[U-Boot-Users] fdt_find_compatible_node() and friends
Wolfgang Grandegger
wg at grandegger.com
Thu May 17 14:31:15 CEST 2007
Jerry Van Baren wrote:
> Wolfgang Grandegger wrote:
>> Hi Jerry,
>>
>> Jerry Van Baren wrote:
>> [...]
>>> Hi Wolfgang G,
>>>
>>> I've applied your patches to my local (working) repository and will
>>> push the changes tonight (my tonight, your tomorrow ;-). I created a
>>> subroutine out of three snippets of code in cmd_fdt.c which your
>>> fdt_find_node_by_path() change fixed up so I had to fix one line in
>>> the new subroutine by hand. Not bad at all considering the changes I
>>> made in that file.
>>
>> Thanks. In the meantime I observed, that
>>
>> fdt_find_node_by_path(fdt, 0, "/");
>>
>> returns an error. Is that by purpose?
> [snip]
>> Wolfgang.
>
> Hmm, interesting observation. The character '/' is a figment of our
> imagination, offset 0 in the tree is the root node. The character '/'
> is the path separator and doesn't actually exist anywhere in the fdt -
> when parsing paths, the stuff between the slashes is searched for and
> the slashes themselves are skipped over.
>
> What you asked in the above call is a node with no name under the root
> node, i.e. "//" in human-speak. That wasn't found, of course. On the
> other hand, it is a pretty obvious "mistake" (I was guilty of making the
> same mistake when I first tried to use fdt_path_offset()).
And it is frequently used in the kernel as the following command reveals:
$ find . -name '*.c' | xargs grep find_node_by_path | grep '"/"'
>
> It seems like I was forever doing a conditional:
>
> 59 if (strcmp(pathp, "/") == 0) {
> 60 nodeoffset = 0;
> 61 } else {
> 62 nodeoffset = fdt_path_offset (fdt, pathp);
> 63 if (nodeoffset < 0) {
>
> <http://www.denx.de/cgi-bin/gitweb.cgi?p=u-boot/u-boot-fdt.git;a=blob;f=common/cmd_fdt.c;h=bf0aef70cedea6ec4a2a446af54c9e1467617a96;hb=HEAD#l55>
I actually wanted to get the string for the property "model" and yes,
fdt_getprop(fdt, 0, "model", NULL) does work.
>
> Trivia: Your patch we are discussing changed exactly this
> fdt_path_offset() into fdt_find_node_by_path() - this is the one place
> your patch didn't apply, because I refactored the three conditionals
> into a single wrapper subroutine.
>
> At the risk of being accused of codling our users, I would propose we
> add the equivalent "/" detection code (above) to
> fdt_find_node_by_path(), (I will do that tonight unless you beat me to
> it). It seems silly to have the caller replicate or wrap the
> conditional since it is going to be such a common idiom/mistake.
Thanks... and no hurry.
Wolfgang.
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