stm32: add TRNG support

Add a driver for the STM32 True Random Number Generator.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BRANCH=none
BUG=chrome-os-partner:62991
TEST=adhoc on STM32L, craft console command and generate/dump
buffers of random numbers.

Change-Id: Ie7ce890cfc36a3b9a277715b17051e3e42fdfc96
Reviewed-on: https://chromium-review.googlesource.com/445777
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Shawn N <shawnn@chromium.org>

chip/stm32/build.mk

@@ -56,6 +56,7 @@ chip-$(CONFIG_ADC)+=adc-$(CHIP_FAMILY).o
 chip-$(CONFIG_STM32_CHARGER_DETECT)+=charger_detect.o
 chip-$(CONFIG_DEBUG_PRINTF)+=debug_printf.o
 chip-$(CONFIG_PWM)+=pwm.o
+chip-$(CONFIG_RNG)+=trng.o
 
 ifeq ($(CHIP_FAMILY),stm32f4)
 chip-$(CONFIG_USB)+=usb_dwc.o usb_endpoints.o

chip/stm32/registers.h

@@ -763,6 +763,7 @@ typedef volatile struct timer_ctlr timer_ctlr_t;
 
 #define STM32_RCC_AHB2ENR		REG32(STM32_RCC_BASE + 0x4C)
 #define STM32_RCC_AHB2ENR_GPIOMASK	(0xff << 0)
+#define STM32_RCC_AHB2ENR_RNGEN		(1 << 18)
 
 #define STM32_RCC_APB1ENR		REG32(STM32_RCC_BASE + 0x58)
 #define STM32_RCC_PWREN                 (1 << 28)
@@ -849,6 +850,12 @@ typedef volatile struct timer_ctlr timer_ctlr_t;
 
 #define STM32_RCC_CSR			REG32(STM32_RCC_BASE + 0x94)
 
+#define STM32_RCC_CRRCR			REG32(STM32_RCC_BASE + 0x98)
+
+#define STM32_RCC_CRRCR_HSI48ON         (1<<0)
+#define STM32_RCC_CRRCR_HSI48RDY        (1<<1)
+#define STM32_RCC_CRRCR_HSI48CAL_MASK   (0x1ff<<7)
+
 #define STM32_RCC_PB2_TIM1		(1 << 11)
 #define STM32_RCC_PB2_TIM8		(1 << 13)
 
@@ -2157,6 +2164,16 @@ typedef volatile struct stm32_dma_regs stm32_dma_regs_t;
 	STM32_USB_EP(n) = (((STM32_USB_EP(n) & (EP_MASK | (mask))) \
 			^ (val)) | (flags))
 
+/* --- TRNG --- */
+#define STM32_RNG_BASE              0x50060800  /* STM32L4 */
+
+#define STM32_RNG_CR                REG32(STM32_RNG_BASE + 0x0)
+#define STM32_RNG_CR_RNGEN          (1<<2)
+#define STM32_RNG_CR_IE             (1<<3)
+#define STM32_RNG_SR                REG32(STM32_RNG_BASE + 0x4)
+#define STM32_RNG_SR_DRDY           (1<<0)
+#define STM32_RNG_DR                REG32(STM32_RNG_BASE + 0x8)
+
 /* --- MISC --- */
 
 #define STM32_UNIQUE_ID             0x1ffff7ac

chip/stm32/trng.c

@@ -0,0 +1,70 @@
+/* Copyright 2017 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/* Hardware Random Number Generator */
+
+#include "common.h"
+#include "panic.h"
+#include "registers.h"
+#include "task.h"
+#include "trng.h"
+#include "util.h"
+
+uint32_t rand(void)
+{
+	int tries = 40;
+	/* Wait for a valid random number */
+	while (!(STM32_RNG_SR & STM32_RNG_SR_DRDY) && --tries)
+		;
+	/* we cannot afford to feed the caller with a dummy number */
+	if (!tries)
+		software_panic(PANIC_SW_BAD_RNG, task_get_current());
+	/* Finally the 32-bit of entropy */
+	return STM32_RNG_DR;
+}
+
+void rand_bytes(void *buffer, size_t len)
+{
+	while (len) {
+		uint32_t number = rand();
+		size_t cnt = 4;
+		/* deal with the lack of alignment guarantee in the API */
+		uintptr_t align = (uintptr_t)buffer & 3;
+
+		if (len < 4 || align) {
+			cnt = MIN(4 - align, len);
+			memcpy(buffer, &number, cnt);
+		} else {
+			*(uint32_t *)buffer = number;
+		}
+		len -= cnt;
+		buffer += cnt;
+	}
+}
+
+void init_trng(void)
+{
+	/* Enable the 48Mhz internal RC oscillator */
+	STM32_RCC_CRRCR |= STM32_RCC_CRRCR_HSI48ON;
+	/* no timeout: we watchdog if the oscillator doesn't start */
+	while (!(STM32_RCC_CRRCR & STM32_RCC_CRRCR_HSI48RDY))
+		;
+
+	/* Clock the TRNG using the HSI48 */
+	STM32_RCC_CCIPR = (STM32_RCC_CCIPR & ~STM32_RCC_CCIPR_CLK48SEL_MASK)
+			| (0 << STM32_RCC_CCIPR_CLK48SEL_SHIFT);
+
+	/* Enable the RNG logic */
+	STM32_RCC_AHB2ENR |= STM32_RCC_AHB2ENR_RNGEN;
+	/* Start the random number generation */
+	STM32_RNG_CR |= STM32_RNG_CR_RNGEN;
+}
+
+void exit_trng(void)
+{
+	STM32_RNG_CR &= ~STM32_RNG_CR_RNGEN;
+	STM32_RCC_AHB2ENR &= ~STM32_RCC_AHB2ENR_RNGEN;
+	STM32_RCC_CRRCR &= ~STM32_RCC_CRRCR_HSI48ON;
+}

include/config.h

@@ -1749,6 +1749,9 @@
 /* Support IR357x Link voltage regulator debugging / reprogramming */
 #undef CONFIG_REGULATOR_IR357X
 
+/* Enable hardware Random Number generator support */
+#undef CONFIG_RNG
+
 /* Support verifying 2048-bit RSA signature */
 #undef CONFIG_RSA
 

include/software_panic.h

@@ -20,5 +20,6 @@
 #define PANIC_SW_PD_CRASH		(PANIC_SW_BASE + 2)
 #define PANIC_SW_ASSERT			(PANIC_SW_BASE + 3)
 #define PANIC_SW_WATCHDOG		(PANIC_SW_BASE + 4)
+#define PANIC_SW_BAD_RNG		(PANIC_SW_BASE + 5)
 
 #endif  /* __CROS_EC_SOFTWARE_PANIC_H */

include/trng.h

@@ -15,6 +15,16 @@
  **/
 void init_trng(void);
 
+/**
+ * Shutdown the true random number generator.
+ *
+ * The opposite operation of init_trng(), disable the hardware resources
+ * used by the TRNG to save power.
+ *
+ * Not supported by all platforms.
+ **/
+void exit_trng(void);
+
 /**
  * Retrieve a 32 bit random value.
  *