Subject: [PATCH] powerpc/scm: Use a specific endian format for storing uuid from the device tree

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 7 Jun 2019 12:17:05 +0530

We used uuid_parse to convert uuid string from device tree to two u64
components. We want to make sure we look at the uuid read from device
tree in an endian-neutral fashion. For now, I am picking little-endian
to be format so that we don't end up doing an additional conversion.

The reason to store in a specific endian format is to enable reading
the namespace created with a little-endian kernel config on a big-endian kernel.
We do store the device tree uuid string as a 64-bit little-endian cookie in the
label area. When booting the kernel we also compare this cookie
against what is read from the device tree. For this, to work we have
to store and compare these values in a CPU endian config independent fashion.

Signed-off-by: Aneesh Kumar K.V <[email-masked]>
---
 arch/powerpc/platforms/pseries/papr_scm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
index 80fbab118ef1..c8ec670ee924 100644
--- a/arch/powerpc/platforms/pseries/papr_scm.c
+++ b/arch/powerpc/platforms/pseries/papr_scm.c
@@ -373,8 +373,15 @@ static int papr_scm_probe(struct platform_device *pdev)
 
 	/* We just need to ensure that set cookies are unique across */
 	uuid_parse(uuid_str, (uuid_t *) uuid);
-	p->nd_set.cookie1 = uuid[0];
-	p->nd_set.cookie2 = uuid[1];
+	/*
+	 * cookie1 and cookie2 are not really little endian
+	 * we store a little endian representation of the
+	 * uuid str so that we can compare this with the label
+	 * area cookie irrespective of the endian config with which
+	 * the kernel is built.
+	 */
+	p->nd_set.cookie1 = cpu_to_le64(uuid[0]);
+	p->nd_set.cookie2 = cpu_to_le64(uuid[1]);
 
 	/* might be zero */
 	p->metadata_size = metadata_size;
-- 
2.21.0