[PATCH v3] powerpc/powernv: Use darn instr for random_seed on p9

[Matt Brown <matthew.brown.dev@gmail.com>]

This adds the powernv_get_random_darn function which utilises the darn
instruction, introduced in POWER9. The powernv_get_random_darn function
is used as the ppc_md.get_random_seed on P9.

The DARN instruction can potentially throw an error, so we attempt to
register the powernv_get_random_darn function up to 10 times before
failing.

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
---
v3:
	- add repeat attempts to register the ppc_md.get_random_seed
	- fixed the PPC_DARN macro
	- move DARN_ERR definition
	- fixed commit message
v2:
	- remove repeat darn attempts
	- move hook to rng_init
---
 arch/powerpc/include/asm/ppc-opcode.h |  4 ++++
 arch/powerpc/platforms/powernv/rng.c  | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index c4ced1d..aabd150 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h
@@ -134,6 +134,7 @@
 #define PPC_INST_COPY			0x7c00060c
 #define PPC_INST_COPY_FIRST		0x7c20060c
 #define PPC_INST_CP_ABORT		0x7c00068c
+#define PPC_INST_DARN			0x7c0005e6
 #define PPC_INST_DCBA			0x7c0005ec
 #define PPC_INST_DCBA_MASK		0xfc0007fe
 #define PPC_INST_DCBAL			0x7c2005ec
@@ -325,6 +326,9 @@
 
 /* Deal with instructions that older assemblers aren't aware of */
 #define	PPC_CP_ABORT		stringify_in_c(.long PPC_INST_CP_ABORT)
+#define PPC_DARN(t, l)		stringify_in_c(.long PPC_INST_DARN |  \
+						___PPC_RT(t)	   |  \
+						(((l) & 0x3) << 16))
 #define	PPC_DCBAL(a, b)		stringify_in_c(.long PPC_INST_DCBAL | \
 					__PPC_RA(a) | __PPC_RB(b))
 #define	PPC_DCBZL(a, b)		stringify_in_c(.long PPC_INST_DCBZL | \
diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
index 5dcbdea..83b925c 100644
--- a/arch/powerpc/platforms/powernv/rng.c
+++ b/arch/powerpc/platforms/powernv/rng.c
@@ -16,11 +16,13 @@
 #include <linux/slab.h>
 #include <linux/smp.h>
 #include <asm/archrandom.h>
+#include <asm/cputable.h>
 #include <asm/io.h>
 #include <asm/prom.h>
 #include <asm/machdep.h>
 #include <asm/smp.h>
 
+#define DARN_ERR 0xFFFFFFFFFFFFFFFFul
 
 struct powernv_rng {
 	void __iomem *regs;
@@ -67,6 +69,21 @@ int powernv_get_random_real_mode(unsigned long *v)
 	return 1;
 }
 
+int powernv_get_random_darn(unsigned long *v)
+{
+	unsigned long val;
+
+	/* Using DARN with L=1 - 64-bit conditioned random number */
+	asm volatile(PPC_DARN(%0, 1) : "=r"(val));
+
+	if (val == DARN_ERR)
+		return 0;
+
+	*v = val;
+
+	return 1;
+}
+
 int powernv_get_random_long(unsigned long *v)
 {
 	struct powernv_rng *rng;
@@ -135,8 +152,9 @@ static __init int rng_create(struct device_node *dn)
 
 static __init int rng_init(void)
 {
+	unsigned long darn_test;
 	struct device_node *dn;
-	int rc;
+	int rc, i;
 
 	for_each_compatible_node(dn, NULL, "ibm,power-rng") {
 		rc = rng_create(dn);
@@ -150,6 +168,21 @@ static __init int rng_init(void)
 		of_platform_device_create(dn, NULL, NULL);
 	}
 
+	if (cpu_has_feature(CPU_FTR_ARCH_300)) {
+		for (i = 0; i < 10; i++) {
+			if (powernv_get_random_darn(&darn_test)) {
+				ppc_md.get_random_seed =
+					powernv_get_random_darn;
+				break;
+			}
+
+			if (i == 9) {
+				pr_warn("Failed to use powernv_get_random_darn"\
+					"as get_random_seed");
+			}
+		}
+	}
+
 	return 0;
 }
 machine_subsys_initcall(powernv, rng_init);
-- 
2.9.3