[dpdk-users] Building app with source in subdirectory ?doesn't work

[Petr Houska <t-pehous@microsoft.com>]

Hi,
I'm having troubles creating a working Makefile for DPDK app that has source files in subdirectories.
Minimal repro I created uses the `helloworld` sample with following changes:

  1.  Add `tst` subdirectory to project's root
  2.  Add empty `tst.c` file to such subdirectory
  3.  Modify original `Makefile`'s SRC-y variable so that it includes `tst/tst.c`
When doing that the fist invoke of `make` works just fine and build the app correctly. Subsequent invokes
(unless either build directory is cleaned manually or `make clean` is invoked), however, don't track changes in
files and always fail on `make[1]: Nothing to be done for 'tst.o'.`.

I tried debugging the `Makefile` via `make D=1` but it doesn't output any debug information before failing on
abovementioned message.

Moving `tst.c` to root folder solved the issue. The issue has been observed with custom Makefiles referencing
`app.rte.mk` as well so it's not just helloworld's issue.

Petr Houska
Azure Intern