hwrng: stm32 - rework error handling in stm32_rng_read()

Try to conceal seed errors when possible. If, despite the error
concealing tries, a seed error is still present, then return an error.

A clock error does not compromise the hardware block and data can
still be read from RNG_DR. Just warn that the RNG clock is too slow
and clear RNG_SR.
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

@@ -43,6 +43,8 @@
 
 #define RNG_HTCR		0x10
 
+#define RNG_NB_RECOVER_TRIES	3
+
 struct stm32_rng_data {
 	u32	cr;
 	u32	nscr;
@@ -162,10 +164,10 @@ static int stm32_rng_conceal_seed_error(struct hwrng *rng)
 
 static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 {
-	struct stm32_rng_private *priv =
-	    container_of(rng, struct stm32_rng_private, rng);
+	struct stm32_rng_private *priv = container_of(rng, struct stm32_rng_private, rng);
+	unsigned int i = 0;
+	int retval = 0, err = 0;
 	u32 sr;
-	int retval = 0;
 
 	pm_runtime_get_sync((struct device *) priv->rng.priv);
 
@@ -174,30 +176,57 @@ static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 
 	while (max >= sizeof(u32)) {
 		sr = readl_relaxed(priv->base + RNG_SR);
-		/* Manage timeout which is based on timer and take */
-		/* care of initial delay time when enabling rng	*/
+		/*
+		 * Manage timeout which is based on timer and take
+		 * care of initial delay time when enabling the RNG.
+		 */
 		if (!sr && wait) {
-			int err;
-
 			err = readl_relaxed_poll_timeout_atomic(priv->base
 								   + RNG_SR,
 								   sr, sr,
 								   10, 50000);
-			if (err)
+			if (err) {
 				dev_err((struct device *)priv->rng.priv,
 					"%s: timeout %x!\n", __func__, sr);
+				break;
+			}
+		} else if (!sr) {
+			/* The FIFO is being filled up */
+			break;
 		}
 
-		/* If error detected or data not ready... */
 		if (sr != RNG_SR_DRDY) {
-			if (WARN_ONCE(sr & (RNG_SR_SEIS | RNG_SR_CEIS),
-					"bad RNG status - %x\n", sr))
+			if (sr & RNG_SR_SEIS) {
+				err = stm32_rng_conceal_seed_error(rng);
+				i++;
+				if (err && i > RNG_NB_RECOVER_TRIES) {
+					dev_err((struct device *)priv->rng.priv,
+						"Couldn't recover from seed error\n");
+					return -ENOTRECOVERABLE;
+				}
+
+				continue;
+			}
+
+			if (WARN_ONCE((sr & RNG_SR_CEIS), "RNG clock too slow - %x\n", sr))
 				writel_relaxed(0, priv->base + RNG_SR);
-			break;
 		}
 
+		/* Late seed error case: DR being 0 is an error status */
 		*(u32 *)data = readl_relaxed(priv->base + RNG_DR);
+		if (!*(u32 *)data) {
+			err = stm32_rng_conceal_seed_error(rng);
+			i++;
+			if (err && i > RNG_NB_RECOVER_TRIES) {
+				dev_err((struct device *)priv->rng.priv,
+					"Couldn't recover from seed error");
+				return -ENOTRECOVERABLE;
+			}
+
+			continue;
+		}
 
+		i = 0;
 		retval += sizeof(u32);
 		data += sizeof(u32);
 		max -= sizeof(u32);