Fix TRNG for NRF52

* When multiple TRNG objects are initialized, destroying the first
  object will turn the TRNG off for the other objects. This fix
  adds a counter to ensure that only when the last object is
  destroyed will it cause the TRNG to be disabled.

* The corner case where a user request 0 bytes is correctly handled
  and will now return immediately.

targets/TARGET_NORDIC/TARGET_NRF5x/trng_api.c

@@ -37,21 +37,39 @@
  */
 
 #if defined(DEVICE_TRNG)
-#include "trng_api.h"
+
+#include "hal/trng_api.h"
+#include "hal/critical_section_api.h"
+
 #include "nrf_drv_rng.h"
 
+/* Keep track of instantiated FlashIAP objects. */
+static int nordic_trng_counter = 0;
+
 void trng_init(trng_t *obj)
 {
-    (void) obj;
+    MBED_ASSERT(obj);
 
-    (void)nrf_drv_rng_init(NULL);
+    /* Increment global counter. */
+    nordic_trng_counter++;
+
+    /* Initialize TRNG on first object only. */
+    if (nordic_trng_counter == 1) {
+        nrf_drv_rng_init(NULL);
+    }
 }
 
 void trng_free(trng_t *obj)
 {
-    (void) obj;
+    MBED_ASSERT(obj);
 
-    nrf_drv_rng_uninit();
+    /* Decrement global counter. */
+    nordic_trng_counter--;
+
+    /* Deinitialize TRNG when all objects have been freed. */
+    if (nordic_trng_counter == 0) {
+        nrf_drv_rng_uninit();
+    }
 }
 
 /* Get random data from NRF5x TRNG peripheral.
@@ -61,30 +79,45 @@ void trng_free(trng_t *obj)
  */
 int trng_get_bytes(trng_t *obj, uint8_t *output, size_t length, size_t *output_length)
 {
-    uint8_t bytes_available;
+    MBED_ASSERT(obj);
+    MBED_ASSERT(output);
+    MBED_ASSERT(output_length);
 
-    (void) obj;
+    int result = 0;
 
-    nrf_drv_rng_bytes_available(&bytes_available);
+    /* Return immediately if requested length is zero. */
+    if (length != 0) {
 
-    if (bytes_available == 0) {
+        /* Query how many bytes are available. */
-        nrf_drv_rng_block_rand(output, 1);
+        uint8_t bytes_available;
-        *output_length = 1;
+        nrf_drv_rng_bytes_available(&bytes_available);
-    } else {
 
-        if (bytes_available > length) {
+        /* If no bytes are cached, block until at least 1 byte is available. */
-            bytes_available = length;
+        if (bytes_available == 0) {
-        }
+            nrf_drv_rng_block_rand(output, 1);
-
+            *output_length = 1;
-        if (nrf_drv_rng_rand(output, bytes_available) != NRF_SUCCESS) {
-            *output_length = 0;
-            return -1;
         } else {
-            *output_length = bytes_available;
+
+            /* Get up to the requested number of bytes. */
+            if (bytes_available > length) {
+                bytes_available = length;
+            }
+
+            ret_code_t result = nrf_drv_rng_rand(output, bytes_available);
+
+            /* Set output length with available bytes. */
+            if (result == NRF_SUCCESS) {
+                *output_length = bytes_available;
+            } else {
+                *output_length = 0;
+            }
         }
+
+        /* Set return value based on how many bytes were read. */
+        result = (*output_length == 0) ? -1 : 0;
     }
 
-    return 0;
+    return result;
 }
 
 #endif

targets/targets.json

@@ -3508,6 +3508,7 @@
             "ITM",
             "TRNG",
             "FLASH",
+            "TRNG",
             "STCLK_OFF_DURING_SLEEP"
         ],
         "extra_labels": [
@@ -3541,7 +3542,7 @@
         "supported_form_factors": ["ARDUINO"],
         "inherits": ["MCU_NRF52832"],
         "macros_add": ["BOARD_PCA10040", "NRF52_PAN_12", "NRF52_PAN_15", "NRF52_PAN_58", "NRF52_PAN_55", "NRF52_PAN_54", "NRF52_PAN_31", "NRF52_PAN_30", "NRF52_PAN_51", "NRF52_PAN_36", "NRF52_PAN_53", "S132", "CONFIG_GPIO_AS_PINRESET", "BLE_STACK_SUPPORT_REQD", "SWI_DISABLE0", "NRF52_PAN_20", "NRF52_PAN_64", "NRF52_PAN_62", "NRF52_PAN_63"],
-        "device_has_add": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE", "TRNG"],
+        "device_has_add": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE"],
         "release_versions": ["5"],
         "device_name": "nRF52832_xxAA"
     },
@@ -3638,6 +3639,7 @@
         "device_has": [
             "ITM",
             "FLASH",
+            "TRNG",
             "STCLK_OFF_DURING_SLEEP"
         ],
         "extra_labels": [
@@ -3671,7 +3673,7 @@
         "supported_form_factors": ["ARDUINO"],
         "inherits": ["MCU_NRF52840"],
         "macros_add": ["BOARD_PCA10056", "CONFIG_GPIO_AS_PINRESET", "SWI_DISABLE0", "NRF52_ERRATA_20"],
-        "device_has_add": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPISLAVE", "TRNG"],
+        "device_has_add": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPISLAVE"],
         "release_versions": ["5"],
         "device_name": "nRF52840_xxAA",
         "bootloader_supported": true