drm/i915/skl+: Fail the flip if ddb min requirement exceeds pipe allocation

DDB minimum requirement of crtc configuration (cumulative of all the
enabled planes in crtc) may exceed the allocated DDB for crtc/pipe.
This patch make changes to fail the flip/ioctl if minimum requirement
for pipe exceeds the total ddb allocated to the pipe.
Previously it succeeded but making alloc_size a negative value. Which
will make subsequent calculations for plane ddb allocation bogus & may
lead to screen corruption or system hang.

Changes from V1:
 - Improve commit message as per Ander's comment
 - Remove extra parentheses (Ander)
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-8-mahesh1.kumar@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4057,6 +4057,7 @@ skl_allocate_pipe_ddb(struct intel_crtc_state *cstate,
 	int num_active;
 	unsigned plane_data_rate[I915_MAX_PLANES] = {};
 	unsigned plane_y_data_rate[I915_MAX_PLANES] = {};
+	uint16_t total_min_blocks = 0;
 
 	/* Clear the partitioning for disabled planes. */
 	memset(ddb->plane[pipe], 0, sizeof(ddb->plane[pipe]));
@@ -4084,10 +4085,18 @@ skl_allocate_pipe_ddb(struct intel_crtc_state *cstate,
 	 */
 
 	for_each_plane_id_on_crtc(intel_crtc, plane_id) {
-		alloc_size -= minimum[plane_id];
-		alloc_size -= y_minimum[plane_id];
+		total_min_blocks += minimum[plane_id];
+		total_min_blocks += y_minimum[plane_id];
 	}
 
+	if (total_min_blocks > alloc_size) {
+		DRM_DEBUG_KMS("Requested display configuration exceeds system DDB limitations");
+		DRM_DEBUG_KMS("minimum required %d/%d\n", total_min_blocks,
+							alloc_size);
+		return -EINVAL;
+	}
+
+	alloc_size -= total_min_blocks;
 	ddb->plane[pipe][PLANE_CURSOR].start = alloc->end - minimum[PLANE_CURSOR];
 	ddb->plane[pipe][PLANE_CURSOR].end = alloc->end;