linux-kernel - Re: [PATCH 3/3] powernv:idle: Set LPCR

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Message-ID: <878tn4aaia.fsf@concordia.ellerman.id.au>
Date:   Thu, 13 Apr 2017 20:05:01 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Michael Neuling <mikey@...ling.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        "Shreyas B. Prabhu" <shreyasbp@...il.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Anton Blanchard <anton@...ba.org>,
        Balbir Singh <bsingharora@...il.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Mahesh J Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] powernv:idle: Set LPCR_UPRT on wakeup from deep-stop

Nicholas Piggin <npiggin@...il.com> writes:

> On Thu, 13 Apr 2017 16:27:34 +1000
> Michael Neuling <mikey@...ling.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.

Yeah can we save it on the way down and restore that value on the way
back up?

The real problem here is that cpu_restore() does not "restore" anything,
it programs a set of fixed values.

We should probably rework it so that it actually does a save/restore to
avoid more bugs like this.

cheers