drm/i915/gt: Optimise aliasing-ppgtt allocations

Since the aliasing-ppgtt remains the default for gen6/gen7, it is worth
optimising the ppgtt allocation for it. In this case, we do not need to
flush the GGTT page directories entries as they are fixed during setup.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200709190111.5492-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/gen6_ppgtt.c

@@ -183,13 +183,11 @@ static int gen6_alloc_va_range(struct i915_address_space *vm,
 	struct gen6_ppgtt *ppgtt = to_gen6_ppgtt(i915_vm_to_ppgtt(vm));
 	struct i915_page_directory * const pd = ppgtt->base.pd;
 	struct i915_page_table *pt, *alloc = NULL;
-	intel_wakeref_t wakeref;
+	bool flush = false;
 	u64 from = start;
 	unsigned int pde;
 	int ret = 0;
 
-	wakeref = intel_runtime_pm_get(&vm->i915->runtime_pm);
-
 	spin_lock(&pd->lock);
 	gen6_for_each_pde(pt, pd, start, length, pde) {
 		const unsigned int count = gen6_pte_count(start, length);
@@ -214,14 +212,20 @@ static int gen6_alloc_va_range(struct i915_address_space *vm,
 				alloc = pt;
 				pt = pd->entry[pde];
 			}
+
+			flush = true;
 		}
 
 		atomic_add(count, &pt->used);
 	}
 	spin_unlock(&pd->lock);
 
-	if (i915_vma_is_bound(ppgtt->vma, I915_VMA_GLOBAL_BIND))
+	if (flush && i915_vma_is_bound(ppgtt->vma, I915_VMA_GLOBAL_BIND)) {
+		intel_wakeref_t wakeref;
+
+		with_intel_runtime_pm(&vm->i915->runtime_pm, wakeref)
 			gen6_flush_pd(ppgtt, from, start);
+	}
 
 	goto out;
 
@@ -230,7 +234,6 @@ static int gen6_alloc_va_range(struct i915_address_space *vm,
 out:
 	if (alloc)
 		free_px(vm, alloc);
-	intel_runtime_pm_put(&vm->i915->runtime_pm, wakeref);
 	return ret;
 }