hwrng: ks-sa - Add minimum sleep time before ready-polling

Current polling timeout is 25 us. The hardware is currently configured to
harvest the entropy for 81920 us. This leads to timeouts even during
blocking read (wait=1).

Log snippet:
[    5.727589] [<c040ffcc>] (ks_sa_rng_probe) from [<c04181e8>] (platform_drv_probe+0x58/0xb4)
...
[    5.727805] hwrng: no data available
...
[   13.157016] random: systemd: uninitialized urandom read (16 bytes read)
[   13.157033] systemd[1]: Initializing machine ID from random generator.
...
[   15.848770] random: fast init done
...
[   15.848807] random: crng init done

After the patch:
[    6.223534] random: systemd: uninitialized urandom read (16 bytes read)
[    6.223551] systemd[1]: Initializing machine ID from random generator.
...
[    6.876075] random: fast init done
...
[    6.954200] random: systemd: uninitialized urandom read (16 bytes read)
[    6.955244] random: systemd: uninitialized urandom read (16 bytes read)
...
[    7.121948] random: crng init done
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/ks-sa-rng.c

@@ -21,6 +21,7 @@
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/delay.h>
+#include <linux/timekeeping.h>
 
 #define SA_CMD_STATUS_OFS			0x8
 
@@ -85,13 +86,36 @@ struct ks_sa_rng {
 	struct clk	*clk;
 	struct regmap	*regmap_cfg;
 	struct trng_regs __iomem *reg_rng;
+	u64 ready_ts;
+	unsigned int refill_delay_ns;
 };
 
+static unsigned int cycles_to_ns(unsigned long clk_rate, unsigned int cycles)
+{
+	return DIV_ROUND_UP_ULL((TRNG_DEF_CLK_DIV_CYCLES + 1) * 1000000000ull *
+				cycles, clk_rate);
+}
+
+static unsigned int startup_delay_ns(unsigned long clk_rate)
+{
+	if (!TRNG_DEF_STARTUP_CYCLES)
+		return cycles_to_ns(clk_rate, BIT(24));
+	return cycles_to_ns(clk_rate, 256 * TRNG_DEF_STARTUP_CYCLES);
+}
+
+static unsigned int refill_delay_ns(unsigned long clk_rate)
+{
+	if (!TRNG_DEF_MAX_REFILL_CYCLES)
+		return cycles_to_ns(clk_rate, BIT(24));
+	return cycles_to_ns(clk_rate, 256 * TRNG_DEF_MAX_REFILL_CYCLES);
+}
+
 static int ks_sa_rng_init(struct hwrng *rng)
 {
 	u32 value;
 	struct device *dev = (struct device *)rng->priv;
 	struct ks_sa_rng *ks_sa_rng = dev_get_drvdata(dev);
+	unsigned long clk_rate = clk_get_rate(ks_sa_rng->clk);
 
 	/* Enable RNG module */
 	regmap_write_bits(ks_sa_rng->regmap_cfg, SA_CMD_STATUS_OFS,
@@ -120,6 +144,10 @@ static int ks_sa_rng_init(struct hwrng *rng)
 	value |= TRNG_CNTL_REG_TRNG_ENABLE;
 	writel(value, &ks_sa_rng->reg_rng->control);
 
+	ks_sa_rng->refill_delay_ns = refill_delay_ns(clk_rate);
+	ks_sa_rng->ready_ts = ktime_get_ns() +
+			      startup_delay_ns(clk_rate);
+
 	return 0;
 }
 
@@ -144,6 +172,7 @@ static int ks_sa_rng_data_read(struct hwrng *rng, u32 *data)
 	data[1] = readl(&ks_sa_rng->reg_rng->output_h);
 
 	writel(TRNG_INTACK_REG_READY, &ks_sa_rng->reg_rng->intack);
+	ks_sa_rng->ready_ts = ktime_get_ns() + ks_sa_rng->refill_delay_ns;
 
 	return sizeof(u32) * 2;
 }
@@ -152,10 +181,19 @@ static int ks_sa_rng_data_present(struct hwrng *rng, int wait)
 {
 	struct device *dev = (struct device *)rng->priv;
 	struct ks_sa_rng *ks_sa_rng = dev_get_drvdata(dev);
+	u64 now = ktime_get_ns();
 
 	u32	ready;
 	int	j;
 
+	if (wait && now < ks_sa_rng->ready_ts) {
+		/* Max delay expected here is 81920000 ns */
+		unsigned long min_delay =
+			DIV_ROUND_UP((u32)(ks_sa_rng->ready_ts - now), 1000);
+
+		usleep_range(min_delay, min_delay + SA_RNG_DATA_RETRY_DELAY);
+	}
+
 	for (j = 0; j < SA_MAX_RNG_DATA_RETRIES; j++) {
 		ready = readl(&ks_sa_rng->reg_rng->status);
 		ready &= TRNG_STATUS_REG_READY;