powerpc/powernv: Use darn instruction for get_random_seed() on Power9

This adds powernv_get_random_darn() which utilises the darn instruction,
introduced in ISA v3.0/POWER9.

The darn instruction can potentially return an error, which is supported
by the get_random_seed() API, in normal usage if we see an error we just
return that to the caller.

However when detecting whether darn is functional at boot we try up to
10 times, before deciding that darn doesn't work and failing the
registration of get_random_seed(). That way an intermittent failure
at boot doesn't deprive the system of randomness until the next reboot.

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
[mpe: Move init into a function, tweak change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 39 insertions, 0 deletions

diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
index 1a9d84371a4d..c5ce3a8bd4c9 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 +69,41 @@ 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;
+}
+
+static int initialise_darn(void)
+{
+	unsigned long val;
+	int i;
+
+	if (!cpu_has_feature(CPU_FTR_ARCH_300))
+		return -ENODEV;
+
+	for (i = 0; i < 10; i++) {
+		if (powernv_get_random_darn(&val)) {
+			ppc_md.get_random_seed = powernv_get_random_darn;
+			return 0;
+		}
+	}
+
+	pr_warn("Unable to use DARN for get_random_seed()\n");
+
+	return -EIO;
+}
+
 int powernv_get_random_long(unsigned long *v)
 {
 	struct powernv_rng *rng;
@@ -150,6 +187,8 @@ static __init int rng_init(void)
 		of_platform_device_create(dn, NULL, NULL);
 	}
 
+	initialise_darn();
+
 	return 0;
 }
 machine_subsys_initcall(powernv, rng_init);