[PATCH] hwrng: bcm2835: Fix hwrng throughput regression

[Stefan Wahren]

Hi Jason,

Am 26.08.23 um 17:48 schrieb Jason A. Donenfeld:
> On Sat, Aug 26, 2023 at 04:01:58PM +0200, Stefan Wahren wrote:
>> 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.
> I'm fine with whatever number you want there. Maybe we need a
> hwrng_usleep_range() that takes into account rng->dying like
> hwrng_msleep() does? (And iproc-rng200 should probably use that too?)
the idea of this patch was to fix the performance regression in upcoming
mainline and backport the fix to Linux 6.1 LTS. After that i'm fine with
the introduction of hwrng_usleep_range().

Best regards

> Jason