ACPI: Add function to write _PPC using NVS

The existing NVS variable for PPCM will be used to
select a dynamic max P-state.

By itself this does not change existing behavior because
the NVS PPCM variable is initialized to zero.

PPCM can be tested by building and booting a modified BIOS
that sets gnvs->ppcm to a value greater than 1 and checking
from the OS that the P-state is limited to that value.

Change-Id: Ia7b3bbc6b84c1aa42349bb236abee5cc92486561
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/1341
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/x86/boot/acpigen.c

@@ -396,6 +396,32 @@ int acpigen_write_PPC(u8 nr)
 	return len;
 }
 
+/* generates a func with max supported P states */
+int acpigen_write_PPC_NVS(void)
+{
+/*
+    Method (_PPC, 0, NotSerialized)
+    {
+        Return (PPCM)
+    }
+*/
+	int len;
+	/* method op */
+	acpigen_emit_byte(0x14);
+	len = acpigen_write_len_f();
+	len += acpigen_emit_namestring("_PPC");
+	/* no fnarg */
+	acpigen_emit_byte(0x00);
+	/* return */
+	acpigen_emit_byte(0xa4);
+	/* arg */
+	len += acpigen_emit_namestring("PPCM");
+	/* add all single bytes */
+	len += 3;
+	acpigen_patch_len(len - 1);
+	return len;
+}
+
 int acpigen_write_TPC(const char *gnvs_tpc_limit)
 {
 /*

src/arch/x86/include/arch/acpigen.h

@@ -42,6 +42,7 @@ int acpigen_write_name_qword(const char *name, uint64_t val);
 int acpigen_write_name_byte(const char *name, uint8_t val);
 int acpigen_write_scope(const char *name);
 int acpigen_write_PPC(u8 nr);
+int acpigen_write_PPC_NVS(void);
 int acpigen_write_empty_PCT(void);
 int acpigen_write_empty_PTC(void);
 int acpigen_write_TPC(const char *gnvs_tpc_limit);

src/cpu/intel/model_206ax/acpi.c

@@ -251,7 +251,7 @@ static int generate_P_state_entries(int core, int cores_per_package)
 	len = acpigen_write_empty_PCT();
 
 	/* Write _PPC with no limit on supported P-state */
-	len += acpigen_write_PPC(0);
+	len += acpigen_write_PPC_NVS();
 
 	/* Write PSD indicating configured coordination type */
 	len += acpigen_write_PSD_package(core, cores_per_package, coord_type);