Date:	Mon, 9 Sep 2013 10:16:07 -0400
From:	Matt Porter <matt.porter@...aro.org>
To:	Jonathan Austin <jonathan.austin@....com>
Cc:	Koen Kooi <koen@...inion.thruhere.net>,
	Mark Rutland <Mark.Rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Pawel Moll <Pawel.Moll@....com>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"tony@...mide.com" <tony@...mide.com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"bcousson@...libre.com" <bcousson@...libre.com>,
	"olof@...om.net" <olof@...om.net>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv3] ARM: OMAP2+: am335x-bone*: add DT for BeagleBone Black

On Mon, Sep 09, 2013 at 09:59:26AM -0400, Matt Porter wrote:
> On Mon, Sep 09, 2013 at 02:51:13PM +0100, Jonathan Austin wrote:
> > Hi Matt,
> > 
> > On 09/09/13 14:31, Matt Porter wrote:
> > >On Sun, Sep 08, 2013 at 01:12:26PM +0200, Koen Kooi wrote:
> > >>The BeagleBone Black is basically a regular BeagleBone with eMMC and HDMI added,
> > >>so create a common dtsi both can use.
> > >>
> > >>IMPORTANT: booting the existing am335x-bone.dts will blow up the HDMI transceiver
> > >>after a dozen boots with an uSD card inserted because LDO will be at 3.3V instead
> > >>of 1.8.
> > >>
> > >>MMC support for AM335x still isn't in, so only the LDO change has been added.
> > >>
> > >>Signed-off-by: Koen Kooi <koen@...inion.thruhere.net>
> > >
> > >Tested-by: Matt Porter <matt.porter@...aro.org>
> > >
> > >Works fine for me on tip and 3.11. I did notice a regression in musb (worked
> > >on 3.11, now failing to probe but this is not related to your new dts as it
> > >happens on am335x-bone.dts too, assuming merge window volatility). One nit,
> > >git-am picked up a whitespace error on that extra line at EOF so you should
> > >trim that out.
> > >
> > >Only thing is...for a clear bug like this that will destroy hardware, it
> > >should be marked Cc: stable@...r.kernel.org to be picked up in stable.
> > >
> > 
> > If I've understood Koen correctly then what he's saying is that if
> > you *were* to use the current (before this patch) am335x-bone.dts on
> > a Beagle Bone Black (which would be wrong, as that's not the board
> > you have...) then things would break.
> > 
> > I don't see that this patch fixes that - as far as I can see, even
> > after the patch, using am335x-bone.dts with a Bone Black will risk
> > the damage?
> > 
> > If so, I don't think this is a 'stable fix' kind of thing, as it
> > doesn't actually fix the problem?
> 
> It fixes the problem by providing the correct dts for BBB which the
> vendor tree has had for sometime. In the absence of a specific dts
> for BBB, it appears everybody (TI and OMAP maintainers, included)
> has assumed that am335x-bone.dts is correct and safe.
> 
> I'm sure there's plenty of systems represented in dts/* where you
> could cause damage by loading another dtb for a similar board from
> the same SoC family...it's a common risk if you get the wrong dtb
> with more-or-less arbitrary regulator settings.

Sorry to reply to myself, but I probably didn't make it 100% clear as
to why this effectively fixes the problem. Both mainline u-boot *and*
the vendor u-boot have findfdt implemented to load an
am335x-boneblack.dtb based on board detection.

Hopefully this makes it clear why this fixes a bug in the kernel. If
you use appended dtb to include the wrong one, well, you shouldn't
be using appended dtb. It's a *hack* and loading it separately
works fine if you use the U-Boot that ships with BBB or mainline.

-Matt

> > Koen - is there a way for a booting kernel to detect which board it
> > is on and avoid any potential damage if someone gives it the wrong
> > DT?
> > 
> > Jonny
> > 
> > >-Matt
> > >
> > >_______________________________________________
> > >linux-arm-kernel mailing list
> > >linux-arm-kernel@...ts.infradead.org
> > >http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> > >
> > 
> > 
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