stm32wl: STM32WL TRNG implementation in crypto/rand

src/crypto/rand/rand_baremetal.go

@@ -0,0 +1,36 @@
+//go:build stm32wle5
+// +build stm32wle5
+
+package rand
+
+import (
+	"machine"
+)
+
+func init() {
+	Reader = &reader{}
+}
+
+type reader struct {
+}
+
+func (r *reader) Read(b []byte) (n int, err error) {
+	if len(b) == 0 {
+		return
+	}
+
+	var randomByte uint32
+	for i := range b {
+		if i%4 == 0 {
+			randomByte, err = machine.GetRNG()
+			if err != nil {
+				return n, err
+			}
+		} else {
+			randomByte >>= 8
+		}
+		b[i] = byte(randomByte)
+	}
+
+	return len(b), nil
+}

src/machine/machine.go

@@ -3,6 +3,7 @@ package machine
 import "errors"
 
 var (
+	ErrTimeoutRNG         = errors.New("machine: RNG Timeout")
 	ErrInvalidInputPin    = errors.New("machine: invalid input pin")
 	ErrInvalidOutputPin   = errors.New("machine: invalid output pin")
 	ErrInvalidClockPin    = errors.New("machine: invalid clock pin")

src/machine/machine_stm32wle5.go

@@ -13,6 +13,8 @@ import (
 	"unsafe"
 )
 
+var rngInitDone = false
+
 const (
 	AF0_SYSTEM             = 0
 	AF1_TIM1_2_LPTIM1      = 1
@@ -30,9 +32,10 @@ const (
 )
 
 const (
-	SYSCLK        = 48e6
-	APB1_TIM_FREQ = SYSCLK
-	APB2_TIM_FREQ = SYSCLK
+	SYSCLK               = 48e6
+	APB1_TIM_FREQ        = SYSCLK
+	APB2_TIM_FREQ        = SYSCLK
+	RNG_MAX_READ_RETRIES = 1000
 )
 
 func CPUFrequency() uint32 {
@@ -396,6 +399,27 @@ func (t *TIM) enableMainOutput() {
 	t.Device.BDTR.SetBits(stm32.TIM_BDTR_MOE)
 }
 
+// GetRNG returns 32 bits of cryptographically secure random data
+func GetRNG() (uint32, error) {
+
+	if !rngInitDone {
+		stm32.RCC.AHB3ENR.SetBits(stm32.RCC_AHB3ENR_RNGEN)
+		stm32.RNG.CR.SetBits(stm32.RNG_CR_RNGEN)
+		rngInitDone = true
+	}
+	cnt := RNG_MAX_READ_RETRIES
+	for cnt > 0 && !stm32.RNG.SR.HasBits(stm32.RNG_SR_DRDY) {
+		cnt--
+	}
+	if cnt == 0 {
+		return 0, ErrTimeoutRNG
+	}
+	ret := stm32.RNG.DR.Get()
+	return ret, nil
+}
+
+//----------
+
 type arrtype = uint32
 type arrRegType = volatile.Register32
 

src/runtime/runtime_stm32wle5.go

@@ -57,7 +57,7 @@ func initCLK() {
 		((PLL_Q - 1) << stm32.RCC_PLLCFGR_PLLQ_Pos) |
 		((PLL_R - 1) << stm32.RCC_PLLCFGR_PLLR_Pos) |
 		((PLL_P - 1) << stm32.RCC_PLLCFGR_PLLP_Pos) |
-		stm32.RCC_PLLCFGR_PLLSRC_HSE32 | stm32.RCC_PLLCFGR_PLLPEN)
+		stm32.RCC_PLLCFGR_PLLSRC_HSE32 | stm32.RCC_PLLCFGR_PLLPEN | stm32.RCC_PLLCFGR_PLLQEN)
 
 	// Enable PLL
 	stm32.RCC.CR.SetBits(stm32.RCC_CR_PLLON)
@@ -98,6 +98,7 @@ func initCLK() {
 	stm32.RCC.CFGR.ReplaceBits(stm32.RCC_CFGR_SW_PLLR, stm32.RCC_CFGR_SW_Msk, 0)
 	for (stm32.RCC.CFGR.Get() & stm32.RCC_CFGR_SWS_Msk) != (stm32.RCC_CFGR_SWS_PLLR << stm32.RCC_CFGR_SWS_Pos) {
 	}
+
 }
 
 func putchar(c byte) {