Date:	Tue, 13 May 2014 16:39:02 +0200
From:	Javier Martinez Canillas <javier@...hile0.org>
To:	Matt Porter <matt.porter@...aro.org>
Cc:	Tom Rini <trini@...com>, Robert Nelson <robertcnelson@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Matt Ranostay <mranostay@...il.com>, robh+dt@...nel.org,
	Mark Rutland <mark.rutland@....com>,
	Russell King <linux@....linux.org.uk>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	devicetree <devicetree@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Pantelis Antoniou <pantelis.antoniou@...il.com>
Subject: Re: [PATCH v2] ARM: dts: am335x-bone-common: Add i2c2 definition

On Tue, May 13, 2014 at 4:22 PM, Matt Porter <matt.porter@...aro.org> wrote:
> On Tue, May 13, 2014 at 04:06:02PM +0200, Javier Martinez Canillas wrote:
>> On Tue, May 13, 2014 at 2:53 PM, Tom Rini <trini@...com> wrote:
>> > On 05/12/2014 04:57 PM, Robert Nelson wrote:
>> >>>> Either case if fine with me.  As who knows when the dtc "overlay" will
>> >>>> every truly make it mainline, as the capemgr was the only real kernel
>> >>>> user of the i2c/at24 eeprom information.
>> >>>
>> >>> Sounds like we should keep it disabled though so u-boot can be used
>> >>> to toggle it while waiting for the capemgr. That's because the board
>> >>> has a header for pins, so it's not exactly limited to just the capes.
>> >>>
>> >>> Anybody working on enabling/disabling cape dtb configurations in u-boot?
>> >>
>> >> Well,
>> >>
>> >> Would Tom even approve of that in mainline u-boot? He didn't want my
>> >> "invert" the gpio to enable the usb hub on the older beagle xm A/B..
>> >>
>> >> http://lists.denx.de/pipermail/u-boot/2014-January/172154.html
>> >>
>> >> http://lists.denx.de/pipermail/u-boot/2014-January/172274.html
>>
>> Using fdt set from the bootloader to use the same FDT for similar
>> boards (like the example with Beagle xM variants) is kind of trying to
>> replicate what we used to do from boards files where it was possible
>> to manage a set of boards using the same platform code.
>>
>> But Device Trees are meant to describe hardware and thus should be
>> static, if two board are almost identical but slightly different, then
>> are two different hardware where each need its proper FDT that
>> describes it.
>>
>> >
>> > I would think that using the 'fdt' command in U-Boot to add all
>> > properties of every cape found on a running system would drive someone
>> > to madness quite quickly.  Moving all of Pantelis' work for dynamic
>> > device trees from the kernel to N bootloaders (U-Boot, barebox, UEFI,
>> > etc) sounds like a step in the wrong direction.
>> >
>>
>> Agreed. I think that until the device tree overlay and the cape
>> manager find their way into mainline we should treat capes as if they
>> were expansion boards attached to a Computer-on-Module. That is, a
>> static based board which its own DTS including the BB{B,W} as an dtsi
>> and not something that can be added on runtime.
>
> It's far more complicated than a SOM plus carrier board. Consider that
> you can have any 4 of these capes stacked on the BBB/BBW in any
> combination (assuming no resource conflicts). Capturing all possible
> combinations in static dtsis is not practical.
>

Right, I forgot that the capes were stackable so is indeed not
practical to model every single combination as DTS in mainline. Even
if stacking was not possible there are just too many capes out there
to have a DTS for every single cape.

My point was that someone who wants to use a BBB + a set of capes can
today write a DTS for its own stacked setup.

Unfortunately I don't have a solution but what I'm pretty sure is that
mangling the DTS from the bootloader is not the right one :-)

> -Matt

Best regards,
Javier
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