/dev/urandom: Add option to replace software PRNG with hardware TRNG.

drivers/Kconfig

@@ -34,8 +34,8 @@ config DEV_URANDOM
 	bool "Enable /dev/urandom"
 	default n
 	---help---
-		Enable support for /dev/urandom provided by a software PRNG
+		Enable support for /dev/urandom provided by either a hardware TRNG or
-		implementation.
+		by a software PRNG implementation.
 
 		NOTE:  This option may not be cryptographially secure and should not
 		be enabled if you are concerned about cyptographically secure
@@ -46,7 +46,8 @@ if DEV_URANDOM
 
 choice
 	prompt "/dev/urandom algorithm"
-	default DEV_URANDOM_XORSHIFT128
+	default DEV_URANDOM_ARCH if ARCH_HAVE_RNG
+	default DEV_URANDOM_XORSHIFT128 if !ARCH_HAVE_RNG
 
 config DEV_URANDOM_XORSHIFT128
 	bool "xorshift128"
@@ -66,6 +67,18 @@ config DEV_URANDOM_CONGRUENTIAL
 
 		NOTE: Not cyptographically secure
 
+config DEV_URANDOM_ARCH
+	bool "Architecture-specific"
+	depends on ARCH_HAVE_RNG
+	---help---
+		The implementation of /dev/urandom is provided in archtecture-
+		specific logic using hardware TRNG logic.  architecture-specific
+		logic must provide the whole implementation in this case, including
+		the function devurandom_register().
+
+		May or may not be cyptographically secure, depending upon the
+		implementation.
+
 endchoice # /dev/urandom algorithm
 endif # DEV_URANDOM
 

drivers/Makefile

@@ -100,9 +100,11 @@ ifeq ($(CONFIG_PWM),y)
 endif
 
 ifeq ($(CONFIG_DEV_URANDOM),y)
+ifneq ($(CONFIG_DEV_URANDOM_ARCH),y)
   CSRCS += dev_urandom.c
 endif
 endif
+endif # CONFIG_NFILE_DESCRIPTORS != 0
 
 AOBJS = $(ASRCS:.S=$(OBJEXT))
 COBJS = $(CSRCS:.c=$(OBJEXT))

drivers/dev_urandom.c

@@ -53,6 +53,8 @@
 #include <nuttx/lib.h>
 #include <nuttx/fs/fs.h>
 
+#if defined(CONFIG_DEV_URANDOM) && !defined(CONFIG_DEV_URANDOM_ARCH)
+
 /****************************************************************************
  * Pre-processor Definitions
  ****************************************************************************/
@@ -297,3 +299,5 @@ void devurandom_register(void)
 
   (void)register_driver("/dev/urandom", &devurand_fops, 0666, NULL);
 }
+
+#endif /* CONFIG_DEV_URANDOM && CONFIG_DEV_URANDOM_ARCH */