drm/i915/skl: Tune IZ hashing when subslices are unbalanced

When one EU is disabled in a particular subslice, we can tune how the
work is spread between subslices to improve EU utilization.

v2: - Use a bitfield to record which subslice(s) has(have) 7 EUs. That
      will also make the machinery work if several sublices have 7 EUs.
      (Jeff Mcgee)
    - Only apply the different hashing algorithm if the slice is
      effectively unbalanced by checking there's a single subslice with
      7 EUs. (Jeff Mcgee)

v3: Fix typo in comment (Jeff Mcgee)

Issue: VIZ-3845
Cc: Jeff Mcgee <jeff.mcgee@intel.com>
Reviewed-by: Jeff Mcgee <jeff.mcgee@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 14 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 9a365b40b50e..f9992ca11d10 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -650,13 +650,24 @@ static void intel_device_info_runtime_init(struct drm_device *dev)
                                 continue;
 
                         for (ss = 0; ss < ss_max; ss++) {
+                                u32 n_disabled;
+
                                 if (ss_disable & (0x1 << ss))
                                         /* skip disabled subslice */
                                         continue;
 
-                                info->eu_total += eu_max -
-                                                  hweight8(eu_disable[s] >>
-                                                           (ss * eu_max));
+                                n_disabled = hweight8(eu_disable[s] >>
+                                                      (ss * eu_max));
+
+                                /*
+                                 * Record which subslice(s) has(have) 7 EUs. we
+                                 * can tune the hash used to spread work among
+                                 * subslices if they are unbalanced.
+                                 */
+                                if (eu_max - n_disabled == 7)
+                                        info->subslice_7eu[s] |= 1 << ss;
+
+                                info->eu_total += eu_max - n_disabled;
                         }
                 }