arm64: add credited/trusted RNG support

Currently arm64 doesn't initialize the primary CRNG in a (potentially)
trusted manner as we only detect the presence of the RNG once secondary
CPUs are up.

Now that the core RNG code distinguishes the early initialization of the
primary CRNG, we can implement arch_get_random_seed_long_early() to
support this.

This patch does so.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200210130015.17664-4-mark.rutland@arm.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

arch/arm64/include/asm/archrandom.h

@@ -4,6 +4,8 @@
 
 #ifdef CONFIG_ARCH_RANDOM
 
+#include <linux/bug.h>
+#include <linux/kernel.h>
 #include <linux/random.h>
 #include <asm/cpufeature.h>
 
@@ -66,6 +68,18 @@ static inline bool __init __early_cpu_has_rndr(void)
 	return (ftr >> ID_AA64ISAR0_RNDR_SHIFT) & 0xf;
 }
 
+static inline bool __init __must_check
+arch_get_random_seed_long_early(unsigned long *v)
+{
+	WARN_ON(system_state != SYSTEM_BOOTING);
+
+	if (!__early_cpu_has_rndr())
+		return false;
+
+	return __arm64_rndr(v);
+}
+#define arch_get_random_seed_long_early arch_get_random_seed_long_early
+
 #else
 
 static inline bool __arm64_rndr(unsigned long *v) { return false; }