comparing_device_trees_with_dtx_diff [Doku Wiki]

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===== Overview =====

How to compare (typically in-kernel) device tree source files with ''dtx_diff''.

===== Preparing for comparison =====

Given a kernel source tree at, say, ''~/k/git/'', prepare for comparison with:

<code>
$ cd ~/k/git
$ PATH=~/k/git/scripts/dtc:$PATH
$ export ARCH=arm
</code>


Note that setting ''PATH'' as above will add access to all of the executables (scripts and possible binary executables) under the kernel's ''scripts/dtc/'' directory, which includes not only the ''dtx_diff'' script but, if you've compiled the kernel scripts, the ''dtc'' device tree compiler program that will exist under there as well.

I mention this so you know that if you already had a ''/usr/bin/dtc'' executable installed via some sort of ''dtc'' package, the one in that scripts directory will take precedence, unless you add that directory at the end of ''PATH'' instead of at the beginning. In any event, you //must// have a ''dtc'' command somewhere for the ''dtx_diff'' script.

===== Comparing two .dts files already in the kernel source tree (Part one) =====

If you're located at the top of the relevant kernel source tree, and you want to compare two device tree source files already there, say, under ''arch/arm/boot/dts/'' (let's say, ''imx6q-sabresd.dts'' and ''imx6qp-sabresd.dts''), you can do this (note the use of shell expansion to save on typing):

<code>
$ dtx_diff arch/arm/boot/dts/imx6q{,p}-sabresd.dts
--- arch/arm/boot/dts/imx6q-sabresd.dts
+++ arch/arm/boot/dts/imx6qp-sabresd.dts
@@ -3,8 +3,8 @@
 / {
        #address-cells = <0x1>;
        #size-cells = <0x1>;
-       compatible = "fsl,imx6q-sabresd", "fsl,imx6q";
-       model = "Freescale i.MX6 Quad SABRE Smart Device Board";
+       compatible = "fsl,imx6qp-sabresd", "fsl,imx6qp";
+       model = "Freescale i.MX6 Quad Plus SABRE Smart Device Board";

        aliases {
                can0 = "/soc/aips-bus@02000000/flexcan@02090000";
@@ -44,14 +44,14 @@
                brightness-levels = <0x0 0x4 0x8 0x10 0x20 0x40 0x80 0xff>;
                compatible = "pwm-backlight";
                default-brightness-level = <0x7>;
-               phandle = <0x7a>;
-               pwms = <0x78 0x0 0x4c4b40>;
+               phandle = <0x82>;
+               pwms = <0x80 0x0 0x4c4b40>;
                status = "okay";
        };
        
        .. etc etc ...
</code>

===== Comparing two .dts files already in the kernel source tree (Part deux) =====

In the above example, you were located in the root directory of the relevant kernel source tree, which is the default for the ''dtx_diff'' script, where it knows how to process device tree source file includes. If, however, you're in the actual underlying ''dts'' directory (or somewhere else in the kernel source tree), you can't just run the script as is since it won't know the kernel source tree to use to process ''include'' directives:

<code>
$ cd arch/arm/boot/dts
$ dtx_diff imx6q{,p}-sabresd.dts

... snip ...

In file included from imx6qp.dtsi:43:0,
                 from imx6qp-sabresd.dts:45:
imx6q.dtsi:11:50: error: no include path in which to search for dt-bindings/interrupt-controller/irq.h
 #include <dt-bindings/interrupt-controller/irq.h>

... snip ...

  ./arch/arm/ does not exist
  Is $ARCH='arm' correct?
  Possible fix: use '-s' option

  ./arch/arm/ does not exist
  Is $ARCH='arm' correct?
  Possible fix: use '-s' option
  Possible fix: use '-S' option

  Possible fix: use '-S' option
$
</code>

Instead, you need to use one of these two variations, depending on whether you're in a Git repository, or just a regular source tree:

<code>
$ dtx_diff -s ~/k/git imx6q{,p}-sabresd.dts    (specify source tree with ''-s'')
$ dtx_diff -S imx6q{,p}-sabresd.dts            (''-S'' means this is a Git repo)
</code>

In short, as long as you're still inside the relevant kernel source tree, comparisons will work -- you just need to make sure the ''dtx_diff'' script is told where the top of the kernel source tree is.

===== Comparing two device tree files out of tree -- apparent bug =====

What doesn't seem to work is comparing two device tree files that are outside the confines of a kernel source tree, even if you identify the kernel source tree. Below, I copy the same two files to ''/tmp'' and try to do the comparison there, referring back to the same kernel source tree:

<code>
$ cp imx6q{,p}-sabresd.dts /tmp
$ cd /tmp
$ dtx_diff -s ~/k/git imx6q{,p}-sabresd.dts
imx6qp-sabresd.dts:45:10: fatal error: imx6qp.dtsi: No such file or directory
 #include "imx6qp.dtsi"
          ^~~~~~~~~~~~~
imx6q-sabresd.dts:15:10: fatal error: imx6q.dtsi: No such file or directory
 #include "imx6q.dtsi"
          ^~~~~~~~~~~~
compilation terminated.
compilation terminated.
Error: imx6qp-sabresd.dts:43.9-10 Error: imx6q-sabresd.dts:13.9-10 syntax errorsyntax error

FATAL ERROR: FATAL ERROR: Unable to parse input tree
Unable to parse input tree

Possible hints to resolve the above error:

  (hints might not fix the problem)
Possible hints to resolve the above error:
  (hints might not fix the problem)


  No hints available.
  No hints available.
$
</code>

I have no idea why this fails -- obviously, even when telling ''dtx_diff'' where the kernel source tree is, the include search path is not being set correctly. I've already reported this as an apparent bug to the device tree mailing list.