[PATCH] hwrng: bcm2835: Fix hwrng throughput regression

[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@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);


I think we still need to use the hwrng_msleep function so that the sleep
remains cancelable. Maybe just change the 1000 to 100?

Jason

>  	}
> 
>  	num_words = rng_readl(priv, RNG_STATUS) >> 24;
> --
> 2.34.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel at lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel