drm/i915/gen9: Add 2us read latency to WM level

According to the updated Bspec, The mailbox response data is not currently
accounting for memory read latency. Add 2 microseconds to the result for
each level.
This patch adds 2us to latency of level 0 for all cases and
for all other levels (1-7) only if latency[level] > 0.

v2: Slightly rework the patch and add a big comment (Damien)
v3: Rebase on top of the renames of the memory latency defines

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Reviewed-by: M, Satheeshakrishna <satheeshakrishna.m@intel.com> (v1)
Cc: Lespiau, Damien <damien.lespiau@intel.com>
Cc: M, Satheeshakrishna <satheeshakrishna.m@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -2286,6 +2286,7 @@ static void intel_read_wm_latency(struct drm_device *dev, uint16_t wm[8])
 	if (IS_GEN9(dev)) {
 		uint32_t val;
 		int ret;
+		int level, max_level = ilk_wm_max_level(dev);
 
 		/* read the first set of memory latencies[0:3] */
 		val = 0; /* data0 to be programmed to 0 for first set */
@@ -2328,6 +2329,21 @@ static void intel_read_wm_latency(struct drm_device *dev, uint16_t wm[8])
 		wm[7] = (val >> GEN9_MEM_LATENCY_LEVEL_3_7_SHIFT) &
 				GEN9_MEM_LATENCY_LEVEL_MASK;
 
+		/*
+		 * 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.
+		 *   - W0 is a bit special in that it's the only level that
+		 *   can't be disabled if we want to have display working, so
+		 *   we always add 2us there.
+		 *   - For levels >=1, punit returns 0us latency when they are
+		 *   disabled, so we respect that and don't add 2us then
+		 */
+		wm[0] += 2;
+		for (level = 1; level <= max_level; level++)
+			if (wm[level] != 0)
+				wm[level] += 2;
+
 	} else if (IS_HASWELL(dev) || IS_BROADWELL(dev)) {
 		uint64_t sskpd = I915_READ64(MCH_SSKPD);