hwrng: bcm2835 - Fix hwrng throughput regression

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/[e-mail]/
Signed-off-by: Stefan Wahren <[e-mail]>
Reviewed-by: Jason A. Donenfeld <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

drivers/char/hw_random/bcm2835-rng.c

···
 	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;

drivers/char/hw_random/core.c

···
 }
 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;

include/linux/hw_random.h

···
 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_ */