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: https://github.com/raspberrypi/linux/issues/5390

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

diff --git a/drivers/char/hw_random/bcm2835-rng.c b/drivers/char/hw_random/bcm2835-rng.c
index 2fc6dd5..84c964b 100644 (file)
--- a/drivers/char/hw_random/bcm2835-rng.c
+++ b/drivers/char/hw_random/bcm2835-rng.c
@@ -13,6 +13,7 @@
 #include <linux/printk.h>
 #include <linux/clk.h>
 #include <linux/reset.h>
+#include <linux/delay.h>
 
 #define RNG_CTRL       0x0
 #define RNG_STATUS     0x4
@@ -27,6 +28,9 @@
 
 #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;
@@ -63,19 +67,23 @@ 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_yield(rng);
+               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);