refactor(trng): cleanup the existing TRNG support

This patch adds the following changes to complete the existing
TRNG implementation:

1. Adds a feature specific scope for buildlog generation.
2. Updates the docs on the build flag "TRNG_SUPPORT" and its values.
3. Makefile update and improves the existing comments at few sections
for better understanding of the underlying logic.

Change-Id: I3f72f0ccd5c94005a2df87158cf23199d2160d37
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>

3 files changed, 31 insertions, 25 deletions

diff --git a/services/std_svc/std_svc_setup.c b/services/std_svc/std_svc_setup.c
index 08d16e21a..2884a3b9e 100644
--- a/services/std_svc/std_svc_setup.c
+++ b/services/std_svc/std_svc_setup.c
@@ -74,7 +74,10 @@ static int32_t std_svc_setup(void)
 	sdei_init();
 #endif
 
+#if TRNG_SUPPORT
+	/* TRNG initialisation */
 	trng_setup();
+#endif /* TRNG_SUPPORT */
 
 #if DRTM_SUPPORT
 	if (drtm_setup() != 0) {
@@ -172,7 +175,8 @@ static uintptr_t std_svc_smc_handler(uint32_t smc_fid,
 		return trng_smc_handler(smc_fid, x1, x2, x3, x4, cookie, handle,
 				flags);
 	}
-#endif
+#endif /* TRNG_SUPPORT */
+
 #if ENABLE_RME
 
 	if (is_rmmd_el3_fid(smc_fid)) {
diff --git a/services/std_svc/trng/trng_entropy_pool.c b/services/std_svc/trng/trng_entropy_pool.c
index ac13b1d7a..30105b3a3 100644
--- a/services/std_svc/trng/trng_entropy_pool.c
+++ b/services/std_svc/trng/trng_entropy_pool.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, ARM Limited. All rights reserved.
+ * Copyright (c) 2021-2022, ARM Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -18,7 +18,7 @@
  * 192 bits of entropy, we don't have to throw out the leftover 1-63 bits of
  * entropy.
  */
-#define WORDS_IN_POOL (4)
+#define WORDS_IN_POOL	(4)
 static uint64_t entropy[WORDS_IN_POOL];
 /* index in bits of the first bit of usable entropy */
 static uint32_t entropy_bit_index;
@@ -27,14 +27,14 @@ static uint32_t entropy_bit_size;
 
 static spinlock_t trng_pool_lock;
 
-#define BITS_PER_WORD (sizeof(entropy[0]) * 8)
-#define BITS_IN_POOL (WORDS_IN_POOL * BITS_PER_WORD)
-#define ENTROPY_MIN_WORD (entropy_bit_index / BITS_PER_WORD)
-#define ENTROPY_FREE_BIT (entropy_bit_size + entropy_bit_index)
-#define _ENTROPY_FREE_WORD (ENTROPY_FREE_BIT / BITS_PER_WORD)
-#define ENTROPY_FREE_INDEX (_ENTROPY_FREE_WORD % WORDS_IN_POOL)
+#define BITS_PER_WORD		(sizeof(entropy[0]) * 8)
+#define BITS_IN_POOL		(WORDS_IN_POOL * BITS_PER_WORD)
+#define ENTROPY_MIN_WORD	(entropy_bit_index / BITS_PER_WORD)
+#define ENTROPY_FREE_BIT	(entropy_bit_size + entropy_bit_index)
+#define _ENTROPY_FREE_WORD	(ENTROPY_FREE_BIT / BITS_PER_WORD)
+#define ENTROPY_FREE_INDEX	(_ENTROPY_FREE_WORD % WORDS_IN_POOL)
 /* ENTROPY_WORD_INDEX(0) includes leftover bits in the lower bits */
-#define ENTROPY_WORD_INDEX(i) ((ENTROPY_MIN_WORD + i) % WORDS_IN_POOL)
+#define ENTROPY_WORD_INDEX(i)	((ENTROPY_MIN_WORD + i) % WORDS_IN_POOL)
 
 /*
  * Fill the entropy pool until we have at least as many bits as requested.
@@ -65,12 +65,12 @@ static bool trng_fill_entropy(uint32_t nbits)
  */
 bool trng_pack_entropy(uint32_t nbits, uint64_t *out)
 {
-	bool success = true;
+	bool ret = true;
 
 	spin_lock(&trng_pool_lock);
 
 	if (!trng_fill_entropy(nbits)) {
-		success = false;
+		ret = false;
 		goto out;
 	}
 
@@ -82,9 +82,8 @@ bool trng_pack_entropy(uint32_t nbits, uint64_t *out)
 	for (word_i = 0; word_i < to_fill; word_i++) {
 		/*
 		 * Repack the entropy from the pool into the passed in out
-		 * buffer. This takes the lower bits from the valid upper bits
-		 * of word_i and the upper bits from the lower bits of
-		 * (word_i + 1).
+		 * buffer. This takes lesser bits from the valid upper bits
+		 * of word_i and more bits from the lower bits of (word_i + 1).
 		 *
 		 * I found the following diagram useful. note: `e` represents
 		 * valid entropy, ` ` represents invalid bits (not entropy) and
@@ -136,7 +135,7 @@ bool trng_pack_entropy(uint32_t nbits, uint64_t *out)
 out:
 	spin_unlock(&trng_pool_lock);
 
-	return success;
+	return ret;
 }
 
 void trng_entropy_pool_setup(void)
diff --git a/services/std_svc/trng/trng_main.c b/services/std_svc/trng/trng_main.c
index 38aa64997..90098a8f2 100644
--- a/services/std_svc/trng/trng_main.c
+++ b/services/std_svc/trng/trng_main.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2021-2022, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -23,9 +23,9 @@ static const uuid_t uuid_null;
 static uintptr_t trng_rnd32(uint32_t nbits, void *handle)
 {
 	uint32_t mask = ~0U;
-	uint64_t ent[2];
+	uint64_t ent[2] = {0};
 
-	if (nbits == 0U || nbits > 96U) {
+	if (nbits == 0U || nbits > TRNG_RND32_ENTROPY_MAXBITS) {
 		SMC_RET1(handle, TRNG_E_INVALID_PARAMS);
 	}
 
@@ -59,9 +59,9 @@ static uintptr_t trng_rnd32(uint32_t nbits, void *handle)
 static uintptr_t trng_rnd64(uint32_t nbits, void *handle)
 {
 	uint64_t mask = ~0ULL;
-	uint64_t ent[3];
+	uint64_t ent[3] = {0};
 
-	if (nbits == 0U || nbits > 192U) {
+	if (nbits == 0U || nbits > TRNG_RND64_ENTROPY_MAXBITS) {
 		SMC_RET1(handle, TRNG_E_INVALID_PARAMS);
 	}
 
@@ -117,9 +117,9 @@ uintptr_t trng_smc_handler(uint32_t smc_fid, u_register_t x1, u_register_t x2,
 	switch (smc_fid) {
 	case ARM_TRNG_VERSION:
 		SMC_RET1(handle, MAKE_SMCCC_VERSION(
-			TRNG_VERSION_MAJOR, TRNG_VERSION_MINOR
-		));
+			TRNG_VERSION_MAJOR, TRNG_VERSION_MINOR));
 		break; /* unreachable */
+
 	case ARM_TRNG_FEATURES:
 		if (is_trng_fid((uint32_t)x1)) {
 			SMC_RET1(handle, TRNG_E_SUCCESS);
@@ -127,16 +127,19 @@ uintptr_t trng_smc_handler(uint32_t smc_fid, u_register_t x1, u_register_t x2,
 			SMC_RET1(handle, TRNG_E_NOT_SUPPORTED);
 		}
 		break; /* unreachable */
+
 	case ARM_TRNG_GET_UUID:
 		SMC_UUID_RET(handle, plat_trng_uuid);
 		break; /* unreachable */
+
 	case ARM_TRNG_RND32:
 		return trng_rnd32((uint32_t)x1, handle);
+
 	case ARM_TRNG_RND64:
 		return trng_rnd64((uint32_t)x1, handle);
+
 	default:
-		WARN("Unimplemented TRNG Service Call: 0x%x\n",
-			smc_fid);
+		WARN("Unimplemented TRNG Service Call: 0x%x\n", smc_fid);
 		SMC_RET1(handle, TRNG_E_NOT_IMPLEMENTED);
 		break; /* unreachable */
 	}