SMCCC: Introduce function to check SMCCC function availability

Currently, 'SMCCC_ARCH_FEATURES' SMC call handler unconditionally
returns 'SMC_OK' for 'SMCCC_ARCH_SOC_ID' function. This seems to
be not correct for the platform which doesn't implement soc-id
functionality i.e. functions to retrieve both soc-version and
soc-revision.
Hence introduced a platform function which will check whether SMCCC
feature is available for the platform.

Also, updated porting guide for the newly added platform function.

Change-Id: I389f0ef6b0837bb24c712aa995b7176117bc7961
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

docs/getting_started/porting-guide.rst

@@ -1130,6 +1130,7 @@ This function returns soc version which mainly consist of below fields
 
     soc_version[30:24] = JEP-106 continuation code for the SiP
     soc_version[23:16] = JEP-106 identification code with parity bit for the SiP
+    soc_version[15:0]  = Implementation defined SoC ID
 
 Function : plat_get_soc_revision()
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -1145,6 +1146,18 @@ This function returns soc revision in below format
 
     soc_revision[0:30] = SOC revision of specific SOC
 
+Function : plat_is_smccc_feature_available()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : u_register_t
+    Return   : int32_t
+
+This function returns SMC_ARCH_CALL_SUCCESS if the platform supports
+the SMCCC function specified in the argument; otherwise returns
+SMC_ARCH_CALL_NOT_SUPPORTED.
+
 Modifications specific to a Boot Loader stage
 ---------------------------------------------
 

include/plat/common/platform.h

@@ -341,4 +341,9 @@ int32_t plat_get_soc_version(void);
  */
 int32_t plat_get_soc_revision(void);
 
+/*
+ * Optional function to check for SMCCC function availability for platform
+ */
+int32_t plat_is_smccc_feature_available(u_register_t fid);
+
 #endif /* PLATFORM_H */

plat/common/plat_bl_common.c

@@ -11,6 +11,7 @@
 #include <common/debug.h>
 #include <lib/xlat_tables/xlat_tables_compat.h>
 #include <plat/common/platform.h>
+#include <services/arm_arch_svc.h>
 #include <smccc_helpers.h>
 #include <tools_share/firmware_encrypted.h>
 
@@ -25,6 +26,7 @@
 #pragma weak bl2_plat_handle_post_image_load
 #pragma weak plat_try_next_boot_source
 #pragma weak plat_get_enc_key_info
+#pragma weak plat_is_smccc_feature_available
 #pragma weak plat_get_soc_version
 #pragma weak plat_get_soc_revision
 
@@ -38,6 +40,11 @@ int32_t plat_get_soc_revision(void)
 	return SMC_ARCH_CALL_NOT_SUPPORTED;
 }
 
+int32_t plat_is_smccc_feature_available(u_register_t fid __unused)
+{
+	return SMC_ARCH_CALL_NOT_SUPPORTED;
+}
+
 void bl2_el3_plat_prepare_exit(void)
 {
 }

services/arm_arch_svc/arm_arch_svc_setup.c

@@ -24,8 +24,9 @@ static int32_t smccc_arch_features(u_register_t arg1)
 	switch (arg1) {
 	case SMCCC_VERSION:
 	case SMCCC_ARCH_FEATURES:
+		return SMC_ARCH_CALL_SUCCESS;
 	case SMCCC_ARCH_SOC_ID:
-		return SMC_OK;
+		return plat_is_smccc_feature_available(arg1);
 #if WORKAROUND_CVE_2017_5715
 	case SMCCC_ARCH_WORKAROUND_1:
 		if (check_wa_cve_2017_5715() == ERRATA_NOT_APPLIES)