dev: linux: add docs explaining how to use the linux target

Add a dedicated linux page behind the developers guide that reminds devs
to use linux for UI testing and informs them about the presence of the
file source.

Signed-off-by: Ryan Turner <ryan@turnrye.com>

Author: turnrye

developers_guide.md

@@ -2,6 +2,7 @@
 
 * [Compilation instructions](compiling.md)
 * [Writing and running test suites](tests.md)
+* [Validating changes using the Linux platform](linux.md)
 * [Software documentation](software.md)
 * [Notes on handling of non-native digital modes](software_modes.md)
 * [Code style guidelines](coding_style.md)

linux.md

@@ -0,0 +1,41 @@
+# Testing on Linux
+
+While it poses limited to no utility for users, the Linux build may be very helpful for developers. 
+
+## User interface testing
+
+The `openrtx_linux` and `openrtx_linux_mod17` targets may be used to validate changes to the UI without having to have a physical device present.
+
+## Testing Software-Decoded Baseband
+
+The Linux build can demodulate a pre-recorded baseband file, which is useful for testing the receive path end-to-end without RF hardware.
+
+### Sample baseband files
+
+The `tests/unit/assets` directory contains the following baseband recordings:
+
+| File                       | Use                                     |
+|----------------------------|-----------------------------------------|
+| `M17_test_baseband.raw`    | Runtime demodulation; src callsign only |
+| `M17_test_baseband_dc.raw` | Runtime demodulation; src callsign only |
+
+
+### Demodulating a raw baseband file with OpenRTX Linux
+
+The Linux build uses a file source driver that reads, on loop, a raw 16-bit, little-endian, mono, 24 kHz file from `/tmp/baseband.raw`. Simply place the sample file in this location and launch the application in order for it to demodulate. Note that since the linux build has no audio driver, it is not expected that received audio can be heard.
+
+```bash
+# Copy the sample to the path the file-source driver reads from:
+cp tests/unit/assets/M17_test_baseband.raw /tmp/baseband.raw
+# Build
+meson setup build_linux
+meson compile -C build_linux openrtx_linux
+# Launch
+./build_linux/openrtx_linux
+```
+
+In this example, a receive screen with callsign OPNRTX is expected.
+
+### Going Further
+
+In theory, you could create a named pipe at `/tmp/baseband.raw` and write to it useing GNU Radio. Something like a source, with resampling to 24kHz if required, then float to short, then a file sink should work.