Hi Jason,
Am 26.08.23 um 14:34 schrieb Jason A. Donenfeld:
On Sat, Aug 26, 2023 at 01:28:28PM +0200, Stefan Wahren wrote:
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@xxxxxxx/
Signed-off-by: Stefan Wahren <wahrenst@xxxxxxx>
---
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);
I think we still need to use the hwrng_msleep function so that the sleep
remains cancelable. Maybe just change the 1000 to 100?
i found that other hwrng driver like iproc-rng200 (Raspberry Pi 4) also
use usleep_range().
Nevertheless here are more numbers:
usleep_range(200,400) : 47776 bytes / sec
hwrng_msleep(20) : 715 bytes / sec
Changing to 100 ms won't be a real gain.
Jason
}
num_words = rng_readl(priv, RNG_STATUS) >> 24;
--
2.34.1
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