Rework FlushSync to return early if commands have been processed since the last update

gpu/command_buffer/common/command_buffer.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_H_
 #define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_H_
 
 #include "../common/buffer.h"
 #include "../common/constants.h"
 
 namespace base {
 class SharedMemory;
 }
 
 namespace gpu {
 
 // Common interface for CommandBuffer implementations.
 class CommandBuffer {
  public:
   enum {
     kMaxCommandBufferSize = 4 * 1024 * 1024
   };
 
   struct State {
     State()
         : num_entries(0),
           get_offset(0),
           put_offset(0),
           token(-1),
-          error(error::kNoError) {
+          error(error::kNoError),
+          generation(0) {
     }
 
     // Size of the command buffer in command buffer entries.
     int32 num_entries;
 
     // The offset (in entries) from which the reader is reading.
     int32 get_offset;
 
     // The offset (in entries) at which the writer is writing.
     int32 put_offset;
 
     // The current token value. This is used by the writer to defer
     // changes to shared memory objects until the reader has reached a certain
     // point in the command buffer. The reader is responsible for updating the
     // token value, for example in response to an asynchronous set token command
     // embedded in the command buffer. The default token value is zero.
     int32 token;
 
     // Error status.
     error::Error error;
+
+    // Generation index of this state. The generation index is incremented every
+    // time a new state is retrieved from the command processor, so that
+    // consistency can be kept even if IPC messages are processed out-of-order.
+    uint32 generation;
   };
 
   CommandBuffer() {
   }
 
   virtual ~CommandBuffer() {
   }
 
   // Initialize the command buffer with the given size.
   virtual bool Initialize(int32 size) = 0;
 
   // Initialize the command buffer using the given preallocated buffer.
   virtual bool Initialize(base::SharedMemory* buffer, int32 size) = 0;
 
   // Gets the ring buffer for the command buffer.
   virtual Buffer GetRingBuffer() = 0;
 
   // Returns the current status.
   virtual State GetState() = 0;
 
   // The writer calls this to update its put offset. This ensures the reader
   // sees the latest added commands, and will eventually process them.
   virtual void Flush(int32 put_offset) = 0;
 
   // The writer calls this to update its put offset. This function returns the
   // reader's most recent get offset. Does not return until after the put offset
   // change callback has been invoked. Returns -1 if the put offset is invalid.
-  // As opposed to Flush(), this function guarantees that the reader has
-  // processed some commands before returning (assuming the command buffer isn't
-  // empty and there is no error).
-  virtual State FlushSync(int32 put_offset) = 0;
+  // If last_known_get is different from the reader's current get pointer, this
+  // function will return immediately, otherwise it guarantees that the reader
+  // has processed some commands before returning (assuming the command buffer
+  // isn't empty and there is no error).
+  virtual State FlushSync(int32 put_offset, int32 last_known_get) = 0;
 
   // Sets the current get offset. This can be called from any thread.
   virtual void SetGetOffset(int32 get_offset) = 0;
 
   // Create a transfer buffer and return a handle that uniquely
   // identifies it or -1 on error. id_request lets the caller request a
   // specific id for the transfer buffer, or -1 if the caller does not care.
   // If the requested id can not be fulfilled, a different id will be returned.
   // id_request must be either -1 or between 0 and 100.
   virtual int32 CreateTransferBuffer(size_t size, int32 id_request) = 0;
@@ skipping 20 matching lines @@
   // Allows the reader to set the current parse error.
   virtual void SetParseError(error::Error) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CommandBuffer);
 };
 
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_H_