drm/i915/display: fix level 0 adjustement on display ver >= 12

We should no longer increment level 0 by 1usec when we have 16Gb DIMMs.
Instead spec says to add 3usec (as opposed to 2) to each valid level
when punit replies 0 to level 0.

So set wm_lv_0_adjust_needed to false for DISPLAY_VER() >= 12 and set
the proper adjustment value when handling WaWmMemoryReadLatency.

Bspec: 49326, 4381
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622212210.3746133-1-lucas.demarchi@intel.com

drivers/gpu/drm/i915/intel_dram.c

@@ -484,8 +484,7 @@ static int gen11_get_dram_info(struct drm_i915_private *i915)
 
 static int gen12_get_dram_info(struct drm_i915_private *i915)
 {
-	/* Always needed for GEN12+ */
-	i915->dram_info.wm_lv_0_adjust_needed = true;
+	i915->dram_info.wm_lv_0_adjust_needed = false;
 
 	return icl_pcode_read_mem_global_info(i915);
 }

drivers/gpu/drm/i915/intel_pm.c

@@ -2913,18 +2913,20 @@ static void intel_read_wm_latency(struct drm_i915_private *dev_priv,
 		}
 
 		/*
-		 * WaWmMemoryReadLatency:skl+,glk
+		 * WaWmMemoryReadLatency
 		 *
 		 * punit doesn't take into account the read latency so we need
-		 * to add 2us to the various latency levels we retrieve from the
-		 * punit when level 0 response data us 0us.
+		 * to add proper adjustement to each valid level we retrieve
+		 * from the punit when level 0 response data is 0us.
 		 */
 		if (wm[0] == 0) {
-			wm[0] += 2;
+			u8 adjust = DISPLAY_VER(dev_priv) >= 12 ? 3 : 2;
+
+			wm[0] += adjust;
 			for (level = 1; level <= max_level; level++) {
 				if (wm[level] == 0)
 					break;
-				wm[level] += 2;
+				wm[level] += adjust;
 			}
 		}
 
@@ -2936,7 +2938,6 @@ static void intel_read_wm_latency(struct drm_i915_private *dev_priv,
 		 */
 		if (dev_priv->dram_info.wm_lv_0_adjust_needed)
 			wm[0] += 1;
-
 	} else if (IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv)) {
 		u64 sskpd = intel_uncore_read64(uncore, MCH_SSKPD);