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linux_device_trees

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Overview

Stuff related to device trees and the Linux kernel, using examples from the current Linux kernel.

To do:

  • overlays

Internal wiki pages

Naming conventions

From Toradex tutorial:

Typically, a device tree is defined at multiple levels and composed of multiple device tree files. Device tree files (dts and dtsi) may include other device tree files known as includable device tree files (dtsi). In this manner, a board level device tree file (dts) generally includes a SoC level device tree file (dtsi). To support the modular approach of Toradex products, our device tree files usually have three levels of inclusion: carrier board, module and SoC level. This is also reflected in the device tree file names, which are composed by the three levels: ${soc}-${module}-${board}.dtb

Examples:

  • imx28-eukrea-mbmx283lc.dts
  • imx28-eukrea-mbmx287lc.dts
  • imx6dl-wandboard.dts
  • imx6dl-wandboard-revb1.dts
  • imx6q-nitrogen6_max.dts
  • imx6q-nitrogen6_som2.dts
  • imx6q-nitrogen6x.dts

Proper device tree design

One board, one .dts file

Every distinct board merits its own .dts file, as in:

  • am335x-boneblack.dts
  • am335x-boneblack-wireless.dts
  • am335x-boneblue.dts
  • am335x-bone.dts
  • am335x-bonegreen.dts
  • am335x-bonegreen-wireless.dts

Proper use of .dtsi files

There is no reason to make local copies of kernel-supplied .dtsi files and alter them; such files should be included as is, and all alterations should be made in the .dts file or a more general board-related .dtsi file supplied by that developer. Any alterations that involve actual removal of included content should be done via one of the two directives:

  • /delete-property/
  • /delete-node/

This functionality was introduced into the Linux kernel file scripts/dtc/dtc-lexer.l file in Sept of 2012:

commit cd296721a9645f9f28800a072490fa15458d1fb7
Author: Stephen Warren <swarren@nvidia.com>
Date:   Fri Sep 28 21:25:59 2012 +0000

    dtc: import latest upstream dtc
    
    This updates scripts/dtc to commit 317a5d9 "dtc: zero out new label
    objects" from git://git.jdl.com/software/dtc.git.
    
    This adds features such as:
    * /bits/ syntax for cell data.
    * Math expressions within cell data.
    * The ability to delete properties or nodes.
    * Support for #line directives in the input file, which allows the use of
      cpp on *.dts.
    * -i command-line option (/include/ path)
    * -W/-E command-line options for error/warning control.
    * Removal of spew to STDOUT containing the filename being compiled.
    * Many additions to the libfdt API.
    
    Signed-off-by: Stephen Warren <swarren@nvidia.com>
    Acked-by: Jon Loeliger <jdl@jdl.com>
    Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Examples can be seen here.

Examples of /delete-node/

imx6dl-tx6s-8034.dts

#include "imx6dl.dtsi"
#include "imx6qdl-tx6.dtsi"

/ {
        model = "Ka-Ro electronics TX6S-8034 Module";
        compatible = "karo,imx6dl-tx6dl", "fsl,imx6dl";

        aliases {
                display = &display;
                ipu1 = &ipu1;
        };

        cpus {
                /delete-node/ cpu@1;
        };

imx6q-utilite-pro.dts

#include <dt-bindings/input/input.h>
#include "imx6q-cm-fx6.dts"

/ {
        model = "CompuLab Utilite Pro";
        compatible = "compulab,utilite-pro", "compulab,cm-fx6", "fsl,imx6q";
        
 ...

};

/*
 * A single IPU is not able to drive both display interfaces available on the
 * Utilite Pro at high resolution due to its bandwidth limitation. Since the
 * tfp410 encoder is wired up to IPU1, sever the link between IPU1 and the
 * SoC-internal Designware HDMI encoder forcing the latter to be connected to
 * IPU2 instead of IPU1.
 */
/delete-node/&ipu1_di0_hdmi;
/delete-node/&hdmi_mux_0;
/delete-node/&ipu1_di1_hdmi;
/delete-node/&hdmi_mux_1;

Examples of /delete-property/

imx6qp.dtsi

#include "imx6q.dtsi"

/ {
        soc {
        
        ...
        
};

&fec {
        /delete-property/interrupts-extended;
        interrupts = <0 118 IRQ_TYPE_LEVEL_HIGH>,
                     <0 119 IRQ_TYPE_LEVEL_HIGH>;
};

imx6ul-tx6ul-mainboard.dts

#include "imx6ul.dtsi"
#include "imx6ul-tx6ul.dtsi"

/ {
        model = "Ka-Ro electronics TXUL-0010 Module on TXUL Mainboard";
        compatible = "karo,imx6ul-tx6ul", "fsl,imx6ul";

        aliases {
                lcdif_24bit_pins_a = &pinctrl_disp0_3;
                mmc0 = &usdhc1;
                /delete-property/ mmc1;
                serial2 = &uart3;
                serial4 = &uart5;
        };
        /delete-node/ sound;
};

...

&usdhc1 {
        pinctrl-0 = <&pinctrl_usdhc1>;
        non-removable;
        /delete-property/ cd-gpios;
        cap-sdio-irq;
};

&uart1 {
        pinctrl-0 = <&pinctrl_uart1>;
        /delete-property/ uart-has-rtscts;
};

&uart2 {
        pinctrl-0 = <&pinctrl_uart2>;
        /delete-property/ uart-has-rtscts;
        status = "okay";
};

Comparing .dts files with dtx_diff

Here's how to properly compare two device tree source files with the dtx_diff script that comes with the Linux kernel source, given an apparent bug that I've already reported and explain below. This script handles all include files when doing the comparison.

Preparing for comparison

First, given a kernel source tree at ~/k/git, prepare for comparison with:

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

Note that setting the 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):

$ 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 ...

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:

$ 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
$

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:

$ 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)

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:

$ 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.
$

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.

linux_device_trees.1542470519.txt.gz · Last modified: 2018/11/17 16:01 by rpjday