efi_loader: get_memory_map: return parameters whenever possible

Currently, if GetMemoryMap API returns EFI_BUFFER_TOO_SMALL, it doesn't
set valid values to other parameters, descriptor_size and
descriptor_version, except memory_map_size.
Some efi applications, however, may use those value; in particular,
xen uses descriptor_size to calculate a size of buffer to be allocated.

While UEFI specification is ambiguous in this point, it would be better
to address this issue proactively to maximize the compatibility with
existing efi applications.

With this patch, for example, xen.efi (and hence linux kernel) can be
started via bootefi without modification.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

lib/efi_loader/efi_memory.c

@@ -627,18 +627,18 @@ efi_status_t efi_get_memory_map(efi_uintn_t *memory_map_size,
 
 	*memory_map_size = map_size;
 
-	if (provided_map_size < map_size)
-		return EFI_BUFFER_TOO_SMALL;
-
-	if (!memory_map)
-		return EFI_INVALID_PARAMETER;
-
 	if (descriptor_size)
 		*descriptor_size = sizeof(struct efi_mem_desc);
 
 	if (descriptor_version)
 		*descriptor_version = EFI_MEMORY_DESCRIPTOR_VERSION;
 
+	if (provided_map_size < map_size)
+		return EFI_BUFFER_TOO_SMALL;
+
+	if (!memory_map)
+		return EFI_INVALID_PARAMETER;
+
 	/* Copy list into array */
 	/* Return the list in ascending order */
 	memory_map = &memory_map[map_entries - 1];