drm/i915: Add distrust_bios_wm flag to dev_priv (v2)

SKL-style platforms can't fully trust the watermark/DDB settings
programmed by the BIOS and need to do extra sanitization on their first
atomic update.  Add a flag to dev_priv that is set during hardware
readout and cleared at the end of the first commit.

Note that for the somewhat common case where everything is turned off
when the driver starts up, we don't need to bother with a recompute...we
know exactly what the DDB should be (all zero's) so just setup the DDB
directly in that case.

v2:
 - Move clearing of distrust_bios_wm up below the swap_state call since
   it's a more natural / self-explanatory location.  (Maarten)
 - Use dev_priv->active_crtcs to test whether any CRTC's are turned on
   during HW WM readout rather than trying to count the active CRTC's
   again ourselves.  (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-9-git-send-email-matthew.d.roper@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -1995,6 +1995,13 @@ struct drm_i915_private {
 		 * cstate->wm.need_postvbl_update.
 		 */
 		struct mutex wm_mutex;
+
+		/*
+		 * Set during HW readout of watermarks/DDB.  Some platforms
+		 * need to know when we're still using BIOS-provided values
+		 * (which we don't fully trust).
+		 */
+		bool distrust_bios_wm;
 	} wm;
 
 	struct i915_runtime_pm pm;

drivers/gpu/drm/i915/intel_display.c

@@ -13594,6 +13594,7 @@ static int intel_atomic_commit(struct drm_device *dev,
 
 	drm_atomic_helper_swap_state(dev, state);
 	dev_priv->wm.config = intel_state->wm_config;
+	dev_priv->wm.distrust_bios_wm = false;
 	intel_shared_dpll_commit(state);
 
 	if (intel_state->modeset) {

drivers/gpu/drm/i915/intel_pm.c

@@ -4026,6 +4026,14 @@ void skl_wm_get_hw_state(struct drm_device *dev)
 	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head)
 		skl_pipe_wm_get_hw_state(crtc);
 
+	if (dev_priv->active_crtcs) {
+		/* Fully recompute DDB on first atomic commit */
+		dev_priv->wm.distrust_bios_wm = true;
+	} else {
+		/* Easy/common case; just sanitize DDB now if everything off */
+		memset(ddb, 0, sizeof(*ddb));
+	}
+
 	/* Calculate plane data rates */
 	for_each_intel_crtc(dev, intel_crtc) {
 		struct intel_crtc_state *cstate = intel_crtc->config;