dmesg documentation

docs/Cli.md

@@ -76,6 +76,7 @@ While connected to the CLI, all Logical Switches are temporarily disabled (5.1.0
 | `bootlog` | Show boot events |
 | `color` | Configure colors |
 | `defaults` | Reset to defaults and reboot |
+| `dmesg` | Show init logs from [serial_printf_debugging](./development/serial_printf_debugging.md) |
 | `dfu` | DFU mode on reboot |
 | `diff` | List configuration changes from default |
 | `dump` | Dump configuration |

docs/development/serial_printf_debugging.md

@@ -4,7 +4,9 @@
 
 INAV offers a function to use serial `printf` style debugging.
 
-This provides a simple and intuitive debugging facility. This facility is only available after the serial sub-system has been initialised, which should be adequate for all but the most hard-core debugging requirements.
+This provides a simple and intuitive debugging facility.
+This facility is only available after the serial sub-system has been initialised, but logs generated prior to serial
+initialization can be obtained via the `dmesg` functionality.
 
 In order to use this feature, the source file must include `common/log.h`.
 
@@ -137,3 +139,12 @@ The output will be formatted as follows:
 For the Configurator, debug messages are shown in the developer console log.
 
 Note: The numeric value in square brackets is the FC uptime in seconds.
+
+To see printf-style log messages generated prior serial initialization, reserve about 2KB-4KB RAM to buffer the
+log) by defining DMESG_SIZE:
+#define DMESG_SIZE 2048
+
+Then `make clean` and `make`.
+
+Then in the CLI you can run `dmesg` to see the buffered log.
+Note dmesg also requires that a serial port be defined for serial debugging.