drm/i915/gen9: fix the WaWmMemoryReadLatency implementation

Bspec says:
  "The mailbox response data may not account for memory read latency.
   If the mailbox response data for level 0 is 0us, add 2 microseconds
   to the result for each valid level."

This means we should only do the +2 in case wm[0] == 0, not always.

So split the sanitizing implementation from the WA implementation and
fix the WA implementation.

v2: Add Fixes tag (Maarten).

Fixes: 367294be7c25 ("drm/i915/gen9: Add 2us read latency to WM level")
Cc: stable@vger.kernel.org
Cc: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-5-git-send-email-paulo.r.zanoni@intel.com
(cherry picked from commit 0727e40a48a1d08cf54ce2c01e120864b92e59bf)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 22 insertions, 20 deletions

diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 93ae27408e49..c90be917deae 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -2127,32 +2127,34 @@ static void intel_read_wm_latency(struct drm_device *dev, uint16_t wm[8])
                                 GEN9_MEM_LATENCY_LEVEL_MASK;
 
                 /*
+                 * If a level n (n > 1) has a 0us latency, all levels m (m >= n)
+                 * need to be disabled. We make sure to sanitize the values out
+                 * of the punit to satisfy this requirement.
+                 */
+                for (level = 1; level <= max_level; level++) {
+                        if (wm[level] == 0) {
+                                for (i = level + 1; i <= max_level; i++)
+                                        wm[i] = 0;
+                                break;
+                        }
+                }
+
+                /*
                  * WaWmMemoryReadLatency:skl
                  *
                  * 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
-                 *
-                 * Additionally, if a level n (n > 1) has a 0us latency, all
-                 * levels m (m >= n) need to be disabled. We make sure to
-                 * sanitize the values out of the punit to satisfy this
-                 * requirement.
+                 * to add 2us to the various latency levels we retrieve from the
+                 * punit when level 0 response data us 0us.
                  */
-                wm[0] += 2;
-                for (level = 1; level <= max_level; level++)
-                        if (wm[level] != 0)
+                if (wm[0] == 0) {
+                        wm[0] += 2;
+                        for (level = 1; level <= max_level; level++) {
+                                if (wm[level] == 0)
+                                        break;
                                 wm[level] += 2;
-                        else {
-                                for (i = level + 1; i <= max_level; i++)
-                                        wm[i] = 0;
-
-                                break;
                         }
+                }
+
         } else if (IS_HASWELL(dev) || IS_BROADWELL(dev)) {
                 uint64_t sskpd = I915_READ64(MCH_SSKPD);