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Date: Tue, 13 Jun 2017 21:56:20 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: svaidy@linux.vnet.ibm.com, "Gautham R . Shenoy"
 <ego@linux.vnet.ibm.com>, linuxppc-dev@lists.ozlabs.org, "Shreyas B .
 Prabhu" <shreyas@linux.vnet.ibm.com>
Subject: Re: [PATCH 13/14] powerpc/64: runlatch CTRL[RUN] set optimisation
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On Tue, 13 Jun 2017 20:04:27 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> writes:
> > * Nicholas Piggin <npiggin@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 <npiggin@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.