Re: [PATCHv3] ARM: OMAP2+: am335x-bone*: add DT for BeagleBone Black

[Jonathan Austin]

Hi Matt,

On 09/09/13 15:16, Matt Porter wrote:
> 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@xxxxxxxxxxxxxxxxxxxxx>
> > > >
> > > > Tested-by: Matt Porter <matt.porter@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxxxxx 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.

This makes more sense now, thanks. Not sure that it is still a good case 
for CC:stable. Are people currently working around findfdt failing, etc? 
If so, do you think backporting the fix will stop them doing that? I 
don't really know what the workflow looks like...

Generally the idea of backporting DT fixes to older kernels gives me the 
Heebie-Jeebies - this case of being able to damage hardware is a great 
example of why it might be scary (though I acknowledge that this 
specific patch is unlikely to have a bad outcome)

Jonny
> 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
> > > >
> > > > _______________________________________________
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> > > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
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> > >
> > >
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