nrf_security: CRACEN: Keep TRNG initialized while fetching entropy

The internal trng_get_entropy() function calls sx_trng_get()
iteratively to get the required number of TRNG bytes since
it's recommended to not request more than 32 bytes of data.

This commit is aimed to increase performance by keeping
TRNG initialized between calls to sx_trng_get() till
the required number of entropy bytes is not read.

For instance, for nRF54LM20A 384-bit seed generation
before this optimization took approx. 8.7ms and 5ms after.

Signed-off-by: Anton Zyma <anton.zyma@nordicsemi.no>

Author: AntonZma

subsys/nrf_security/src/drivers/cracen/cracenpsa/src/cracen_psa_ctr_drbg.c

@@ -67,18 +67,17 @@ NRF_SECURITY_MUTEX_DEFINE(cracen_prng_trng_mutex);
  *
  * @return 0 on success, nonzero on failure.
  */
-static int trng_get_entropy(uint8_t *dst, int num_trng_bytes)
+static int trng_get_entropy(uint8_t *dst, size_t num_trng_bytes)
 {
-	int sx_err;
+	int sx_err = SX_ERR_RESET_NEEDED;
 	struct sx_trng trng;
-	int trng_chunk_size;
+	size_t trng_chunk_size;
 	bool loop_continue;
 
 	while (num_trng_bytes) {
 		/* The sx_trng_get function suggests to return <= 32 bytes at a time */
 		trng_chunk_size = MIN(num_trng_bytes, 32);
 
-		sx_err = SX_ERR_RESET_NEEDED;
 		loop_continue = true;
 		while (loop_continue) {
 			switch (sx_err) {
@@ -104,13 +103,15 @@ static int trng_get_entropy(uint8_t *dst, int num_trng_bytes)
 			}
 		}
 
-		(void)sx_trng_close(&trng);
-		if (sx_err != SX_OK) {
-			return sx_err;
-		}
-
 		num_trng_bytes -= trng_chunk_size;
 		dst += trng_chunk_size;
+
+		if (num_trng_bytes && sx_err == SX_OK) {
+			sx_err = SX_ERR_HW_PROCESSING;
+		} else {
+			(void)sx_trng_close(&trng);
+			return sx_err;
+		}
 	}
 
 	return SX_OK;