[PATCH] hwrng: bcm2835: Fix hwrng throughput regression

[Stefan Wahren]

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@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?
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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