soc/intel/common: sanity check ebda signature

It's possible for chipsets utilizing ebda to cache the cbmem_top()
value to be called prior to the object being entirely setup. As
such it's important to check the signature to ensure the object
has been initialized. Do that in a newly introduced function,
retrieve_ebda_object(), which will zero out the object if the
signature doesn't match.

Change-Id: I66b07c36f46ed9112bc39b44914c860ba68677ae
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/21995
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

src/soc/intel/common/block/ebda/ebda.c

@@ -15,6 +15,7 @@
 
 #include <arch/ebda.h>
 #include <intelblocks/ebda.h>
+#include <string.h>
 
 /*
  * Mainboard Override function
@@ -47,3 +48,11 @@ void fill_ebda_area(void)
 	create_mainboard_ebda(cfg);
 	write_ebda_data(cfg, sizeof(*cfg));
 }
+
+void retrieve_ebda_object(struct ebda_config *cfg)
+{
+	read_ebda_data(cfg, sizeof(*cfg));
+
+	if (cfg->signature != EBDA_SIGNATURE)
+		memset(cfg, 0, sizeof(*cfg));
+}

src/soc/intel/common/block/include/intelblocks/ebda.h

@@ -18,7 +18,7 @@
 
 #include <soc/ebda.h>
 
-#define EBDA_SIGNATURE 0xebda
+#define EBDA_SIGNATURE 0xebdaebda
 
 /*
  * Mainboard Override function
@@ -45,6 +45,10 @@ void fill_soc_memmap_ebda(struct ebda_config *cfg);
  */
 void fill_ebda_area(void);
 
+/* Fill the ebda object pointed to by cfg. Object will be zero filled
+ * if signature check fails. */
+void retrieve_ebda_object(struct ebda_config *cfg);
+
 /*
  * EBDA structure
  *

src/soc/intel/skylake/memmap.c

@@ -348,7 +348,7 @@ void *cbmem_top(void)
 	if (sa_get_tseg_base() == 0)
 		return NULL;
 
-	read_ebda_data(cfg, sizeof(*cfg));
+	retrieve_ebda_object(cfg);
 
 	return (void *)(uintptr_t)cfg->tolum_base;
 }