firmware: smccc: Introduce SMCCC TRNG framework

The ARM DEN0098 document describe an SMCCC based firmware service to
deliver hardware generated random numbers. Its existence is advertised
according to the SMCCC v1.1 specification.

Add a (dummy) call to probe functions implemented in each architecture
(ARM and arm64), to determine the existence of this interface.
For now this return false, but this will be overwritten by each
architecture's support patch.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

arch/arm/include/asm/archrandom.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARCHRANDOM_H
+#define _ASM_ARCHRANDOM_H
+
+static inline bool __init smccc_probe_trng(void)
+{
+	return false;
+}
+
+#endif /* _ASM_ARCHRANDOM_H */

arch/arm64/include/asm/archrandom.h

@@ -8,6 +8,11 @@
 #include <linux/kernel.h>
 #include <asm/cpufeature.h>
 
+static inline bool __init smccc_probe_trng(void)
+{
+	return false;
+}
+
 static inline bool __arm64_rndr(unsigned long *v)
 {
 	bool ok;
@@ -79,5 +84,12 @@ arch_get_random_seed_long_early(unsigned long *v)
 }
 #define arch_get_random_seed_long_early arch_get_random_seed_long_early
 
+#else /* !CONFIG_ARCH_RANDOM */
+
+static inline bool __init smccc_probe_trng(void)
+{
+	return false;
+}
+
 #endif /* CONFIG_ARCH_RANDOM */
 #endif /* _ASM_ARCHRANDOM_H */

drivers/firmware/smccc/smccc.c

@@ -7,14 +7,19 @@
 
 #include <linux/init.h>
 #include <linux/arm-smccc.h>
+#include <asm/archrandom.h>
 
 static u32 smccc_version = ARM_SMCCC_VERSION_1_0;
 static enum arm_smccc_conduit smccc_conduit = SMCCC_CONDUIT_NONE;
 
+bool __ro_after_init smccc_trng_available = false;
+
 void __init arm_smccc_version_init(u32 version, enum arm_smccc_conduit conduit)
 {
 	smccc_version = version;
 	smccc_conduit = conduit;
+
+	smccc_trng_available = smccc_probe_trng();
 }
 
 enum arm_smccc_conduit arm_smccc_1_1_get_conduit(void)