drm/i915: Protect DSPARB registers with a spinlock

Each DSPARB register can house bits for two separate pipes, hence
we must protect the registers during reprogramming so that parallel
FIFO reconfigurations happening simultaneosly on multiple pipes won't
corrupt each others values.

We'll use a new spinlock for this instead of the wm_mutex since we'll
have to move the DSPARB programming to happen from the vblank evade
critical section, and we can't use mutexes in there.

v2: Document why we use a spinlock instead of a mutex (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480947208-18468-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -801,6 +801,7 @@ static int i915_driver_init_early(struct drm_i915_private *dev_priv,
 	spin_lock_init(&dev_priv->uncore.lock);
 	spin_lock_init(&dev_priv->mm.object_stat_lock);
 	spin_lock_init(&dev_priv->mmio_flip_lock);
+	spin_lock_init(&dev_priv->wm.dsparb_lock);
 	mutex_init(&dev_priv->sb_lock);
 	mutex_init(&dev_priv->modeset_restore_lock);
 	mutex_init(&dev_priv->av_mutex);

drivers/gpu/drm/i915/i915_drv.h

@@ -2191,6 +2191,9 @@ struct drm_i915_private {
 	} sagv_status;
 
 	struct {
+		/* protects DSPARB registers on pre-g4x/vlv/chv */
+		spinlock_t dsparb_lock;
+
 		/*
 		 * Raw watermark latency values:
 		 * in 0.1us units for WM0,

drivers/gpu/drm/i915/intel_pm.c

@@ -1223,6 +1223,8 @@ static void vlv_pipe_set_fifo_size(struct intel_crtc *crtc)
 		      pipe_name(crtc->pipe), sprite0_start,
 		      sprite1_start, fifo_size);
 
+	spin_lock(&dev_priv->wm.dsparb_lock);
+
 	switch (crtc->pipe) {
 		uint32_t dsparb, dsparb2, dsparb3;
 	case PIPE_A:
@@ -1279,6 +1281,10 @@ static void vlv_pipe_set_fifo_size(struct intel_crtc *crtc)
 	default:
 		break;
 	}
+
+	POSTING_READ(DSPARB);
+
+	spin_unlock(&dev_priv->wm.dsparb_lock);
 }
 
 #undef VLV_FIFO