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

["Pali Rohár" <pali.rohar@gmail.com>]

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@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:
> > > > > > On Sat, Nov 28, 2015 at 12:34:23PM -0500, Nicolas Pitre wrote:
> > > > > > > Good. And Arnd likes the idea too. So we might be converging at
> > > > > > > last which is a good thing.
> > > > > > 
> > > > > > I disagree with the idea that there is convergence.  There might be
> > > > > > convergence towards an idea, but... Here's a mail extract, from
> > > > > > July 7th, from earlier in this very thread:
> > > > > > 
> > > > > > Pali:
> > > > > > > Me:
> > > > > > > > Are the ATAGs at a fixed address on the N900?
> > > > > > > 
> > > > > > > Yes, in board-rx51.c is:
> > > > > > > 
> > > > > > > .atag_offset    = 0x100
> > > > > > > 
> > > > > > > and Nokia Bootloader (proprietary) store them to that address.
> > > > > > > 
> > > > > > > > Can that be handled in
> > > > > > > > some kind of legacy file for the N900 which calls save_atags()
> > > > > > > > on it, so we don't end up introducing yet more stuff that we
> > > > > > > > have to maintain into the distant future?  If not, what about
> > > > > > > > copying a known working atag structure into a legacy file for
> > > > > > > > the N900?
> > > > > > > 
> > > > > > > I already asked question if it is possible to read ATAGs from DT
> > > > > > > booted kernel. And somebody (do not remember who) wrote to ML,
> > > > > > > that it is not possible and it can be done in that uncompress
> > > > > > > code.
> > > > > 
> > > > > Who is that somebody? If ever it happened to be me then objection is
> > > > > withdrawn. Otherwise that somebody should come forth and speak up
> > > > > again.
> > > > > 
> > > > 
> > > > ... do not remember ... this discussion were in more email threads and 
> > > > takes more then one year... sorry but my memory is not excellent
> > > 
> > > Yes this certainly seems like the best solution. I think we got into
> > > the atags-to-dt track as some of the atags are already being translated.
> > > 
> > > In this case there's no need to translate them AFAIK. You can just
> > > parse them and have them available for the user space. So as long as
> > > nothing trashes the atags at the atag_offset, you should be able to
> > > call a function to parse them in the n900 specific init_machine.
> > > 
> > > Regards,
> > > 
> > > Tony
> > 
> > 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.
> 
> 
> Nicolas

So am I understand correctly that solution would be to hack
arch/arm/mm/mmu.c to not overwrite page at PHYS_OFFSET?

And should be this just when when we detect Nokia N900 in DT? Or for all
OMAP2 boards? Or all ARM boards?

-- 
Pali Rohár
pali.rohar@gmail.com