machine/stm32f4, stm32f7, stm32l4: implement TRNG for randomness

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/crypto/rand/rand_baremetal.go

@@ -1,5 +1,5 @@
-//go:build stm32wlx || (sam && atsamd51) || (sam && atsame5x)
-// +build stm32wlx sam,atsamd51 sam,atsame5x
+//go:build stm32wlx || stm32f4 || stm32f7 || stm32l4 || (sam && atsamd51) || (sam && atsame5x)
+// +build stm32wlx stm32f4 stm32f7 stm32l4 sam,atsamd51 sam,atsame5x
 
 package rand
 

src/machine/machine_stm32_rng.go

@@ -0,0 +1,29 @@
+//go:build stm32f4 || stm32f7 || stm32l4
+// +build stm32f4 stm32f7 stm32l4
+
+package machine
+
+import "device/stm32"
+
+var rngInitDone = false
+
+const RNG_MAX_READ_RETRIES = 1000
+
+// GetRNG returns 32 bits of cryptographically secure random data
+func GetRNG() (uint32, error) {
+	if !rngInitDone {
+		initRNG()
+		rngInitDone = true
+	}
+
+	cnt := RNG_MAX_READ_RETRIES
+	for !stm32.RNG.SR.HasBits(stm32.RNG_SR_DRDY) {
+		cnt--
+		if cnt == 0 {
+			return 0, ErrTimeoutRNG
+		}
+	}
+
+	ret := stm32.RNG.DR.Get()
+	return ret, nil
+}

src/machine/machine_stm32f4.go

@@ -1,3 +1,4 @@
+//go:build stm32f4
 // +build stm32f4
 
 package machine
@@ -586,3 +587,8 @@ const (
 	ARR_MAX = 0x10000
 	PSC_MAX = 0x10000
 )
+
+func initRNG() {
+	stm32.RCC.AHB2ENR.SetBits(stm32.RCC_AHB2ENR_RNGEN)
+	stm32.RNG.CR.SetBits(stm32.RNG_CR_RNGEN)
+}

src/machine/machine_stm32f7.go

@@ -1,3 +1,4 @@
+//go:build stm32f7
 // +build stm32f7
 
 package machine
@@ -716,3 +717,8 @@ const (
 	ARR_MAX = 0x10000
 	PSC_MAX = 0x10000
 )
+
+func initRNG() {
+	stm32.RCC.AHB2ENR.SetBits(stm32.RCC_AHB2ENR_RNGEN)
+	stm32.RNG.CR.SetBits(stm32.RNG_CR_RNGEN)
+}

src/machine/machine_stm32l0.go

@@ -1,3 +1,4 @@
+//go:build stm32l0
 // +build stm32l0
 
 package machine

src/machine/machine_stm32l4.go

@@ -1,3 +1,4 @@
+//go:build stm32l4
 // +build stm32l4
 
 package machine
@@ -494,3 +495,12 @@ const (
 	ARR_MAX = 0x10000
 	PSC_MAX = 0x10000
 )
+
+func initRNG() {
+	stm32.RCC.CRRCR.SetBits(stm32.RCC_CRRCR_HSI48ON)
+	for !stm32.RCC.CRRCR.HasBits(stm32.RCC_CRRCR_HSI48RDY) {
+	}
+
+	stm32.RCC.AHB2ENR.SetBits(stm32.RCC_AHB2ENR_RNGEN)
+	stm32.RNG.CR.SetBits(stm32.RNG_CR_RNGEN)
+}