[PATCH] hwrng: bcm2835: Fix hwrng throughput regression

[Stefan Wahren]

The recent RCU stall fix caused a massive throughput regression of the
hwrng on Raspberry Pi 0 - 3. So try to restore a similiar throughput
as before the RCU stall fix.

Some performance measurements on Raspberry Pi 3B+ (arm64/defconfig):

sudo dd if=/dev/hwrng of=/dev/null count=1 bs=10000

cpu_relax              ~138025 Bytes / sec
hwrng_msleep(1000)         ~13 Bytes / sec
usleep_range(100,200)   ~92141 Bytes / sec

Fixes: 96cb9d055445 ("hwrng: bcm2835 - use hwrng_msleep() instead of cpu_relax()")
Link: https://lore.kernel.org/linux-arm-kernel/bc97ece5-44a3-4c4e-77da-2db3eb66b128@gmx.net/
Signed-off-by: Stefan Wahren <wahrenst at gmx.net>
---
 drivers/char/hw_random/bcm2835-rng.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/bcm2835-rng.c b/drivers/char/hw_random/bcm2835-rng.c
index e98fcac578d6..3f1b6aaa98ee 100644
--- a/drivers/char/hw_random/bcm2835-rng.c
+++ b/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
@@ -71,7 +72,7 @@ static int bcm2835_rng_read(struct hwrng *rng, void *buf, size_t max,
 	while ((rng_readl(priv, RNG_STATUS) >> 24) == 0) {
 		if (!wait)
 			return 0;
-		hwrng_msleep(rng, 1000);
+		usleep_range(100, 200);
 	}

 	num_words = rng_readl(priv, RNG_STATUS) >> 24;
--
2.34.1