drm/i915/gt: Onion unwind for scratch page allocation failure

In switching to using objects for our ppGTT scratch pages, care was not
taken to avoid trying to unref NULL objects on failure. And for gen6
ppGTT, it appears we forgot entirely to unwind after a partial allocation
failure.

Fixes: 8935192 ("drm/i915/gt: Switch to object allocations for page directories")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201019083444.1286-1-chris@chris-wilson.co.uk

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

@@ -239,18 +239,24 @@ static int gen6_ppgtt_init_scratch(struct gen6_ppgtt *ppgtt)
 			       I915_CACHE_NONE, PTE_READ_ONLY);
 
 	vm->scratch[1] = vm->alloc_pt_dma(vm, I915_GTT_PAGE_SIZE_4K);
-	if (IS_ERR(vm->scratch[1]))
-		return PTR_ERR(vm->scratch[1]);
+	if (IS_ERR(vm->scratch[1])) {
+		ret = PTR_ERR(vm->scratch[1]);
+		goto err_scratch0;
+	}
 
 	ret = pin_pt_dma(vm, vm->scratch[1]);
-	if (ret) {
-		i915_gem_object_put(vm->scratch[1]);
-		return ret;
-	}
+	if (ret)
+		goto err_scratch1;
 
 	fill32_px(vm->scratch[1], vm->scratch[0]->encode);
 
 	return 0;
+
+err_scratch1:
+	i915_gem_object_put(vm->scratch[1]);
+err_scratch0:
+	i915_gem_object_put(vm->scratch[0]);
+	return ret;
 }
 
 static void gen6_ppgtt_free_pd(struct gen6_ppgtt *ppgtt)

drivers/gpu/drm/i915/gt/gen8_ppgtt.c

@@ -607,7 +607,8 @@ static int gen8_init_scratch(struct i915_address_space *vm)
 	return 0;
 
 free_scratch:
-	free_scratch(vm);
+	while (i--)
+		i915_gem_object_put(vm->scratch[i]);
 	return -ENOMEM;
 }