday 30 November 2015 11:09:42 Nicolas Pitre wrote:
> > On Mon, 30 Nov 2015, Pali Roh?r wrote:
> > > On Monday 30 November 2015 07:23:53 Tony Lindgren wrote:
> > > > * Pali Roh?r <pali.rohar@gmail.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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