Vaidyanathan Srinivasan <sva...@linux.vnet.ibm.com> writes:
> * Nicholas Piggin <npig...@gmail.com> [2017-06-12 09:58:34]:
>
>> The CTRL register is read-only except bit 63 which is the run latch
>> control. This means it can be updated with a mtspr rather than
>> mfspr/mtspr.
>>
>> Signed-off-by: Nicholas Piggin <npig...@gmail.com>
>
>> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
>> index baae104b16c7..a44ea034c226 100644
>> --- a/arch/powerpc/kernel/process.c
>> +++ b/arch/powerpc/kernel/process.c
>> @@ -1973,13 +1969,9 @@ void notrace __ppc64_runlatch_on(void)
>> void notrace __ppc64_runlatch_off(void)
>> {
>> struct thread_info *ti = current_thread_info();
>> - unsigned long ctrl;
>>
>> ti->local_flags &= ~_TLF_RUNLATCH;
>> -
>> - ctrl = mfspr(SPRN_CTRLF);
>> - ctrl &= ~CTRL_RUNLATCH;
>> - mtspr(SPRN_CTRLT, ctrl);
>> + mtspr(SPRN_CTRLT, 0);
>
> Good idea. Writing to CTRL register can change only the RUN field.
> Was this any different in older generations?
No AFAICS back to 2.02.
> Anton and Ben kept the mfspr/mtspr part in earlier updates to this
> routine.
Doing the read/modify write is forward compatible vs a new writable
field, whereas writing the whole register with a known value is not.
cheers
