drm/i915/dsi: be defensive about out of bounds operation byte

Untie the VBT based generic panel driver from the VBT parsing, so that
the two don't have to be updated in lockstep.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/01c71ac89a9db8bc7b8ae0fb05c50a5fae362dc4.1450702954.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dsi_panel_vbt.c

@@ -232,11 +232,9 @@ static const u8 *mipi_exec_gpio(struct intel_dsi *intel_dsi, const u8 *data)
 typedef const u8 * (*fn_mipi_elem_exec)(struct intel_dsi *intel_dsi,
 					const u8 *data);
 static const fn_mipi_elem_exec exec_elem[] = {
-	NULL, /* reserved */
-	mipi_exec_send_packet,
-	mipi_exec_delay,
-	mipi_exec_gpio,
-	NULL, /* status read; later */
+	[MIPI_SEQ_ELEM_SEND_PKT] = mipi_exec_send_packet,
+	[MIPI_SEQ_ELEM_DELAY] = mipi_exec_delay,
+	[MIPI_SEQ_ELEM_GPIO] = mipi_exec_gpio,
 };
 
 /*
@@ -264,7 +262,6 @@ static const char *sequence_name(enum mipi_seq seq_id)
 static void generic_exec_sequence(struct intel_dsi *intel_dsi, const u8 *data)
 {
 	fn_mipi_elem_exec mipi_elem_exec;
-	int index;
 
 	if (!data)
 		return;
@@ -277,15 +274,14 @@ static void generic_exec_sequence(struct intel_dsi *intel_dsi, const u8 *data)
 
 	/* parse each byte till we reach end of sequence byte - 0x00 */
 	while (1) {
-		index = *data;
-		mipi_elem_exec = exec_elem[index];
-		if (!mipi_elem_exec) {
-			DRM_ERROR("Unsupported MIPI element, skipping sequence execution\n");
+		u8 operation_byte = *data++;
+		if (operation_byte >= ARRAY_SIZE(exec_elem) ||
+		    !exec_elem[operation_byte]) {
+			DRM_ERROR("Unsupported MIPI operation byte %u\n",
+				  operation_byte);
 			return;
 		}
-
-		/* goto element payload */
-		data++;
+		mipi_elem_exec = exec_elem[operation_byte];
 
 		/* execute the element specific rotines */
 		data = mipi_elem_exec(intel_dsi, data);