powerpc/mm: Add a parameter to disable 1TB segs

This patch adds the kernel command line parameter "no_tb_segs" which
forces the kernel to use 256MB rather than 1TB segments. Forcing the use
of 256MB segments makes it considerably easier to test code that depends
on an SLB miss occurring.
Suggested-by: Michael Neuling <mikey@neuling.org>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/kernel-parameters.txt

@@ -920,6 +920,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	dhash_entries=	[KNL]
 			Set number of hash buckets for dentry cache.
 
+	disable_1tb_segments [PPC]
+			Disables the use of 1TB hash page table segments. This
+			causes the kernel to fall back to 256MB segments which
+			can be useful when debugging issues that require an SLB
+			miss to occur.
+
 	disable=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 

arch/powerpc/mm/hash_utils_64.c

@@ -317,6 +317,15 @@ int htab_remove_mapping(unsigned long vstart, unsigned long vend,
 	return ret;
 }
 
+static bool disable_1tb_segments = false;
+
+static int __init parse_disable_1tb_segments(char *p)
+{
+	disable_1tb_segments = true;
+	return 0;
+}
+early_param("disable_1tb_segments", parse_disable_1tb_segments);
+
 static int __init htab_dt_scan_seg_sizes(unsigned long node,
 					 const char *uname, int depth,
 					 void *data)
@@ -335,6 +344,12 @@ static int __init htab_dt_scan_seg_sizes(unsigned long node,
 	for (; size >= 4; size -= 4, ++prop) {
 		if (be32_to_cpu(prop[0]) == 40) {
 			DBG("1T segment support detected\n");
+
+			if (disable_1tb_segments) {
+				DBG("1T segments disabled by command line\n");
+				break;
+			}
+
 			cur_cpu_spec->mmu_features |= MMU_FTR_1T_SEGMENT;
 			return 1;
 		}