pw_chrono: Improve SystemClock C API

Changes the SystemClock C Api to:
1) Use a pw_chrono_SystemClock_Duration struct instead of aliasing
   an int64_t under pw_chrono_SystemClock_TickCount which could
   accidentally permit direct tick usage.
2) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H} and
   PW_SYSTEM_CLOCK_{MS,S,MIN,H}_CEIL to permit C API users to
   create durations which round up to the nearest tick for deadlines
   and timeouts, mirroring std::chrono::ceil.
3) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H}_FLOOR to permit C API users to
   create durations which round down to the nearest tick for oddball
   corner cases, mirroring std::chrono::floor.
4) In order to enable said macros, the system_clock_config.h backend
   config was changed to require the clock period as a preprocessor
   defines instead of a std::ratio<>.
5) Renames pw_chrono_SystemClock_TimeDelta to
   pw_chrono_SystemClock_TimeElapsed to make the argument ordering
   make more sense.
6) Changes existing std::chrono::duration_cast usage to
   std::chrono::ceil and std::chrono:floor to set a good example
   to be explicit on rounding.
7) Renames pw_chrono_SystemClock_TickCountsToNsTruncate accordingly to
   pw_chrono_SystemClock_DurationToNsFloor.

Requires: pigweed-internal:9340
Change-Id: Ia628dceac53f964eda7c4aacd3d790b8b0e92207
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31280
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

2 files changed, 18 insertions, 11 deletions

diff --git a/pw_chrono_threadx/public/pw_chrono_threadx/config.h b/pw_chrono_threadx/public/pw_chrono_threadx/config.h
index a4681c5a2..bfb21098d 100644
--- a/pw_chrono_threadx/public/pw_chrono_threadx/config.h
+++ b/pw_chrono_threadx/public/pw_chrono_threadx/config.h
@@ -31,8 +31,10 @@
 #define PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR 100
 #endif  // PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR
 
-static_assert(PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_NUMERATOR >= 1);
-static_assert(PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR >= 1);
+static_assert(PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_NUMERATOR >= 1,
+              "the numerator must be positive and cannot be fractional");
+static_assert(PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR >= 1,
+              "the denominator must be positive and cannot be fractional");
 
 // Because the SystemClock::now() implementation requires the user to invoke it
 // more than once per overflow period, the max timeout is set to ensure that
@@ -43,4 +45,5 @@ static_assert(PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR >= 1);
 #endif  // PW_CHRONO_THREADX_CFG_MAX_TIMEOUT
 
 static_assert((PW_CHRONO_THREADX_CFG_MAX_TIMEOUT > 0) &&
-              (PW_CHRONO_THREADX_CFG_MAX_TIMEOUT <= (TX_WAIT_FOREVER - 1)));
+                  (PW_CHRONO_THREADX_CFG_MAX_TIMEOUT <= (TX_WAIT_FOREVER - 1)),
+              "the timeout must be greater than 0 and less than the sentinel");
diff --git a/pw_chrono_threadx/public/pw_chrono_threadx/system_clock_config.h b/pw_chrono_threadx/public/pw_chrono_threadx/system_clock_config.h
index 3e8a676ba..6ab829aa6 100644
--- a/pw_chrono_threadx/public/pw_chrono_threadx/system_clock_config.h
+++ b/pw_chrono_threadx/public/pw_chrono_threadx/system_clock_config.h
@@ -13,18 +13,20 @@
 // the License.
 #pragma once
 
-#include <ratio>
-
-#include "pw_chrono/epoch.h"
 #include "pw_chrono_threadx/config.h"
 
-namespace pw::chrono::backend {
-
 // ThreadX does not have an API to determine the tick rate/period, instead
 // require the user to specify this through the configuration.
-using SystemClockPeriodSecondsRatio =
-    std::ratio<PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_NUMERATOR,
-               PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR>;
+#define PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_NUMERATOR \
+  PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_NUMERATOR
+#define PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_DENOMINATOR \
+  PW_CHRONO_THREADX_CFG_CLOCK_PERIOD_SECONDS_DENOMINATOR
+
+#ifdef __cplusplus
+
+#include "pw_chrono/epoch.h"
+
+namespace pw::chrono::backend {
 
 // The ThreadX clock starts at zero during initialization, approximately the
 // time since boot.
@@ -37,3 +39,5 @@ constexpr inline bool kSystemClockNmiSafe = false;
 constexpr inline bool kSystemClockFreeRunning = false;
 
 }  // namespace pw::chrono::backend
+
+#endif  // __cplusplus