stm32:rng: add helper to actually get random numbers

Simplified blocking API, with an async routine if you really need it.
Follows as best as I can understand the reference manual, but testing
those conditions will be difficult.

include/libopencm3/stm32/common/rng_common_v1.h

@@ -28,6 +28,9 @@ specific memorymap.h header before including this header file.*/
 #ifndef LIBOPENCM3_RNG_V1_H
 #define LIBOPENCM3_RNG_V1_H
 
+#include <stdbool.h>
+#include <stdint.h>
+
 /**@{*/
 
 /* --- Random number generator registers ----------------------------------- */
@@ -72,6 +75,8 @@ BEGIN_DECLS
 
 void rng_enable(void);
 void rng_disable(void);
+bool rng_get_random(uint32_t *rand_nr);
+uint32_t rng_get_random_blocking(void);
 
 END_DECLS
 

lib/stm32/common/rng_common_v1.c

@@ -41,4 +41,51 @@ void rng_enable(void)
     RNG_CR |= RNG_CR_RNGEN;
 }
 
+/** Randomizes a number (non-blocking).
+ * Can fail if a clock error or seed error is detected. Consult the Reference
+ * Manual, but "try again", potentially after resetting the peripheral
+ * @param pointer to a uint32_t that will be randomized.
+ * @returns true on success, pointer is only written to on success
+ * @sa rng_get_random_blocking
+ */
+bool rng_get_random(uint32_t *rand_nr)
+{
+        /* data ready */
+        if (!(RNG_SR & RNG_SR_DRDY)) {
+                return false;
+        }
+
+        /* Check for errors */
+        if (RNG_SR & (RNG_SR_CECS | RNG_SR_SECS)) {
+                return false;
+        }
+
+        *rand_nr = RNG_DR;
+
+        return true;
+}
+
+
+/**
+ * Get a random number and block until it works.
+ * Unless you have a clock problem, this should always return "promptly"
+ * If you have a clock problem, you will wait here forever!
+ * @returns a random 32bit number
+ */
+uint32_t rng_get_random_blocking(void)
+{
+        uint32_t rv;
+        bool done;
+        do {
+                if (RNG_SR & RNG_SR_SECS) {
+                        rng_disable();
+                        rng_enable();
+                }
+                done = rng_get_random(&rv);
+        } while (!done);
+
+        return rv;
+}
+
+
 /**@}*/