Date:	Wed, 28 Jan 2015 00:17:11 +0100
From:	Pavel Machek <pavel@....cz>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Pali Rohár <pali.rohar@...il.com>,
	linux-omap <linux-omap@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	Sebastian Reichel <sre@...nel.org>,
	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	linux-arm-kernel@...ts.infradead.org,
	Laura Abbott <lauraa@...eaurora.org>
Subject: Re: N900 v3.19-rc5 arm atags_to_fdt.c is broken

On Tue 2015-01-27 13:50:22, Nicolas Pitre wrote:
> On Tue, 27 Jan 2015, Russell King - ARM Linux wrote:
> 
> > On Tue, Jan 27, 2015 at 10:16:24AM -0500, Nicolas Pitre wrote:
> > > On Tue, 27 Jan 2015, Pavel Machek wrote:
> > > > (Note, that in 3.19 dts for n900 got too big, so we are actually
> > > > triggering old bugs. That means that this is a regression fix, and
> > > > should go in ASAP).
> > > 
> > > It is queued here:
> > > http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=8294/1
> > 
> > Irrespective of that patch, I commented on another thread (about adding
> > the serial number to DT):
> > 
> > | How about something a little more radical.
> > | 
> > | Rather than trying to squeeze various ATAGs into DT, why don't we add a
> > | standard ATAG to contain the DT and pass that through into the kernel.
> > | This is IMHO how we _should_ have done the ATAG compatibility from the
> > | start.
> > | 
> > | That means we could get rid of most of the libfdt in the decompressor,
> > | and instead resolve the differences in the kernel.
> > 
> > That would allow us to get rid of all the FDT compatibility code in the
> > decompressor, and when we encounter these special ATAGs, we can deal with
> > them in the kernel instead of having to mess around with the DT.  I'd
> > also assume (although I haven't checked) that it would be much easier to
> > add additional DT properties in the kernel environment, rather than in
> > the rather restrictive decompressor environment.
> 
> Well...
> 
> I was initially against any ATAG-to-DT compatibility at all.  IMHO the 
> "right" solution is to update the bootloader.
> 
> However people complained that updating the bootloader wasn't possible. 
> I therefore suggested they go with a shim layer I termed the "impedance 
> matcher" acting like an intermediate boot stage converting their (often 
> unofficial and out-of-tree) ATAGs into proper DT nodes.  Some people did 
> exactly that and it works wonderfully for them. It also works 
> wonderfully for kernel maintenance as the platform specific tweaks are 
> kept out of the kernel tree.
> 
> Still, that wasn't good enough for some cases as the move to DT 
> regressed some platforms that just used to work before.  Concatenating a 
> DTB to zImage was tolerable but installing an additional shim layer was 
> too much.  Hence the ATAG-to-DT compat code in the decompressor.  This 
> was meant to smooth things around the transition to DT, etc.  After all, 
> those devices with non-replaceable bootloaders where shim layers are not 
> possible should get out of commission eventually?

N900 is still phone that is best-supported by the mainline,
AFAICT. Yes, it is rather sad, but it means that no, it is probably
not going out of commission anytime soon.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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