drm/i915: WARN when trying to enabled an unused crtc

This is the first tiny step towards cross-checking the entire modeset
state machine with WARNs. A crtc can only be enabled when it's
actually in use, i.e. crtc->active imlies crtc->enabled.

Unfortunately we can't (yet) check this when disabling the crtc,
because the crtc helpers are a bit slopy with updating state and
unconditionally update crtc->enabled before changing the hw state.

Fixing that requires quite some more work.
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3215,6 +3215,8 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 	u32 temp;
 	bool is_pch_port;
 
+	WARN_ON(!crtc->enabled);
+
 	/* XXX: For compatability with the crtc helper code, call the encoder's
 	 * enable function unconditionally for now. */
 	if (intel_crtc->active)
@@ -3391,6 +3393,8 @@ static void i9xx_crtc_enable(struct drm_crtc *crtc)
 	int pipe = intel_crtc->pipe;
 	int plane = intel_crtc->plane;
 
+	WARN_ON(!crtc->enabled);
+
 	/* XXX: For compatability with the crtc helper code, call the encoder's
 	 * enable function unconditionally for now. */
 	if (intel_crtc->active)

drivers/gpu/drm/i915/intel_drv.h

@@ -158,7 +158,15 @@ struct intel_crtc {
 	enum plane plane;
 	u8 lut_r[256], lut_g[256], lut_b[256];
 	int dpms_mode;
-	bool active; /* is the crtc on? independent of the dpms mode */
+	/*
+	 * Whether the crtc and the connected output pipeline is active. Implies
+	 * that crtc->enabled is set, i.e. the current mode configuration has
+	 * some outputs connected to this crtc.
+	 *
+	 * Atm crtc->enabled is unconditionally updated _before_ the hw state is
+	 * changed, hence we can only check this when enabling the crtc.
+	 */
+	bool active;
 	bool primary_disabled; /* is the crtc obscured by a plane? */
 	bool lowfreq_avail;
 	struct intel_overlay *overlay;