x86: provide consistent cbmem_top() for CONFIG_LATE_CBMEM_INIT

For x86 systems employing CONFIG_LATE_CBMEM_INIT, set_top_of_ram() is
called in ramstage to note the upper address of the 32-bit address
space. This in turn is consumed by cbmem. However, in this scenario
cbmem_top() cannot always be relied upon because get_top_of_ram()
doesn't return the same value provided to set_top_of_ram().
To fix the inconsistency in ramstage save the value passed in
to set_top_of_ram() and defer to it as the return value for
cbmem_top().

Change-Id: Ida796fb836c59b9776019e7f8b3f2cd71156f0e5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/10313
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins)

src/arch/x86/boot/cbmem.c

@@ -34,10 +34,23 @@ void __attribute__((weak)) backup_top_of_ram(uint64_t ramtop)
 	/* Do nothing. Chipset may have implementation to save ramtop in NVRAM. */
 }
 
+static void *ramtop_pointer;
+
 void set_top_of_ram(uint64_t ramtop)
 {
 	backup_top_of_ram(ramtop);
-	cbmem_set_top((void*)(uintptr_t)ramtop);
+	ramtop_pointer = (void *)(uintptr_t)ramtop;
+	cbmem_set_top(ramtop_pointer);
+}
+
+static inline void *saved_ramtop(void)
+{
+	return ramtop_pointer;
+}
+#else
+static inline void *saved_ramtop(void)
+{
+	return NULL;
 }
 #endif /* !__PRE_RAM__ */
 
@@ -50,6 +63,11 @@ unsigned long __attribute__((weak)) get_top_of_ram(void)
 void *cbmem_top(void)
 {
 	/* Top of cbmem is at lowest usable DRAM address below 4GiB. */
+	void *ptr = saved_ramtop();
+
+	if (ptr != NULL)
+		return ptr;
+
 	return (void *)get_top_of_ram();
 }
 

src/include/cbmem.h

@@ -221,6 +221,9 @@ void cbmem_list(void);
  * early features like COLLECT_TIMESTAMPS and CBMEM_CONSOLE.
  */
 #if IS_ENABLED(CONFIG_ARCH_X86) && IS_ENABLED(CONFIG_LATE_CBMEM_INIT)
+/* Note that many of the current providers of get_top_of_ram() conditionally
+ * return 0 when the sleep type is non S3. i.e. cold and warm boots would
+ * return 0 from get_top_of_ram(). */
 unsigned long get_top_of_ram(void);
 void set_top_of_ram(uint64_t ramtop);
 void backup_top_of_ram(uint64_t ramtop);