drm/i915: Combine multiple internal plists into the same i915_priolist bucket

As we are about to allow ourselves to slightly bump the user priority
into a few different sublevels, packthose internal priority lists
into the same i915_priolist to keep the rbtree compact and avoid having
to allocate the default user priority even after the internal bumping.
The downside to having an requests[] rather than a node per active list,
is that we then have to walk over the empty higher priority lists. To
compensate, we track the active buckets and use a small bitmap to skip
over any inactive ones.

v2: Use MASK of internal levels to simplify our usage.
v3: Prevent overflow when SHIFT is zero.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001123204.23982-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -190,11 +190,22 @@ enum intel_engine_id {
 };
 
 struct i915_priolist {
+	struct list_head requests[I915_PRIORITY_COUNT];
 	struct rb_node node;
-	struct list_head requests;
+	unsigned long used;
 	int priority;
 };
 
+#define priolist_for_each_request(it, plist, idx) \
+	for (idx = 0; idx < ARRAY_SIZE((plist)->requests); idx++) \
+		list_for_each_entry(it, &(plist)->requests[idx], sched.link)
+
+#define priolist_for_each_request_consume(it, n, plist, idx) \
+	for (; (idx = ffs((plist)->used)); (plist)->used &= ~BIT(idx - 1)) \
+		list_for_each_entry_safe(it, n, \
+					 &(plist)->requests[idx - 1], \
+					 sched.link)
+
 struct st_preempt_hang {
 	struct completion completion;
 	bool inject_hang;