Question about make coverage-html

Hi hackers.

The example of test coverage in the documentation [1] works as advertised.

But I wanted to generate test coverage results only of some TAP tests
in src/test/subscription.

The documentation [1] also says "The make commands also work in
subdirectories." so I tried running them all in that folder.

However, when I run "make coverage-html" in that subdirectory
src/test/subscription it does not work:

=====
[postgres(at)CentOS7-x64 subscription]$ make coverage-html
/usr/local/bin/lcov --gcov-tool /usr/bin/gcov -q --no-external -c -i
-d . -d . -o lcov_base.info
geninfo: WARNING: no .gcno files found in . - skipping!
geninfo: WARNING: no .gcno files found in . - skipping!
/usr/local/bin/lcov --gcov-tool /usr/bin/gcov -q --no-external -c -d .
-d . -o lcov_test.info
geninfo: WARNING: no .gcda files found in . - skipping!
geninfo: WARNING: no .gcda files found in . - skipping!
rm -rf coverage
/usr/local/bin/genhtml -q --legend -o coverage --title='PostgreSQL
14devel' --num-spaces=4 --prefix='/home/postgres/oss_postgres_2PC'
lcov_base.info lcov_test.info
genhtml: ERROR: no valid records found in tracefile lcov_base.info
make: *** [coverage-html-stamp] Error 255
[postgres(at)CentOS7-x64 subscription]$
=====

OTOH, running the "make coverage-html" at the top folder after running
my TAP tests does give the desired coverage results.

~

QUESTION:

Was that documentation [1] just being misleading by saying it can work
in the subdirectories?
e.g. Are you only supposed to run "make coverage-html" from the top folder?

Or is it supposed to work but I did something wrong?

--

[1] https://www.postgresql.org/docs/13/regress-coverage.html

Kind Regards.
Peter Smith
Fujitsu Australia.