greybus: enforce a buffer headroom maximum size

Define a maximum size that a host device can use for its private
area ahead of the payload space used by Greybus in a message buffer.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/es1-ap-usb.c

@@ -127,6 +127,7 @@ static void hd_buffer_constraints(struct greybus_host_device *hd)
 	 */
 	hd->buffer_headroom = sizeof(u32);	/* For cport id */
 	hd->buffer_size_max = ES1_GBUF_MSG_SIZE_MAX;
+	BUILD_BUG_ON(hd->buffer_headroom > GB_BUFFER_HEADROOM_MAX);
 }
 
 #define ES1_TIMEOUT	500	/* 500 ms for the SVC to do something */

drivers/staging/greybus/greybus.h

@@ -69,6 +69,14 @@
 struct greybus_host_device;
 struct svc_msg;
 
+/*
+ * When the Greybus code allocates a buffer it sets aside bytes
+ * prior to the beginning of the payload area for the host device's
+ * exclusive use.  The size is specified by hd->buffer_headroom, and
+ * which can't be greater than GB_BUFFER_HEADROOM_MAX.
+ */
+#define GB_BUFFER_HEADROOM_MAX		sizeof(u64)
+
 /* Buffers allocated from the host driver will be aligned to this multiple */
 #define GB_BUFFER_ALIGN	sizeof(u32)