[PATCH V2] hwrng: bcm2835: Fix hwrng throughput regression

[Linux regression tracking (Thorsten Leemhuis)]

On 15.09.23 12:45, Herbert Xu wrote:
> On Wed, Sep 06, 2023 at 01:27:57AM +0200, Stefan Wahren wrote:
>> 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/bc97ece5-44a3-4c4e-77da-2db3eb66b128@gmx.net/
>> Signed-off-by: Stefan Wahren <wahrenst at gmx.net>
>> ---
>>
>> Changes in V2:
>> - introduce hwrng_yield and use it
>>
>>  drivers/char/hw_random/bcm2835-rng.c | 2 +-
>>  drivers/char/hw_random/core.c        | 6 ++++++
>>  include/linux/hw_random.h            | 1 +
>>  3 files changed, 8 insertions(+), 1 deletion(-)
> 
> Patch applied.  Thanks.

Hi Herbert, I there a strong reason why you merged this to what from
here looks like the branch that targets the next merge window? The patch
fixes a regression introduced during the last 12 months and thus
normally should not wait for the next merge window. For details see
"Expectations and best practices for fixing regressions" in
Documentation/process/handling-regressions.rst; or if you want to hear
it from Linus directly, check these out:
https://lore.kernel.org/all/CAHk-=wis_qQy4oDNynNKi5b7Qhosmxtoj1jxo5wmB6SRUwQUBQ@mail.gmail.com/
https://lore.kernel.org/all/CAHk-=wgD98pmSK3ZyHk_d9kZ2bhgN6DuNZMAJaV0WTtbkf=RDw@mail.gmail.com/

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
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