powerpc/mm/radix: Add a kernel command line to disable radix

This patch adds the kernel command line disable_radix which disable
the radix MMU mode even if firmware indicates radix support via
ibm,pa-features device tree node.

This helps in testing different MMU mode easily.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/kernel-parameters.txt

@@ -929,6 +929,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	disable=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 
+	disable_radix	[PPC]
+			Disable RADIX MMU mode on POWER9
+
 	disable_cpu_apicid= [X86,APIC,SMP]
 			Format: <int>
 			The number of initial APIC ID for the

arch/powerpc/kernel/prom.c

@@ -646,6 +646,14 @@ static void __init early_reserve_mem(void)
 #endif
 }
 
+static bool disable_radix;
+static int __init parse_disable_radix(char *p)
+{
+	disable_radix = true;
+	return 0;
+}
+early_param("disable_radix", parse_disable_radix);
+
 void __init early_init_devtree(void *params)
 {
 	phys_addr_t limit;
@@ -735,6 +743,11 @@ void __init early_init_devtree(void *params)
 	 */
 	spinning_secondaries = boot_cpu_count - 1;
 #endif
+	/*
+	 * now fixup radix MMU mode based on kernel command line
+	 */
+	if (disable_radix)
+		cur_cpu_spec->mmu_features &= ~MMU_FTR_RADIX;
 
 #ifdef CONFIG_PPC_POWERNV
 	/* Scan and build the list of machine check recoverable ranges */