From ca7c12643c646bbf6fd2ec8e936d74059d33f5c4 Mon Sep 17 00:00:00 2001
From: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Date: Tue, 13 Jul 2021 16:51:00 +0200
Subject: [PATCH] tests/periph_uart_mode: Drop dep to periph_timer

Use xtimer only optionally (when periph_timer is available) for the
test. This makes this test applicable to freshly ported boards that
do not yet have a peripheral timer driver.
---
 tests/periph_uart_mode/Makefile           |  3 +--
 tests/periph_uart_mode/Makefile.board.dep |  3 +++
 tests/periph_uart_mode/main.c             | 23 ++++++++++++++++++++++-
 3 files changed, 26 insertions(+), 3 deletions(-)
 create mode 100644 tests/periph_uart_mode/Makefile.board.dep

diff --git a/tests/periph_uart_mode/Makefile b/tests/periph_uart_mode/Makefile
index 04882515c0..329a1ac637 100644
--- a/tests/periph_uart_mode/Makefile
+++ b/tests/periph_uart_mode/Makefile
@@ -2,8 +2,7 @@ include ../Makefile.tests_common
 
 FEATURES_REQUIRED += periph_uart
 FEATURES_REQUIRED += periph_uart_modecfg
-
-USEMODULE += xtimer
+FEATURES_OPTIONAL += periph_timer
 
 # Set this to prevent welcome message from printing and confusing output
 CFLAGS+=-DLOG_LEVEL=LOG_NONE
diff --git a/tests/periph_uart_mode/Makefile.board.dep b/tests/periph_uart_mode/Makefile.board.dep
new file mode 100644
index 0000000000..1da3893d4d
--- /dev/null
+++ b/tests/periph_uart_mode/Makefile.board.dep
@@ -0,0 +1,3 @@
+ifneq (,$(filter periph_timer,$(FEATURES_USED)))
+  USEMODULE += xtimer
+endif
diff --git a/tests/periph_uart_mode/main.c b/tests/periph_uart_mode/main.c
index d15df67ba0..b37d44796b 100644
--- a/tests/periph_uart_mode/main.c
+++ b/tests/periph_uart_mode/main.c
@@ -22,7 +22,9 @@
 #include <string.h>
 #include <stdlib.h>
 
+#include "board.h"
 #include "periph/uart.h"
+#include "periph_conf.h"
 #include "stdio_uart.h"
 #include "xtimer.h"
 
@@ -52,6 +54,25 @@
 /* Stores each mode string for printing at the end of the test */
 static char mode_strings[TOTAL_OPTIONS][MODE_STR_LEN];
 
+static void _delay(void)
+{
+    if (IS_USED(MODULE_XTIMER)) {
+        xtimer_usleep(DELAY_US);
+    }
+    else {
+        /*
+         * As fallback for freshly ported boards with no timer drivers written
+         * yet, we just use the CPU to delay execution and assume that roughly
+         * 20 CPU cycles are spend per loop iteration.
+         *
+         * Note that the volatile qualifier disables compiler optimizations for
+         * all accesses to the counter variable. Without volatile, modern
+         * compilers would detect that the loop is only wasting CPU cycles and
+         * optimize it out - but here the wasting of CPU cycles is desired.
+         */
+        for (volatile uint32_t i = 0; i < CLOCK_CORECLOCK / 20; i++) { }
+    }
+}
 
 static void _get_mode(const uart_data_bits_t data_bits,
                       const uart_parity_t parity, const uart_stop_bits_t stop_bits, char* mode_str) {
@@ -153,7 +174,7 @@ int main(void)
                 if (status == UART_OK) {
                     results[ridx] = true;
                     printf("%s\n", mode_strings[ridx]);
-                    xtimer_usleep(DELAY_US);
+                    _delay();
                 }
                 else {
                     results[ridx] = false;