linux-kernel - Re: [PATCH 09/32] bcm2837-rpi-3-b.dts for 32bit arm

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Date:	Thu, 02 Jun 2016 11:53:13 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@...r.kernel.org>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Russell King <linux@...linux.org.uk>,
	open list <linux-kernel@...r.kernel.org>,
	Eric Anholt <eric@...olt.net>,
	Rob Herring <robh+dt@...nel.org>,
	linux-rpi-kernel@...ts.infradead.org,
	Kumar Gala <galak@...eaurora.org>
Subject: Re: [PATCH 09/32] bcm2837-rpi-3-b.dts for 32bit arm

On Thursday, June 2, 2016 11:14:30 AM CEST Gerd Hoffmann wrote:
>   Hi,
> 
> > > Well, it just includes the arm64 version as-is, so we don't have
> > > duplication.  I'm open to suggestions to how handle this better.  
> > > 
> > > Symbolic link?
> > > 
> > > Reference to ../../../arm64/boot/dts/broadcom/bcm2837-rpi-3-b.dts
> > > directly in the Makefile?
> > > 
> > > I've seen arch/arm/boot/dts/Makefile uses globs on *.dts, so I suspect
> > > the later wouldn't be that straight forward.
> > 
> > It's just weird that the arm64 file includes the .dtsi files from arch/arm/
> > and is then again included back from another file there.
> > 
> > I can see two possible ways to handle this better:
> > 
> > - leave the complete set of bcm2837 files in arch/arm and then have one
> >   reference from arch/arm64 per .dts that refers to just that file.
> 
> So basically do it the other way around.  Would be a bit less messy
> indeed.
> 
> > - come up with a rule to also build the .dtb files in arch/arm64 when
> >   we run 'make dtbs' for arch/arm and leave this file there. The argument
> >   that one could use the same dtb with a 32-bit kernel should basically
> >   hold true for any arm64 system, it's not specific to rpi-3 really.
> 
> Yes, in theory.  No, in practice.  As far I know the rpi3 is the only
> 64bit soc where a almost identical 32bit version exists, so running
> 32bit kernels on a 64bit processor actually happens in practice and I
> expect this to continue.  If you want create sdcard images which run on
> any rpi variant this is pretty much the only reasonable way to do it.

I think the Allwinner A64 and the Samsung s5p6818 are other examples
for this, where the initial run of boards all run 32-bit kernels
for much of the same reasons. If users want to run a 32-bit distro
on rpi-3 and on e.g. orange-pi, I don't see why they wouldn't also run
the same binary on A64.

	Arnd