Re: [V2, 68/68] powerpc/mm/radix: Use firmware feature to disable radix
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2016-04-20 02:59:54
On Sat, 2016-09-04 at 06:14:04 UTC, "Aneesh Kumar K.V" wrote:
We can depend on ibm,pa-features to enable/disable radix. This gives us a nice way to test p9 hash config, by changing device tree property.
I think we might want to be more careful here. You set MMU_FTR_RADIX in the cputable entry. So it's on by default on P9 cpus. Then if there is an ibm,pa-features property *and* it is >= 41 bytes long, the below feature entry will hit. In that case the firmware controls whether it's on or off. I think it would be clearer if we removed RADIX from the cputable, and the below became the only way to turn it on. Would that break anything?
quoted hunk ↗ jump to hunk
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c index 7030b035905d..a4d1f44364b8 100644 --- a/arch/powerpc/kernel/prom.c +++ b/arch/powerpc/kernel/prom.c@@ -165,6 +165,7 @@ static struct ibm_pa_feature { * which is 0 if the kernel doesn't support TM. */ {CPU_FTR_TM_COMP, 0, 0, 22, 0, 0}, + {0, MMU_FTR_RADIX, 0, 40, 0, 0},
So that says bit 0 of byte 40 enables MMU_FTR_RADIX. Where is that documented? cheers