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Date: Thu, 13 Apr 2017 22:08:41 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Gautham R Shenoy <ego@linux.vnet.ibm.com>
Cc: Michael Neuling <mikey@neuling.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
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        Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] powernv:idle: Set LPCR_UPRT on wakeup from
 deep-stop
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On Thu, 13 Apr 2017 17:24:34 +0530
Gautham R Shenoy <ego@linux.vnet.ibm.com> wrote:

> On Thu, Apr 13, 2017 at 05:18:17PM +1000, Nicholas Piggin wrote:
> > On Thu, 13 Apr 2017 16:27:34 +1000
> > Michael Neuling <mikey@neuling.org> wrote:
> >   
> > > On Thu, 2017-04-13 at 14:12 +1000, Benjamin Herrenschmidt wrote:  
> > > > On Thu, 2017-04-13 at 09:28 +0530, Aneesh Kumar K.V wrote:    
> > > > > >   #endif
> > > > > >        mtctr   r12
> > > > > >        bctrl
> > > > > > +/*
> > > > > > + * cur_cpu_spec->cpu_restore would restore LPCR to a
> > > > > > + * sane value that is set at early boot time,
> > > > > > + * thereby clearing LPCR_UPRT.
> > > > > > + * LPCR_UPRT is required if we are running in Radix mode.
> > > > > > + * Set it here if that be the case.
> > > > > > + */
> > > > > > +BEGIN_MMU_FTR_SECTION
> > > > > > +     mfspr   r3, SPRN_LPCR
> > > > > > +     LOAD_REG_IMMEDIATE(r4, LPCR_UPRT)
> > > > > > +     or      r3, r3, r4
> > > > > > +     mtspr   SPRN_LPCR, r3
> > > > > > +END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)    
> > > > 
> > > > We are probably better off saving the value somewhere during boot
> > > > and just "blasting" it whole back.    
> > > 
> > > We seem to touch LPCR in a bunch of places these days.  Not sure when "sometimes
> > >  during boot" should actually be.  
> > 
> > In the short term, what if we just save LPCR and restore it after calling
> > cpu_restore? As you say there are a lot of things that touch LPCR we're
> > not catching here.  
> 
> In that case can we skip calling cpu_restore in the idle_exit path
> altogether and simply restore LPCR to the value that the thread had
> before executing stop ?

Good question. For a minimal fix I would keep calling cpu_restore. I'd
like to get rid of it if we can though, but that might take a bit more
work.