mbedtls: Add full platform implementation of timing

When MBEDTLS_TIMING_C and MBEDTLS_TIMING_ALT are enabled,
the Arm Compiler generates errors like the following (one for
each missing symbol):

    Error: L6218E: Undefined symbol mbedtls_timing_get_delay

Reason:

The function `mbedtls_timing_self_test()` in the Mbed TLS default
`timing.c` always gets compiled, if MBEDTLS_SELF_TEST is defined.
And MBEDTLS_SELF_TEST is always defined, as we have a Greentea test
to run some of the Mbed TLS self tests. (In the future we should try
not to enable MBEDTLS_SELF_TEST except for tests, but it requires
a rework in our test flow.)

`mbedtls_timing_self_test()` tests (calls) the full API declared in
`timing.h`, and the ARM Compiler requires all symbols referenced by
all functions to be defined, even those not used by the final
application. This is unlike GCC_ARM which resolves what are required.

Solution:

To fix the "undefined symbol" errors, we add an implementation of
`mbedtls_timing_get_timer()` based on Mbed OS `LowPowerTimer` or
`Timer` (depending on which one is available), and copy Mbed TLS's
default `mbedtls_timing_set_delay()` and `mbedtls_timing_get_delay()`
which are built on top of `mbedtls_timing_get_timer()`. This will also
benefit user applications that need to enable timing in Mbed TLS.

connectivity/mbedtls/platform/src/timing_mbed.cpp

@@ -1,6 +1,7 @@
 /*
  *  timing.cpp
  *
+ *  Copyright The Mbed TLS Contributors
  *  Copyright (C) 2021, Arm Limited, All Rights Reserved
  *  SPDX-License-Identifier: Apache-2.0
  *
@@ -46,20 +47,23 @@ extern "C" void mbedtls_set_alarm(int seconds)
     t.attach(handle_alarm, std::chrono::seconds(seconds));
 }
 
-#if !defined(HAVE_HARDCLOCK)
-#define HAVE_HARDCLOCK
-static int timer_init = 0;
-
-extern "C" unsigned long mbedtls_timing_hardclock(void)
-{
+// The static Mbed timer here is initialized once only.
+// Mbed TLS can have multiple timers (mbedtls_timing_hr_time) derived
+// from the Mbed timer.
 #if DEVICE_LPTICKER
-    static mbed::LowPowerTimer timer;
+static mbed::LowPowerTimer timer;
 #elif DEVICE_USTICKER
-    static mbed::Timer timer;
+static mbed::Timer timer;
 #else
 #error "MBEDTLS_TIMING_C requires either LPTICKER or USTICKER"
 #endif
+static int timer_init = 0;
 
+#if !defined(HAVE_HARDCLOCK)
+#define HAVE_HARDCLOCK
+
+extern "C" unsigned long mbedtls_timing_hardclock(void)
+{
     if (timer_init == 0) {
         timer.reset();
         timer.start();
@@ -69,3 +73,60 @@ extern "C" unsigned long mbedtls_timing_hardclock(void)
     return timer.elapsed_time().count();
 }
 #endif /* !HAVE_HARDCLOCK */
+
+extern "C" unsigned long mbedtls_timing_get_timer(struct mbedtls_timing_hr_time *val, int reset)
+{
+    if (timer_init == 0) {
+        timer.reset();
+        timer.start();
+        timer_init = 1;
+    }
+
+    if (reset) {
+        val->start = std::chrono::duration_cast<std::chrono::milliseconds>(timer.elapsed_time()).count();
+        return 0;
+    } else {
+        return std::chrono::duration_cast<std::chrono::milliseconds>(timer.elapsed_time()).count() - val->start;
+    }
+}
+
+/**
+ * Note: The following implementations come from the default timing.c
+ * from Mbed TLS. They are disabled in timing.c when MBEDTLS_TIMING_ALT
+ * is defined, but the implementation is nonetheless applicable to
+ * Mbed OS, so we copy them over.
+ */
+
+extern "C" void mbedtls_timing_set_delay(void *data, uint32_t int_ms, uint32_t fin_ms)
+{
+    mbedtls_timing_delay_context *ctx = (mbedtls_timing_delay_context *) data;
+
+    ctx->int_ms = int_ms;
+    ctx->fin_ms = fin_ms;
+
+    if (fin_ms != 0) {
+        (void) mbedtls_timing_get_timer(&ctx->timer, 1);
+    }
+}
+
+extern "C" int mbedtls_timing_get_delay(void *data)
+{
+    mbedtls_timing_delay_context *ctx = (mbedtls_timing_delay_context *) data;
+    unsigned long elapsed_ms;
+
+    if (ctx->fin_ms == 0) {
+        return -1;
+    }
+
+    elapsed_ms = mbedtls_timing_get_timer(&ctx->timer, 0);
+
+    if (elapsed_ms >= ctx->fin_ms) {
+        return 2;
+    }
+
+    if (elapsed_ms >= ctx->int_ms) {
+        return 1;
+    }
+
+    return 0;
+}