amd64: Allow early ACPICA table subsystem init

Add a method for initializing the ACPICA table subsystem earlier than
the initilization of the entire ACPICA subsystem. Also add a method to
walk through the subtables of an ACPI table

bsps/x86_64/amd64/acpi/acpi.c

@@ -37,19 +37,57 @@
 
 #include <assert.h>
 #include <acpi/acpi.h>
-#include <acpi/acpica/acpi.h>
 #include <rtems/sysinit.h>
 
 uint64_t acpi_rsdp_addr = 0;
 
+static bool acpi_tables_initialized = false;
+
+bool acpi_tables_initialize(void)
+{
+  ACPI_STATUS status;
+
+  status = AcpiInitializeTables(NULL, ACPI_MAX_INIT_TABLES, FALSE);
+
+  if (status == (AE_OK)) {
+    acpi_tables_initialized = true;
+    return true;
+  }
+  return false;
+}
+
+void acpi_walk_subtables(
+  ACPI_TABLE_HEADER* table,
+  size_t size_of_header,
+  void (*handler)(ACPI_SUBTABLE_HEADER*)
+)
+{
+  ACPI_SUBTABLE_HEADER* entry;
+  ACPI_SUBTABLE_HEADER* end;
+
+  if (table == NULL) {
+    return;
+  }
+
+  entry = (ACPI_SUBTABLE_HEADER*) ((uint8_t*) table + size_of_header);
+  end = (ACPI_SUBTABLE_HEADER*) ((uint8_t*) table + table->Length);
+
+  while (entry < end) {
+    handler(entry);
+    entry = (ACPI_SUBTABLE_HEADER*) ((uint8_t*) entry + entry->Length);
+  }
+}
+
 static void initialize_acpi(void)
 {
   ACPI_STATUS status;
   status = AcpiInitializeSubsystem();
   assert(status == (AE_OK));
 
+  if (acpi_tables_initialized == false) {
     status = AcpiInitializeTables(NULL, ACPI_MAX_INIT_TABLES, FALSE);
     assert(status == (AE_OK));
+  }
 
   status = AcpiLoadTables();
   assert(status == (AE_OK));

bsps/x86_64/amd64/include/acpi/acpi.h

@@ -39,9 +39,36 @@
 #define _AMD64_ACPI_H_
 
 #include <bsp.h>
+#include <acpi/acpica/acpi.h>
 
 #define ACPI_MAX_INIT_TABLES 16
 
 extern uint64_t acpi_rsdp_addr;
 
+/**
+ * @brief Initializes the ACPICA Table Manager. Requires dynamic memory.
+ *
+ * The ACPICA Table Manager is independent of the rest of the ACPICA subsystem
+ * and only requires dynamic memory to be initialized (unless a statically
+ * allocated array for the tables is provided). Since access to ACPI tables
+ * may be required before the entire ACPICA subsystem can be initialized
+ * this routine can be used earlier in the system intialization.
+ *
+ * @return true if successful.
+ */
+bool acpi_tables_initialize(void);
+
+/**
+ * @brief Walks through the subtables of an ACPI table.
+ *
+ * @param table Pointer to the table.
+ * @param size_of_header Size of the table header (used to offset into first subtable).
+ * @param handler Handler that is called for each subtable found.
+ */
+void acpi_walk_subtables(
+  ACPI_TABLE_HEADER *table,
+  size_t size_of_header,
+  void (*handler)(ACPI_SUBTABLE_HEADER*)
+);
+
 #endif /* _AMD64_ACPI_H_ */