powerpc/microwatt: Add support for hardware random number generator

Microwatt's hardware RNG is accessed using the DARN instruction.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YMwXPHlV/ZleiQUY@thinks.paulus.ozlabs.org

arch/powerpc/platforms/microwatt/Kconfig

@@ -7,6 +7,7 @@ config PPC_MICROWATT
 	select PPC_ICP_NATIVE
 	select PPC_NATIVE
 	select PPC_UDBG_16550
+	select ARCH_RANDOM
 	help
           This option enables support for FPGA-based Microwatt implementations.
 

arch/powerpc/platforms/microwatt/Makefile

-obj-y	+= setup.o
+obj-y	+= setup.o rng.o

arch/powerpc/platforms/microwatt/rng.c

+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Derived from arch/powerpc/platforms/powernv/rng.c, which is:
+ * Copyright 2013, Michael Ellerman, IBM Corporation.
+ */
+
+#define pr_fmt(fmt)	"microwatt-rng: " fmt
+
+#include <linux/kernel.h>
+#include <linux/smp.h>
+#include <asm/archrandom.h>
+#include <asm/cputable.h>
+#include <asm/machdep.h>
+
+#define DARN_ERR 0xFFFFFFFFFFFFFFFFul
+
+int microwatt_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 __init int rng_init(void)
+{
+	unsigned long val;
+	int i;
+
+	for (i = 0; i < 10; i++) {
+		if (microwatt_get_random_darn(&val)) {
+			ppc_md.get_random_seed = microwatt_get_random_darn;
+			return 0;
+		}
+	}
+
+	pr_warn("Unable to use DARN for get_random_seed()\n");
+
+	return -EIO;
+}
+machine_subsys_initcall(, rng_init);