Client side display item cache flag

third_party/WebKit/Source/platform/graphics/paint/README.md

Index: third_party/WebKit/Source/platform/graphics/paint/README.md
diff --git a/third_party/WebKit/Source/platform/graphics/paint/README.md b/third_party/WebKit/Source/platform/graphics/paint/README.md
index 5e5fb62e611d3da49e2fc75fabbf30f04aef0a56..89b97695cd8cf3095032d155a2e654def63cd6aa 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/README.md
+++ b/third_party/WebKit/Source/platform/graphics/paint/README.md
@@ -179,9 +179,48 @@ replaced with the cached content from the previous artifact.
 
 At this point, the paint artifact is ready to be drawn or composited.
 
-*** aside
-TODO(jbroman): Explain invalidation.
-***
+### Paint result caching and invalidation
+
+See [Display item caching](../../../core/paint/README.md#paint-result-caching)
+and [Paint invalidation](../../../core/paint/README.md#paint-invalidation) for
+more details about how caching and invalidation are handled in blink core
+module using `PaintController` API.
+
+We use 'cache generation' which is a unique id of cache status in each
+`DisplayItemClient` and `PaintController` to determine if the client is validly
+cached by a `PaintController`.
+
+A paint controller sets its cache generation to
+`DisplayItemCacheGeneration::next()` at the end of each
+`commitNewDisplayItems()`, and updates the cache generation of each client with
+cached drawings by calling `DisplayItemClient::setDisplayItemsCached()`.
+A display item is treated as validly cached in a paint controller if its cache
+generation matches the paint controller's cache generation.
+
+`kInvalidCacheGeneration` is a special cache generation value which matches no
+other cache generations. When a `DisplayItemClient` is invalidated, we set its
+cache generation to `kInvalidCacheGeneration`. When a `PaintController` is
+cleared (e.g. when the corresponding `GraphicsLayer` is fully invalidated), we
+also set its cache generation to `kInvalidCacheGeneration`.
+
+For now we use a uint32_t variable to store cache generation. Assuming there is
+an animation in 60fps needing main-thread repaint, the cache generation will
+overflow after 2^32/86400/60 = 828 days. When it overflows, we may treat some

[chrishtr, 2016/04/29 15:55:17]

This is only with one PaintController. Could be less with more.

[Xianzhu, 2016/04/29 16:37:11]

Yes. Added a sentence for this fact.

+object that is not cached as cached if the following conditions are all met:
+*   the object was painted when the cache generation was n;
+*   the object has not been painted since cache generation n;
+*   when the cache generation wraps back to exact n (after several years), the
+    object happens to be painted again and treated as cached.
+As the paint controller doesn't have cached display items for the object, there
+will be corrupted painting or assertion failure. The chance is too low to be
+concerned.
+
+SPv1 only: If a display item is painted on multiple paint controllers, because
+cache generations are unique, the client's cache generation matches the last
+paint controller only. The client will be treated as invalid on other paint
+controllers regardless if it's validly cached by these paint controllers.
+The situation is very rare (about 0.07% clients were painted on multiple paint
+controllers) so the performance penalty is trivial.
 
 ## Paint artifact compositor