How does HAL code get compiled, built, and used? (Pixel 9)

joSS

I have been using GrapheneOS on my Pixel 9 and was looking into the source code at 'hardware/interfaces/' to learn how it works, as I have interest in contributing to some code related to this. After reviewing the project files, I have started getting a rough idea of how it works, which I will share below. But I would like some confirmation, assistance in filling in some gaps in knowledge, etc. with my understanding of the code.

So the code in 'hardware/interfaces/' seems to be the equivalent of "HAL source code".
After reviewing "Makefile-like" files in the project (such as at 'out/soong/Android-tokay.mk' and 'out/soong/installs-tokay.mk'), it seems like I can pass different targets to the 'm' command such as 'android.hardware.gnss@2.0-service' and 'android.hardware.gnss@1.0-impl'. After that, I can see .so files (which are object files?) created like 'out/target/product/tokay/vendor/lib64/hw/android.hardware.gnss@1.0-impl.so'.
-- I'm skeptical if, by default, the project builds objects and executables related to this code. I have not been able to make small changes to the logs in that directory, and have the resulting build on the Pixel show those logs in the system logs. Even after trying to build these other targets and create another zip, I still cannot see my logs in the system logs.
Then it becomes unclear to me if/how the object files get linked into some binary or executable that is placed elsewhere in the 'out/' directory. Going off of the GNSS example, one of those targets will create out/target/product/tokay/vendor/bin/hw/android.hardware.gnss@2.0-service. This seems like it would be the executable, but I'm not too sure.
-- I compare that "executable" with vendor/google_devices/tokay/proprietary/vendor/bin/hw/gnssd (which seems to be a file extracted from the vendor files). But it has quite a different file size (100KB vs ₆,000KB) and when inspecting the files with 'nm' command, android.hardware.gnss@2.0-service will report no symbols and gnssd will have output.
-- I also am guessing that gnssd is something independent of the "executables" created from these targets, and that there is potentially some interdependence (or perhaps linking) between these two.

Again, if I could get some clarifications, corrections (or some pointers for me to do more research), etc. on my interpretation of how this code gets used for building, I would appreciate it.

wizoatk

There's a fair amount of magic in adevtool that replaces device source as Google/AOSP no longer provides device trees.

https://github.com/GrapheneOS/adevtool

joSS

wizoatk So does that mean that the code from 'hardware/interfaces/' is obsolete and/or meant for older devices, which do have device trees from Google/AOSP?

Or perhaps the 'hardware/interfaces/' code can still be used? The Github README mentions "building modules from source (whenever possible)" so perhaps there's a setting in there that I need to enable?

Also, you mention that it "replaces device source". But wouldn't it not be a true replacement, as the source isn't openly available? From my understanding, it seems to just provide binaries/files (i.e., the output from building the source), but not the source itself.