[PATCH V2] hwrng: bcm2835: Fix hwrng throughput regression

[Stefan Wahren <wahrenst@xxxxxxx>]

The last RCU stall fix caused a massive throughput regression of the
hwrng on Raspberry Pi 0 - 3. hwrng_msleep doesn't sleep precisely enough
and usleep_range doesn't allow scheduling. So try to restore the
best possible throughput by introducing hwrng_yield which interruptable
sleeps for one jiffy.

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
hwrng_yield              ~2510 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@xxxxxxx/
Signed-off-by: Stefan Wahren <wahrenst@xxxxxxx>
---

Changes in V2:
- introduce hwrng_yield and use it

 drivers/char/hw_random/bcm2835-rng.c | 2 +-
 drivers/char/hw_random/core.c        | 6 ++++++
 include/linux/hw_random.h            | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/bcm2835-rng.c b/drivers/char/hw_random/bcm2835-rng.c
index e98fcac578d6..634eab4776f3 100644
--- a/drivers/char/hw_random/bcm2835-rng.c
+++ b/drivers/char/hw_random/bcm2835-rng.c
@@ -71,7 +71,7 @@ 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);
+		hwrng_yield(rng);
 	}

 	num_words = rng_readl(priv, RNG_STATUS) >> 24;
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index f34d356fe2c0..599a4bc2c548 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -679,6 +679,12 @@ long hwrng_msleep(struct hwrng *rng, unsigned int msecs)
 }
 EXPORT_SYMBOL_GPL(hwrng_msleep);

+long hwrng_yield(struct hwrng *rng)
+{
+	return wait_for_completion_interruptible_timeout(&rng->dying, 1);
+}
+EXPORT_SYMBOL_GPL(hwrng_yield);
+
 static int __init hwrng_modinit(void)
 {
 	int ret;
diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index 8a3115516a1b..136e9842120e 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -63,5 +63,6 @@ extern void hwrng_unregister(struct hwrng *rng);
 extern void devm_hwrng_unregister(struct device *dve, struct hwrng *rng);

 extern long hwrng_msleep(struct hwrng *rng, unsigned int msecs);
+extern long hwrng_yield(struct hwrng *rng);

 #endif /* LINUX_HWRANDOM_H_ */
--
2.34.1