tests/drivers/candev: minor cleanup

- Do not hard code the number of the CAN interface to use. (This
  also allows specifying it via make command line / environment
  variable.)
- Make less use of preprocessor and rely on compiler to eliminate dead
  branches and unused variables.

tests/drivers/candev/Makefile

@@ -6,4 +6,9 @@ USEMODULE += shell
 USEMODULE += can
 USEMODULE += isrpipe
 
+# The number of the CAN bus to use can be controlled via environment
+# variable CAN_DEV:
+CAN_DEV ?= 0
+CFLAGS += -DCONFIG_CAN_DEV=$(CAN_DEV)
+
 include $(RIOTBASE)/Makefile.include

tests/drivers/candev/main.c

@@ -19,42 +19,38 @@
  * @}
  */
 
-#include <assert.h>
-#include <errno.h>
 #include <isrpipe.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include "shell.h"
-#include "test_utils/expect.h"
-#include "can/device.h"
-
-#if IS_USED(MODULE_PERIPH_CAN)
 
-#include "periph/can.h"
-#include "can_params.h"
 #include "board.h"
-#include "periph_conf.h"
-#include "periph/gpio.h"
-
-static can_t periph_dev;
-
-#elif defined(MODULE_MCP2515)
+#include "can_params.h"
 #include "candev_mcp2515.h"
-#include "mcp2515_params.h"
+#include "periph/can.h"
+#include "periph_conf.h"
+#include "shell.h"
+#include "test_utils/expect.h"
 
-static candev_mcp2515_t mcp2515_dev;
+/* The params header is only in the include path when the module is used */
+#if MODULE_MCP2515
+#  include "mcp2515_params.h"
+#endif
 
-#else
-/* add includes for other candev drivers here */
+#ifdef BOARD_SAME54_XPRO
+#  include "periph/gpio.h"
 #endif
 
 #define ENABLE_DEBUG 0
-#include <debug.h>
+#include "debug.h"
 
 /* Default is not using loopback test mode */
 #ifndef CONFIG_USE_LOOPBACK_MODE
-#define CONFIG_USE_LOOPBACK_MODE        0
+#  define CONFIG_USE_LOOPBACK_MODE  0
+#endif
+
+#ifndef CONFIG_CAN_DEV
+#  define CONFIG_CAN_DEV 0
 #endif
 
 #define RX_RINGBUFFER_SIZE 128      /* Needs to be a power of 2! */
@@ -63,6 +59,20 @@ static uint8_t rx_ringbuf[RX_RINGBUFFER_SIZE];
 
 static candev_t *candev = NULL;
 
+/* Only one of them is actually used, depending on the driver selected.
+ * We rely on the compiler to garbage collect the unused */
+static can_t periph_dev;
+static candev_mcp2515_t mcp2515_dev;
+
+/* The params header is only in the include path when the module is used,
+ * so we fall back to a NULL ptr if not */
+#if MODULE_MCP2515
+static candev_mcp2515_conf_t *mcp2515_conf = &candev_mcp2515_conf[CONFIG_CAN_DEV];
+#else
+static candev_mcp2515_conf_t *mcp2515_conf = NULL;
+#endif
+
+
 static int _send(int argc, char **argv)
 {
     int ret = 0;
@@ -275,26 +285,33 @@ static void _can_event_callback(candev_t *dev, candev_event_t event, void *arg)
 
 int main(void)
 {
-
     puts("candev test application\n");
 
     isrpipe_init(&rxbuf, (uint8_t *)rx_ringbuf, sizeof(rx_ringbuf));
-#if IS_USED(MODULE_PERIPH_CAN)
-    puts("Initializing CAN periph device");
-    can_init(&periph_dev, &(candev_conf[0]));
-    candev = &(periph_dev.candev);
+    if (IS_USED(MODULE_PERIPH_CAN)) {
+        puts("Initializing CAN periph device");
+        can_init(&periph_dev, &(candev_conf[CONFIG_CAN_DEV]));
+        candev = &(periph_dev.candev);
 #if defined(BOARD_SAME54_XPRO)
-    gpio_init(AT6561_STBY_PIN, GPIO_OUT);
-    gpio_clear(AT6561_STBY_PIN);
-#endif
-#elif defined(MODULE_MCP2515)
-    puts("Initializing MCP2515");
-    candev_mcp2515_init(&mcp2515_dev, &candev_mcp2515_conf[0]);
-    candev = (candev_t *)&mcp2515_dev;
-
-#else
-    /* add initialization for other candev drivers here */
+        gpio_init(AT6561_STBY_PIN, GPIO_OUT);
+        gpio_clear(AT6561_STBY_PIN);
 #endif
+    }
+    else if (IS_USED(MODULE_MCP2515)) {
+        puts("Initializing MCP2515");
+        candev_mcp2515_init(&mcp2515_dev, mcp2515_conf);
+        candev = &mcp2515_dev.candev;
+    }
+    else {
+        /* No CAN driver is used or used CAN driver is not integrated in this
+         * test yet. We use an undefined function name to let this fail at
+         * compile time. The conditions above are all compile time constants
+         * and the compiler will eliminate the dead branches. So if any of them
+         * matched, this function call will not be part of the compiled object
+         * file and linking will work. */
+        extern void the_can_test_apps_depends_on_a_supported_can_driver_but_none_is_used(void);
+        the_can_test_apps_depends_on_a_supported_can_driver_but_none_is_used();
+    }
 
     expect(candev);