build: Define DECL_CONSTANT mechanism for defining exported constants

Add a DECL_CONSTANT macro to allow the firmware to define constants
that are to be exported to the host during the "identify" phase.  This
replaces the existing hardcoded mechanism of scanning the Kconfig
header file for certain constants.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/serial.c

@@ -9,6 +9,7 @@
 #include "autoconf.h" // CONFIG_SERIAL_BAUD
 #include "board/io.h" // readb
 #include "board/misc.h" // console_get_input
+#include "command.h" // DECL_CONSTANT
 #include "sched.h" // DECL_INIT
 #include "irq.h" // irq_save
 
@@ -22,6 +23,8 @@ static uint8_t transmit_pos, transmit_max;
  * Serial hardware
  ****************************************************************/
 
+DECL_CONSTANT(SERIAL_BAUD, CONFIG_SERIAL_BAUD);
+
 static void
 serial_init(void)
 {

src/avr/timer.c

@@ -16,6 +16,9 @@
  * Low level timer code
  ****************************************************************/
 
+DECL_CONSTANT(CLOCK_FREQ, F_CPU);
+DECL_CONSTANT(MCU, CONFIG_MCU);
+
 // Return the number of clock ticks for a given number of microseconds
 uint32_t
 timer_from_us(uint32_t us)

src/command.h

@@ -15,6 +15,10 @@
 // Flags for command handler declarations.
 #define HF_IN_SHUTDOWN   0x01   // Handler can run even when in emergency stop
 
+// Declare a constant exported to the host
+#define DECL_CONSTANT(NAME, VALUE)              \
+    _DECL_CONSTANT(NAME, VALUE)
+
 // Send an output message (and declare a static message type for it)
 #define output(FMT, args...)                    \
     _sendf(_DECL_OUTPUT(FMT) , ##args )
@@ -59,6 +63,11 @@ extern const uint32_t command_identify_size;
         = "_DECL_COMMAND " __stringify(FUNC) " " __stringify(FLAGS) " " MSG; \
     void __visible FUNC(uint32_t*)
 
+#define _DECL_CONSTANT(NAME, VALUE)             \
+    char __PASTE(_DECLC_ ## NAME ## _, __LINE__) []                     \
+        __visible __section(".compile_time_request")                    \
+        = "_DECL_CONSTANT " __stringify(NAME) " " __stringify(VALUE)
+
 // Create a compile time request and return a unique (incrementing id)
 // for that request.
 #define _DECL_REQUEST_ID(REQUEST, ID_SECTION) ({                \

src/sam3x8e/Kconfig

@@ -6,10 +6,6 @@ config BOARD_DIRECTORY
     string
     default "sam3x8e"
 
-config MCU
-    string
-    default "sam3x8e"
-
 config CLOCK_FREQ
     int
     default 42000000 # 84000000/2

src/sam3x8e/serial.c

@@ -9,6 +9,7 @@
 #include "board/gpio.h" // gpio_peripheral
 #include "board/io.h" // readb
 #include "board/misc.h" // console_get_input
+#include "command.h" // DECL_CONSTANT
 #include "sam3x8e.h" // UART
 #include "sched.h" // DECL_INIT
 #include "irq.h" // irq_save
@@ -24,6 +25,8 @@ static uint32_t transmit_pos, transmit_max;
  * Serial hardware
  ****************************************************************/
 
+DECL_CONSTANT(SERIAL_BAUD, CONFIG_SERIAL_BAUD);
+
 static void
 serial_init(void)
 {

src/sam3x8e/timer.c

@@ -16,6 +16,9 @@
  * Low level timer code
  ****************************************************************/
 
+DECL_CONSTANT(CLOCK_FREQ, CONFIG_CLOCK_FREQ);
+DECL_CONSTANT(MCU, "sam3x8e");
+
 // Return the number of clock ticks for a given number of microseconds
 uint32_t
 timer_from_us(uint32_t us)