Re: puzzled by online explanation as to how to "overwrite" DT node

On Wed, 18 Oct 2017, Rob Herring wrote:

On Wed, Oct 18, 2017 at 11:40 AM, Robert P. J. Day
[off-list ref] wrote:

quoted

  (WARNING: sort of newbie alert, but you know that, right? :-)

  was perusing online tutorial on customizing device trees here:

    http://developer.toradex.com/device-tree-customization

Never seen that one.

  apparently, i picked a bad example to ask about, then. :-)

quoted

and got a bit confused about the part describing how to (as i read
it) "overwrite" entire nodes.

  first, part on "overwriting properties" makes perfect sense --
here's an example in that piece about overwriting default USB
mode:

  &usb0 {
      dr_mode = "host";
  };

so, as i've always understood it, the above will do one of two
things:

  1) if that property is not set in the node referenced by that label,
     it will be *added* to what is already there, or

  2) if that property is already set (to whatever), it will be
     overwritten

so far, so good?

Yes.

  right, that was the easy part.

quoted

  as a special example of that right below that, there's a short
paragraph called "Activating/Deactivating Devices" that uses this
example:

  &uart4 {
      status = "okay";
  };

but, really, that's just a special case of the first example so
there's nothing new here. but it's the next section called
"Overwriting nodes" that seems confusing:

  "Whole nodes can be overwritten by simply redefining them. Like
  overwriting properties, latter definitions overwrite earlier
  definitions."

I don't think that's right. I think you have to use /delete-node/
first.

  right, that was also my limited understanding, so you can
understand my confusion based on what could be interpreted as a claim
that entire nodes could be overwritten.

quoted

wait, what? entire nodes can be overwritten by redefining them?
the example given looks like this:

  "E.g. to overwrite the pin configuration of Vybrids UART2 (UART_B)
  overwrite the uart2grp node by simply redefining it in your device
  tree ..."

with the device tree content

  &iomuxc {
      vf610-colibri {
          pinctrl_uart2: uart2grp {
              fsl,pins = <
                  VF610_PAD_PTD0__UART2_TX        0x21a2
                  VF610_PAD_PTD1__UART2_RX        0x21a1
              >;
          };
          ...
      };
  };

hang on ... isn't "vf610-colibri" also a "node" in this device
tree? does the above mean that the *entire* vf610-colibri node is
being overwritten? and that i would have to supply the entire new
content of that node, including simply duplicating the rest of the
content that is already there that i want to keep? (is that what
the "..." represents?)

No, you should get a merged vf610-colibri node with the original
contents and any new or overwritten properties from this chunk.

IOW, only properties are overwritten.

  good, that makes *way* more sense, thanks for clarifying that.
followup question coming shortly ...

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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