From 8ddd2c72ee7dd6f45e5375a197774b8671b982b7 Mon Sep 17 00:00:00 2001
From: Mathias Agopian <mathias@google.com>
Date: Fri, 1 Oct 2010 16:22:41 -0700
Subject: fix [2835280] Add support for cancelling buffers to ANativeWindow

There is a new ANativeWindow::cancelBuffer() API that can be used to
cancel any dequeued buffer, BEFORE it's been enqueued. The buffer is
returned to the list of availlable buffers. dequeue and cancel are not
mutually thread safe, they must be called from the same thread or
external synchronization must be used.

Change-Id: I86cc7985bace8b6a93ad2c75d2bef5c3c2cb4d61
---
 libs/surfaceflinger_client/SharedBufferStack.cpp | 30 +++++++++++++++---------
 1 file changed, 19 insertions(+), 11 deletions(-)

(limited to 'libs/surfaceflinger_client/SharedBufferStack.cpp')

diff --git a/libs/surfaceflinger_client/SharedBufferStack.cpp b/libs/surfaceflinger_client/SharedBufferStack.cpp
index 38b2fae..4bc5d9e 100644
--- a/libs/surfaceflinger_client/SharedBufferStack.cpp
+++ b/libs/surfaceflinger_client/SharedBufferStack.cpp
@@ -285,10 +285,12 @@ ssize_t SharedBufferClient::DequeueUpdate::operator()() {
     return NO_ERROR;
 }
 
-SharedBufferClient::UndoDequeueUpdate::UndoDequeueUpdate(SharedBufferBase* sbb)
-    : UpdateBase(sbb) {    
+SharedBufferClient::CancelUpdate::CancelUpdate(SharedBufferBase* sbb,
+        int tail, int buf)
+    : UpdateBase(sbb), tail(tail), buf(buf) {
 }
-ssize_t SharedBufferClient::UndoDequeueUpdate::operator()() {
+ssize_t SharedBufferClient::CancelUpdate::operator()() {
+    stack.index[tail] = buf;
     android_atomic_inc(&stack.available);
     return NO_ERROR;
 }
@@ -319,7 +321,7 @@ ssize_t SharedBufferServer::RetireUpdate::operator()() {
         return BAD_VALUE;
 
     // Preventively lock the current buffer before updating queued.
-    android_atomic_write(stack.index[head], &stack.inUse);
+    android_atomic_write(stack.headBuf, &stack.inUse);
 
     // Decrement the number of queued buffers 
     int32_t queued;
@@ -334,7 +336,9 @@ ssize_t SharedBufferServer::RetireUpdate::operator()() {
     // the buffer we preventively locked upon entering this function
 
     head = (head + 1) % numBuffers;
-    android_atomic_write(stack.index[head], &stack.inUse);
+    const int8_t headBuf = stack.index[head];
+    stack.headBuf = headBuf;
+    android_atomic_write(headBuf, &stack.inUse);
 
     // head is only modified here, so we don't need to use cmpxchg
     android_atomic_write(head, &stack.head);
@@ -359,7 +363,7 @@ ssize_t SharedBufferServer::StatusUpdate::operator()() {
 SharedBufferClient::SharedBufferClient(SharedClient* sharedClient,
         int surface, int num, int32_t identity)
     : SharedBufferBase(sharedClient, surface, identity),
-      mNumBuffers(num), tail(0), undoDequeueTail(0)
+      mNumBuffers(num), tail(0)
 {
     SharedBufferStack& stack( *mSharedStack );
     tail = computeTail();
@@ -395,7 +399,6 @@ ssize_t SharedBufferClient::dequeue()
     DequeueUpdate update(this);
     updateCondition( update );
 
-    undoDequeueTail = tail;
     int dequeued = stack.index[tail];
     tail = ((tail+1 >= mNumBuffers) ? 0 : tail+1);
     LOGD_IF(DEBUG_ATOMICS, "dequeued=%d, tail++=%d, %s",
@@ -408,14 +411,19 @@ ssize_t SharedBufferClient::dequeue()
 
 status_t SharedBufferClient::undoDequeue(int buf)
 {
+    return cancel(buf);
+}
+
+status_t SharedBufferClient::cancel(int buf)
+{
     RWLock::AutoRLock _rd(mLock);
 
-    // TODO: we can only undo the previous dequeue, we should
-    // enforce that in the api
-    UndoDequeueUpdate update(this);
+    // calculate the new position of the tail index (essentially tail--)
+    int localTail = (tail + mNumBuffers - 1) % mNumBuffers;
+    CancelUpdate update(this, localTail, buf);
     status_t err = updateCondition( update );
     if (err == NO_ERROR) {
-        tail = undoDequeueTail;
+        tail = localTail;
     }
     return err;
 }
-- 
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