hwrng: stm32 - support RNG configuration locking mechanism

If "st,rng-lock-conf" DT binding property is set for a stm32-rng node,
the RNG configuration will be locked until next hardware block reset
or platform reset.
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

@@ -62,6 +62,7 @@ struct stm32_rng_private {
 	struct reset_control *rst;
 	const struct stm32_rng_data *data;
 	bool ced;
+	bool lock_conf;
 };
 
 /*
@@ -301,6 +302,9 @@ static int stm32_rng_init(struct hwrng *rng)
 
 		reg &= ~RNG_CR_CONDRST;
 		reg |= RNG_CR_RNGEN;
+		if (priv->lock_conf)
+			reg |= RNG_CR_CONFLOCK;
+
 		writel_relaxed(reg, priv->base + RNG_CR);
 
 		err = readl_relaxed_poll_timeout_atomic(priv->base + RNG_CR, reg,
@@ -439,6 +443,7 @@ static int stm32_rng_probe(struct platform_device *ofdev)
 	}
 
 	priv->ced = of_property_read_bool(np, "clock-error-detect");
+	priv->lock_conf = of_property_read_bool(np, "st,rng-lock-conf");
 
 	priv->data = of_device_get_match_data(dev);
 	if (!priv->data)