Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

Also allow control of advertising unknown features to userspace and
with this parameter, and remove the clunky CONFIG option.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 10 ++++++
 arch/powerpc/Kconfig                            |  5 ---
 arch/powerpc/kernel/dt_cpu_ftrs.c               | 41 +++++++++++++++++++------
 3 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index facc20a3f962..7a7c1cc80f9f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -867,6 +867,16 @@
 
 	dscc4.setup=	[NET]
 
+	dt_cpu_ftrs=	[PPC]
+			Format: {"off" | "known"}
+			Control how the dt_cpu_ftrs device-tree binding is
+			used for CPU feature discovery and setup (if it
+			exists).
+			off: Do not use it, fall back to legacy cpu table.
+			known: Do not pass through unknown features to guests
+			or userspace, only those that the kernel is not aware
+			of.
+
 	dump_apple_properties	[X86]
 			Dump name and content of EFI device properties on
 			x86 Macs.  Useful for driver authors to determine

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 5bd868f2e813..c924710e5bc7 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -391,11 +391,6 @@ config PPC_DT_CPU_FTRS
 	  firmware provides this binding.
 	  If you're not sure say Y.
 
-config PPC_CPUFEATURES_ENABLE_UNKNOWN
-	bool "cpufeatures pass through unknown features to guest/userspace"
-	depends on PPC_DT_CPU_FTRS
-	default y
-
 config HIGHMEM
 	bool "High memory support"
 	depends on PPC32

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index fcc7588a96d6..050925b5b451 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -671,12 +671,24 @@ static struct dt_cpu_feature_match __initdata
 	{"wait-v3", feat_enable, 0},
 };
 
-/* XXX: how to configure this? Default + boot time? */
-#ifdef CONFIG_PPC_CPUFEATURES_ENABLE_UNKNOWN
-#define CPU_FEATURE_ENABLE_UNKNOWN 1
-#else
-#define CPU_FEATURE_ENABLE_UNKNOWN 0
-#endif
+static bool __initdata using_dt_cpu_ftrs = true;
+static bool __initdata dt_cpu_ftrs_enable_unknown = true;
+
+static int __init dt_cpu_ftrs_parse(char *str)
+{
+	if (!str)
+		return 0;
+
+	if (!strcmp(str, "off"))
+		using_dt_cpu_ftrs = false;
+	else if (!strcmp(str, "known"))
+		dt_cpu_ftrs_enable_unknown = false;
+	else
+		return 1;
+
+	return 0;
+}
+early_param("dt_cpu_ftrs", dt_cpu_ftrs_parse);