kernel: crude k_busy_wait() implementation

This allows for builds with CONFIG_SYS_CLOCK_EXISTS=n in which case
busy waits are achieved with a crude CPU loop. If ever accuracy is
needed even with such a configuration then implementing arch_busy_wait()
should be considered.

Signed-off-by: Nicolas Pitre <[email protected]>

Author: Nicolas Pitre

kernel/Kconfig

@@ -755,6 +755,18 @@ config SYS_CLOCK_MAX_TIMEOUT_DAYS
 	  algorithm is selected for conversion if maximum timeout represented in
 	  source frequency domain multiplied by target frequency fits in 64 bits.
 
+config BUSYWAIT_CPU_LOOPS_PER_USEC
+	int "Number of CPU loops per microsecond for crude busy looping"
+	depends on !SYS_CLOCK_EXISTS && !ARCH_HAS_CUSTOM_BUSY_WAIT
+	default 500
+	help
+	  Calibration for crude CPU based busy loop duration. The default
+	  is assuming 1 GHz CPU and 2 cycles per loop. Reality is certainly
+	  much worse but all we want here is a ball-park figure that ought
+	  to be good enough for the purpose of being able to configure out
+	  system timer support. If accuracy is very important then
+	  implementing arch_busy_wait() should be considered.
+
 config XIP
 	bool "Execute in place"
 	help

kernel/busy_wait.c

@@ -19,7 +19,7 @@ void z_impl_k_busy_wait(uint32_t usec_to_wait)
 
 #if defined(CONFIG_ARCH_HAS_CUSTOM_BUSY_WAIT)
 	arch_busy_wait(usec_to_wait);
-#else
+#elif defined(CONFIG_SYS_CLOCK_EXISTS)
 	uint32_t start_cycles = k_cycle_get_32();
 
 	/* use 64-bit math to prevent overflow when multiplying */
@@ -37,6 +37,17 @@ void z_impl_k_busy_wait(uint32_t usec_to_wait)
 			break;
 		}
 	}
+#else
+	/*
+	 * Crude busy loop for the purpose of being able to configure out
+	 * system timer support.
+	 */
+	unsigned int loops_per_usec = CONFIG_BUSYWAIT_CPU_LOOPS_PER_USEC;
+	unsigned int loops = loops_per_usec * usec_to_wait;
+
+	while (loops-- > 0) {
+		arch_nop();
+	}
 #endif
 
 	SYS_PORT_TRACING_FUNC_EXIT(k_thread, busy_wait, usec_to_wait);