Subject: [PATCH] powerpc/mm: Add a parameter to disable 1TB segs

From: Oliver O'Halloran <[email-masked]>
Date: Mon, 4 Jul 2016 10:44:04 +1000

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 <[email-masked]>
Suggested-by: Michael Ellerman <[email-masked]>
Signed-off-by: Oliver O'Halloran <[email-masked]>
---
 arch/powerpc/mm/hash_utils_64.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index 5b22ba0b58bc..6da1a9d18e15 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -321,6 +321,15 @@ int htab_remove_mapping(unsigned long vstart, unsigned long vend,
 	return ret;
 }
 
+static bool no_tb_segs = false;
+
+static int __init parse_no_tb_segs(char *p)
+{
+	no_tb_segs = true;
+	return 0;
+}
+early_param("no_tb_segs", parse_no_tb_segs);
+
 static int __init htab_dt_scan_seg_sizes(unsigned long node,
 					 const char *uname, int depth,
 					 void *data)
@@ -339,6 +348,12 @@ 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 (no_tb_segs) {
+				DBG("Forcing 256MB segments\n");
+				break;
+			}
+
 			cur_cpu_spec->mmu_features |= MMU_FTR_1T_SEGMENT;
 			return 1;
 		}
-- 
2.5.5