On Tue, 13 Jun 2017 20:04:27 +1000
Michael Ellerman <m...@ellerman.id.au> wrote:
> 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.
Can we call that an incompatible arch change and not worry about
it? ISA says we may expect TS (read-only) field to expand, but I
guess they could shoehorn something else in there.
