drm/i915: Merge LP1+ watermarks in safer way

On ILK when we disable a particular watermark level, we must
maintain the actual watermark values for that level for some time
(until the next vblank possibly). Otherwise we risk underruns.

In order to achieve that result we must merge the LP1+ watermarks a
bit differently since we must also merge levels that are to be
disabled. We must also make sure we don't overflow the fields in the
watermark registers in case the calculated watermarks come out too
big to fit.

As early as possbile we mark all computed watermark levels as
disabled if they would exceed the register maximums. We make sure
to leave the actual watermarks for such levels zeroed out. Then during
merging, we take the maxium values for every level, regardless if
they're disabled or not. That may seem a bit pointless since at the
moment all the watermark levels we merge should have their values
zeroed if the level is already disabled. However soon we will be
dealing with intermediate watermarks that, in addition to the new
watermark values, also contain the previous watermark values, and so
levels that are disabled may no longer be zeroed out.

v2: Split the patch in two (Paulo)
    Use if() instead of & when merging ->enable (Paulo)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix commit message as noted by Paulo.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -2252,6 +2252,8 @@ static void ilk_merge_wm_level(struct drm_device *dev,
 {
 	const struct intel_crtc *intel_crtc;
 
+	ret_wm->enable = true;
+
 	list_for_each_entry(intel_crtc, &dev->mode_config.crtc_list, base.head) {
 		const struct intel_pipe_wm *active = &intel_crtc->wm.active;
 		const struct intel_wm_level *wm = &active->wm[level];
@@ -2259,16 +2261,19 @@ static void ilk_merge_wm_level(struct drm_device *dev,
 		if (!active->pipe_enabled)
 			continue;
 
+		/*
+		 * The watermark values may have been used in the past,
+		 * so we must maintain them in the registers for some
+		 * time even if the level is now disabled.
+		 */
 		if (!wm->enable)
-			return;
+			ret_wm->enable = false;
 
 		ret_wm->pri_val = max(ret_wm->pri_val, wm->pri_val);
 		ret_wm->spr_val = max(ret_wm->spr_val, wm->spr_val);
 		ret_wm->cur_val = max(ret_wm->cur_val, wm->cur_val);
 		ret_wm->fbc_val = max(ret_wm->fbc_val, wm->fbc_val);
 	}
-
-	ret_wm->enable = true;
 }
 
 /*
@@ -2280,6 +2285,7 @@ static void ilk_wm_merge(struct drm_device *dev,
 			 struct intel_pipe_wm *merged)
 {
 	int level, max_level = ilk_wm_max_level(dev);
+	int last_enabled_level = max_level;
 
 	/* ILK/SNB/IVB: LP1+ watermarks only w/ single pipe */
 	if ((INTEL_INFO(dev)->gen <= 6 || IS_IVYBRIDGE(dev)) &&
@@ -2295,14 +2301,18 @@ static void ilk_wm_merge(struct drm_device *dev,
 
 		ilk_merge_wm_level(dev, level, wm);
 
-		if (!ilk_validate_wm_level(level, max, wm))
-			break;
+		if (level > last_enabled_level)
+			wm->enable = false;
+		else if (!ilk_validate_wm_level(level, max, wm))
+			/* make sure all following levels get disabled */
+			last_enabled_level = level - 1;
 
 		/*
 		 * The spec says it is preferred to disable
 		 * FBC WMs instead of disabling a WM level.
 		 */
 		if (wm->fbc_val > max->fbc) {
+			if (wm->enable)
 				merged->fbc_wm_enabled = false;
 			wm->fbc_val = 0;
 		}
@@ -2358,14 +2368,19 @@ static void ilk_compute_wm_results(struct drm_device *dev,
 		level = ilk_wm_lp_to_level(wm_lp, merged);
 
 		r = &merged->wm[level];
-		if (!r->enable)
-			break;
 
-		results->wm_lp[wm_lp - 1] = WM3_LP_EN |
+		/*
+		 * Maintain the watermark values even if the level is
+		 * disabled. Doing otherwise could cause underruns.
+		 */
+		results->wm_lp[wm_lp - 1] =
 			(ilk_wm_lp_latency(dev, level) << WM1_LP_LATENCY_SHIFT) |
 			(r->pri_val << WM1_LP_SR_SHIFT) |
 			r->cur_val;
 
+		if (r->enable)
+			results->wm_lp[wm_lp - 1] |= WM1_LP_SR_EN;
+
 		if (INTEL_INFO(dev)->gen >= 8)
 			results->wm_lp[wm_lp - 1] |=
 				r->fbc_val << WM1_LP_FBC_SHIFT_BDW;
@@ -2373,6 +2388,10 @@ static void ilk_compute_wm_results(struct drm_device *dev,
 			results->wm_lp[wm_lp - 1] |=
 				r->fbc_val << WM1_LP_FBC_SHIFT;
 
+		/*
+		 * Always set WM1S_LP_EN when spr_val != 0, even if the
+		 * level is disabled. Doing otherwise could cause underruns.
+		 */
 		if (INTEL_INFO(dev)->gen <= 6 && r->spr_val) {
 			WARN_ON(wm_lp != 1);
 			results->wm_lp_spr[wm_lp - 1] = WM1S_LP_EN | r->spr_val;