hwrng: stm32 - restrain RNG noise source clock

For NIST certification the noise source sampling may need to be
restrained.

This change implements an algorithm that gets the rate of the RNG
clock and apply the correct value in CLKDIV field in RNG_CR register
to force the RNG clock rate to be "max_clock_rate" maximum.

As it is platform-specific, implement it as a compat data.
Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/stm32-rng.c

@@ -23,11 +23,13 @@
 #define RNG_CR_CONFIG1		GENMASK(11, 8)
 #define RNG_CR_NISTC		BIT(12)
 #define RNG_CR_CONFIG2		GENMASK(15, 13)
+#define RNG_CR_CLKDIV_SHIFT	16
+#define RNG_CR_CLKDIV		GENMASK(19, 16)
 #define RNG_CR_CONFIG3		GENMASK(25, 20)
 #define RNG_CR_CONDRST		BIT(30)
 #define RNG_CR_CONFLOCK		BIT(31)
 #define RNG_CR_ENTROPY_SRC_MASK	(RNG_CR_CONFIG1 | RNG_CR_NISTC | RNG_CR_CONFIG2 | RNG_CR_CONFIG3)
-#define RNG_CR_CONFIG_MASK	(RNG_CR_ENTROPY_SRC_MASK | RNG_CR_CED)
+#define RNG_CR_CONFIG_MASK	(RNG_CR_ENTROPY_SRC_MASK | RNG_CR_CED | RNG_CR_CLKDIV)
 
 #define RNG_SR			0x04
 #define RNG_SR_DRDY		BIT(0)
@@ -46,6 +48,7 @@
 #define RNG_NB_RECOVER_TRIES	3
 
 struct stm32_rng_data {
+	uint	max_clock_rate;
 	u32	cr;
 	u32	nscr;
 	u32	htcr;
@@ -238,6 +241,28 @@ static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 	return retval || !wait ? retval : -EIO;
 }
 
+static uint stm32_rng_clock_freq_restrain(struct hwrng *rng)
+{
+	struct stm32_rng_private *priv =
+	    container_of(rng, struct stm32_rng_private, rng);
+	unsigned long clock_rate = 0;
+	uint clock_div = 0;
+
+	clock_rate = clk_get_rate(priv->clk);
+
+	/*
+	 * Get the exponent to apply on the CLKDIV field in RNG_CR register
+	 * No need to handle the case when clock-div > 0xF as it is physically
+	 * impossible
+	 */
+	while ((clock_rate >> clock_div) > priv->data->max_clock_rate)
+		clock_div++;
+
+	pr_debug("RNG clk rate : %lu\n", clk_get_rate(priv->clk) >> clock_div);
+
+	return clock_div;
+}
+
 static int stm32_rng_init(struct hwrng *rng)
 {
 	struct stm32_rng_private *priv =
@@ -259,8 +284,11 @@ static int stm32_rng_init(struct hwrng *rng)
 	 * 0 is an invalid value as it disables all entropy sources.
 	 */
 	if (priv->data->has_cond_reset && priv->data->cr) {
+		uint clock_div = stm32_rng_clock_freq_restrain(rng);
+
 		reg &= ~RNG_CR_CONFIG_MASK;
-		reg |= RNG_CR_CONDRST | (priv->data->cr & RNG_CR_ENTROPY_SRC_MASK);
+		reg |= RNG_CR_CONDRST | (priv->data->cr & RNG_CR_ENTROPY_SRC_MASK) |
+		       (clock_div << RNG_CR_CLKDIV_SHIFT);
 		if (priv->ced)
 			reg &= ~RNG_CR_CED;
 		else
@@ -360,6 +388,7 @@ static const struct dev_pm_ops stm32_rng_pm_ops = {
 
 static const struct stm32_rng_data stm32mp13_rng_data = {
 	.has_cond_reset = true,
+	.max_clock_rate = 48000000,
 	.cr = 0x00F00D00,
 	.nscr = 0x2B5BB,
 	.htcr = 0x969D,
@@ -367,6 +396,7 @@ static const struct stm32_rng_data stm32mp13_rng_data = {
 
 static const struct stm32_rng_data stm32_rng_data = {
 	.has_cond_reset = false,
+	.max_clock_rate = 3000000,
 };
 
 static const struct of_device_id stm32_rng_match[] = {