hwrng: stm32 - implement error concealment

The RNG driver should be capable of recovering from an error. Implement
an error concealment API. This avoids irrecoverable RNG state.
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

@@ -29,10 +29,12 @@
 #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_SR		0x04
-#define RNG_SR_SEIS	BIT(6)
-#define RNG_SR_CEIS	BIT(5)
-#define RNG_SR_DRDY	BIT(0)
+#define RNG_SR			0x04
+#define RNG_SR_DRDY		BIT(0)
+#define RNG_SR_CECS		BIT(1)
+#define RNG_SR_SECS		BIT(2)
+#define RNG_SR_CEIS		BIT(5)
+#define RNG_SR_SEIS		BIT(6)
 
 #define RNG_DR			0x08
 
@@ -57,6 +59,107 @@ struct stm32_rng_private {
 	bool ced;
 };
 
+/*
+ * Extracts from the STM32 RNG specification when RNG supports CONDRST.
+ *
+ * When a noise source (or seed) error occurs, the RNG stops generating
+ * random numbers and sets to “1” both SEIS and SECS bits to indicate
+ * that a seed error occurred. (...)
+ *
+ * 1. Software reset by writing CONDRST at 1 and at 0 (see bitfield
+ * description for details). This step is needed only if SECS is set.
+ * Indeed, when SEIS is set and SECS is cleared it means RNG performed
+ * the reset automatically (auto-reset).
+ * 2. If SECS was set in step 1 (no auto-reset) wait for CONDRST
+ * to be cleared in the RNG_CR register, then confirm that SEIS is
+ * cleared in the RNG_SR register. Otherwise just clear SEIS bit in
+ * the RNG_SR register.
+ * 3. If SECS was set in step 1 (no auto-reset) wait for SECS to be
+ * cleared by RNG. The random number generation is now back to normal.
+ */
+static int stm32_rng_conceal_seed_error_cond_reset(struct stm32_rng_private *priv)
+{
+	struct device *dev = (struct device *)priv->rng.priv;
+	u32 sr = readl_relaxed(priv->base + RNG_SR);
+	u32 cr = readl_relaxed(priv->base + RNG_CR);
+	int err;
+
+	if (sr & RNG_SR_SECS) {
+		/* Conceal by resetting the subsystem (step 1.) */
+		writel_relaxed(cr | RNG_CR_CONDRST, priv->base + RNG_CR);
+		writel_relaxed(cr & ~RNG_CR_CONDRST, priv->base + RNG_CR);
+	} else {
+		/* RNG auto-reset (step 2.) */
+		writel_relaxed(sr & ~RNG_SR_SEIS, priv->base + RNG_SR);
+		goto end;
+	}
+
+	err = readl_relaxed_poll_timeout_atomic(priv->base + RNG_CR, cr, !(cr & RNG_CR_CONDRST), 10,
+						100000);
+	if (err) {
+		dev_err(dev, "%s: timeout %x\n", __func__, sr);
+		return err;
+	}
+
+	/* Check SEIS is cleared (step 2.) */
+	if (readl_relaxed(priv->base + RNG_SR) & RNG_SR_SEIS)
+		return -EINVAL;
+
+	err = readl_relaxed_poll_timeout_atomic(priv->base + RNG_SR, sr, !(sr & RNG_SR_SECS), 10,
+						100000);
+	if (err) {
+		dev_err(dev, "%s: timeout %x\n", __func__, sr);
+		return err;
+	}
+
+end:
+	return 0;
+}
+
+/*
+ * Extracts from the STM32 RNG specification, when CONDRST is not supported
+ *
+ * When a noise source (or seed) error occurs, the RNG stops generating
+ * random numbers and sets to “1” both SEIS and SECS bits to indicate
+ * that a seed error occurred. (...)
+ *
+ * The following sequence shall be used to fully recover from a seed
+ * error after the RNG initialization:
+ * 1. Clear the SEIS bit by writing it to “0”.
+ * 2. Read out 12 words from the RNG_DR register, and discard each of
+ * them in order to clean the pipeline.
+ * 3. Confirm that SEIS is still cleared. Random number generation is
+ * back to normal.
+ */
+static int stm32_rng_conceal_seed_error_sw_reset(struct stm32_rng_private *priv)
+{
+	unsigned int i = 0;
+	u32 sr = readl_relaxed(priv->base + RNG_SR);
+
+	writel_relaxed(sr & ~RNG_SR_SEIS, priv->base + RNG_SR);
+
+	for (i = 12; i != 0; i--)
+		(void)readl_relaxed(priv->base + RNG_DR);
+
+	if (readl_relaxed(priv->base + RNG_SR) & RNG_SR_SEIS)
+		return -EINVAL;
+
+	return 0;
+}
+
+static int stm32_rng_conceal_seed_error(struct hwrng *rng)
+{
+	struct stm32_rng_private *priv = container_of(rng, struct stm32_rng_private, rng);
+
+	dev_dbg((struct device *)priv->rng.priv, "Concealing seed error\n");
+
+	if (priv->data->has_cond_reset)
+		return stm32_rng_conceal_seed_error_cond_reset(priv);
+	else
+		return stm32_rng_conceal_seed_error_sw_reset(priv);
+};
+
+
 static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 {
 	struct stm32_rng_private *priv =
@@ -66,6 +169,9 @@ static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 
 	pm_runtime_get_sync((struct device *) priv->rng.priv);
 
+	if (readl_relaxed(priv->base + RNG_SR) & RNG_SR_SEIS)
+		stm32_rng_conceal_seed_error(rng);
+
 	while (max >= sizeof(u32)) {
 		sr = readl_relaxed(priv->base + RNG_SR);
 		/* Manage timeout which is based on timer and take */