src/ui/lib/escher/impl/uniform_buffer_pool.h

// Copyright 2016 The Fuchsia 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 SRC_UI_LIB_ESCHER_IMPL_UNIFORM_BUFFER_POOL_H_
#define SRC_UI_LIB_ESCHER_IMPL_UNIFORM_BUFFER_POOL_H_

#include <vector>
#include <vulkan/vulkan.hpp>

#include "src/lib/fxl/memory/weak_ptr.h"
#include "src/ui/lib/escher/forward_declarations.h"
#include "src/ui/lib/escher/resources/resource_manager.h"
#include "src/ui/lib/escher/vk/buffer.h"

namespace escher {
namespace impl {

// Vends host-accessible Buffers whose resources are automatically returned
// to the pool upon destruction.  If necessary, it will grow by creating new
// buffers (and allocating backing memory for them).  |additional_flags| allows
// the user to customize the memory that is allocated by the pool; by default,
// only eHostVisible is used.  Not thread-safe.
class UniformBufferPool : public ResourceManager {
 public:
  UniformBufferPool(EscherWeakPtr escher, size_t ring_size,
                    // If no allocator is provided, Escher's default allocator will be used.
                    GpuAllocator* allocator = nullptr,
                    vk::MemoryPropertyFlags additional_flags = vk::MemoryPropertyFlags());
  ~UniformBufferPool();

  UniformBufferPoolWeakPtr GetWeakPtr() { return weak_factory_.GetWeakPtr(); }

  // Allocate a buffer that will be returned to this pool when the last
  // reference to it is dropped.
  BufferPtr Allocate();

  // Rotate the ring buffer so that buffers freed in previous frames are moved
  // toward the front.
  void BeginFrame();

  // Return the size of buffers allocated by this allocator; constant over the
  // allocator's lifetime.
  //
  // NOTE: this value is currently always 64kB, which is a typical max-size for
  // Vulkan uniform buffers.
  vk::DeviceSize buffer_size() const { return buffer_size_; }

 private:
  // Implement ResourceManager::OnReceiveOwnable().
  void OnReceiveOwnable(std::unique_ptr<Resource> resource) override;

  // Create a batch of new buffers, which are added to free_buffers_.
  void InternalAllocate();

  // Used to allocate backing memory for the pool's buffers.
  GpuAllocator* const allocator_;

  // Specify the properties of the memory used to back the pool's buffers (e.g.
  // host-visible and coherent).
  const vk::MemoryPropertyFlags flags_;

  // The size of each allocated buffer.
  const vk::DeviceSize buffer_size_;

  using FreeBuffers = std::vector<std::unique_ptr<Buffer>>;

  // Queue of FreeBuffers that will become available for allocation once they
  // reach the front.  This allows reuse to be deferred for a number of frames,
  // so that the memory isn't stomped while it is still in use.
  static constexpr size_t kMaxRingSize = 25;
  FreeBuffers ring_[kMaxRingSize];
  size_t ring_size_;

  // See comment in InternalAllocate().
  bool is_allocating_ = false;

  fxl::WeakPtrFactory<UniformBufferPool> weak_factory_;  // must be last

  FXL_DISALLOW_COPY_AND_ASSIGN(UniformBufferPool);
};

}  // namespace impl
}  // namespace escher

#endif  // SRC_UI_LIB_ESCHER_IMPL_UNIFORM_BUFFER_POOL_H_