[PATCH] powerpc: Fix booting P9 hash with CONFIG_PPC_RADIX_MMU=N

[Michael Ellerman]

Michael Neuling <mikey at neuling.org> writes:

> On Wed, 2017-05-24 at 14:26 +0530, Aneesh Kumar K.V wrote:
>> Michael Neuling <mikey at neuling.org> writes:
>> 
>> > Currently if you disable CONFIG_PPC_RADIX_MMU you'll crash on boot on
>> > a P9. This is because we still set MMU_FTR_TYPE_RADIX via
>> > ibm,pa-features and MMU_FTR_TYPE_RADIX is what's used for code patching
>> > in much of the asm code (ie. slb_miss_realmode)
>> > 
>> > This patch fixes the problem by stopping MMU_FTR_TYPE_RADIX from being
>> > set from ibm.pa-features.
>> > 
>> > We may eventually end up removing the CONFIG_PPC_RADIX_MMU option
>> > completely but until then this fixes the issue.
>> > 
>> > Signed-off-by: Michael Neuling <mikey at neuling.org>
>> > ---
>> >  arch/powerpc/kernel/prom.c | 2 ++
>> >  1 file changed, 2 insertions(+)
>> > 
>> > diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
>> > index 40c4887c27..f830562974 100644
>> > --- a/arch/powerpc/kernel/prom.c
>> > +++ b/arch/powerpc/kernel/prom.c
>> > @@ -161,7 +161,9 @@ static struct ibm_pa_feature {
>> >  	{ .pabyte = 0,  .pabit = 3, .cpu_features  = CPU_FTR_CTRL },
>> >  	{ .pabyte = 0,  .pabit = 6, .cpu_features  = CPU_FTR_NOEXECUTE },
>> >  	{ .pabyte = 1,  .pabit = 2, .mmu_features  = MMU_FTR_CI_LARGE_PAGE
>> > },
>> > +#ifdef CONFIG_PPC_RADIX_MMU
>> >  	{ .pabyte = 40, .pabit = 0, .mmu_features  = MMU_FTR_TYPE_RADIX },
>> > +#endif
>> >  	{ .pabyte = 1,  .pabit = 1, .invert = 1, .cpu_features =
>> > CPU_FTR_NODSISRALIGN },
>> >  	{ .pabyte = 5,  .pabit = 0, .cpu_features  = CPU_FTR_REAL_LE,
>> >  				    .cpu_user_ftrs = PPC_FEATURE_TRUE_LE },
>> > -- 
>> > 2.11.0
>> 
>> Instead can we do that feature removal in mmu_early_init_devtree. ie,
>> something like
>
> It looks like mmu_early_init_devtree() gets called after parsing ibm,pa_features
> so that should work.

But why is doing it later preferable?

Mikey's patch means it will never be set at any point during boot, which
seems obviously better to me.

cheers