Adds description for using out-of-tree message definition

[ksschwabe]

Related to PR PX4/PX4-Autopilot#9073.

I wonder whether we should possibly rename this page from out_of_tree_modules.md to something that now represents that it now also includes the out-of-tree uORB message definitions?

@hamishwillee : Do you think it is necessary for me to explain that the new uORB headers and sources will be generated in the same location as the normal uORB messages?

[hamishwillee]

Hi @ksschwabe

Thanks for this!

I wonder whether we should possibly rename this page from out_of_tree_modules.md to something that now represents that it now also includes the out-of-tree uORB message definitions?

No, it is fine.

@hamishwillee : Do you think it is necessary for me to explain that the new uORB headers and sources will be generated in the same location as the normal uORB messages?

I think that is a good idea.

Other things:

1. I think that it is worth updating https://dev.px4.io/en/middleware/uorb.html with a note or section on "out of tree messages". This can simply contain link to the new doc. I can add later if you like.
2. There will be some minor layout issues from this, but I will fix in a post-process.

[hamishwillee]

Outside of scope for this fix - @bkueng (and @ksschwabe )

This doc could use a little more context on the when/why/limitations of external modules. So can you tell me:

• what are external modules typically used for? When would you use one?
• Do external modules get included in firmware builds when you build PX4?
• Are there things you can't do with them?

Where do you think it should live in the devguide tree (currently this doc is an "Advanced Topic - but I'm trying to reduce that folder to a minimum, grouping topics by "what they are for")?

• As I see it we have 3 topics on apps/modules - Writing an application, Out of Tree Modules, and your new module template (see Document template for new modules #467).
• We could leave this where it is - after all it is an advanced topic
• We could move it into "Getting Started" alongside the others
• We could remove Writing an application from getting started, and group all the related topics on writing modules in a "Writing Modules" section.
• Other?

Thoughts?

[bkueng]

Looks good.

what are external modules typically used for? When would you use one?

It's for downstream adopters: people/companies who want to either replace or add one or more (proprietary) module - for example a vehicle-specific position controller. It makes it easier to see which modules are company-internal, and which ones are upstream (of course we encourage people to bring their changes upstream, but this is a method that can be used for cases where they can't/don't want to).

Do external modules get included in firmware builds when you build PX4?

I'm not sure I understand. They get included when following the instructions here.

Are there things you can't do with them?

Hmm, not sure. You can access all the usual PX4 API's, so you have pretty much all the flexibility.

We could leave this where it is - after all it is an advanced topic

I think that makes sense. It's not something that you need to go though when getting started.

[ksschwabe]

@hamishwillee: I have updated my PR with the description related to where the topics are generated.

I also added a link in https://dev.px4.io/en/middleware/uorb.html to point to the new out-of-tree uORB section.

[hamishwillee]

@ksschwabe Thanks very much! I've merged this now. @bkueng Thanks for the review. I'll do a further minor tidy up with your additional info.