hwrng: bcm2835 - sleep more intelligently

While waiting for random data, use sleeps that are proportional
to the amount of data expected. Prevent indefinite waits by
giving up if nothing is received for a second.

See: #5390

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/char/hw_random/bcm2835-rng.c

@@ -14,6 +14,7 @@
 #include <linux/printk.h>
 #include <linux/clk.h>
 #include <linux/reset.h>
+#include <linux/delay.h>
 
 #define RNG_CTRL	0x0
 #define RNG_STATUS	0x4
@@ -28,6 +29,9 @@
 
 #define RNG_INT_OFF	0x1
 
+#define RNG_FIFO_WORDS	4
+#define RNG_US_PER_WORD	34 /* Tuned for throughput */
+
 struct bcm2835_rng_priv {
 	struct hwrng rng;
 	void __iomem *base;
@@ -64,19 +68,23 @@ static inline void rng_writel(struct bcm2835_rng_priv *priv, u32 val,
 static int bcm2835_rng_read(struct hwrng *rng, void *buf, size_t max,
 			       bool wait)
 {
+	u32 retries = 1000000/(RNG_FIFO_WORDS * RNG_US_PER_WORD);
 	struct bcm2835_rng_priv *priv = to_rng_priv(rng);
 	u32 max_words = max / sizeof(u32);
 	u32 num_words, count;
 
-	while ((rng_readl(priv, RNG_STATUS) >> 24) == 0) {
-		if (!wait)
+	num_words = rng_readl(priv, RNG_STATUS) >> 24;
+
+	while (!num_words) {
+		if (!wait || !retries)
 			return 0;
-		hwrng_msleep(rng, 1000);
+		retries--;
+		usleep_range((u32)RNG_US_PER_WORD,
+			     (u32)RNG_US_PER_WORD * RNG_FIFO_WORDS);
+		num_words = rng_readl(priv, RNG_STATUS) >> 24;
 	}
 
-	num_words = rng_readl(priv, RNG_STATUS) >> 24;
-	if (num_words > max_words)
-		num_words = max_words;
+	num_words = min(num_words, max_words);
 
 	for (count = 0; count < num_words; count++)
 		((u32 *)buf)[count] = rng_readl(priv, RNG_DATA);