Re: dtsi vs dts files, where to better understand this

On Wed, Mar 21, 2012 at 02:50:09PM -0600, John Linn wrote:

quoted

-----Original Message-----
From: Stephen Warren [mailto:swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org]
Sent: Wednesday, March 21, 2012 1:37 PM
To: John Linn
Cc: devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org
Subject: Re: dtsi vs dts files, where to better understand this

On 03/21/2012 12:36 PM, John Linn wrote:

quoted

Hi,

I've done some digging and not found any info, maybe I looked in the
wrong places.

I'm trying to better understand how the dtsi file works with the dts
file and what can go in each file.

The basic idea is that what's inside the SoC is identical across all
boards using that SoC. This information is put into the .dtsi file so
it
can be included by the .dts file for any boards using the SoC.

Anything that's board-specific goes into the board's individual .dts
file. This might be new nodes to e.g. define which chips are connected
to an I2C bus hosted by the SoC, and much more.

Where properties existing in both, the most recent value in parsing
order overrides any earlier values. So for example, if the .dtsi file
said that 'status = "disabled"' for a particular HW module, the board
.dts could later override it by saying 'status = "okay"' within the
same
node.

One other factor: Something like an SDHCI controller may have
properties
that are defined by the SoC (e.g. compatible, reg, interrupts), and
properties that are defined by the board (e.g. the GPIO ID to use for
CD, WP, etc.). Just set the relevant properties in the .dtsi and .dts
files and they'll get merged together to form the final device tree.

Was that what you were looking for?

That was very helpful.  I was hoping there were details written
somewhere I had missed.  My apologies if I'm just not getting it yet :)

It's not clear to me how the hierarchy is maintained across both files
such as an i2c controller would be in the dtsi, while the i2c eeprom on
the bus is in the board file.

Do I have to duplicate the i2c controller (and the bus it's on also) in
the board file? It's not clear how smart the merge is.

The examples in the 3.3 tree don't seem that clear to me yet.

The model used by dtc for this is a "stack of overlays".  Once the
/include/s are processed you effectively have one big dts file with
several partial trees specified one after another.  The first one
always starts at /, the later ones can start at / but more likely
start at a subtree of the first tree, specified by a path or a label.

dtc then starts with the first tree and lays the later trees over it
in order.  If the same property is specified in a later tree, it
replaces the value from an earlier tree.  If the same node is
specified, its properties and subnodes are merged into the existing
version.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson