Add offset support to BitstreamBuffer.

content/common/gpu/media/shared_memory_region.cc

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.

[kcwu, 2016/03/16 06:20:53]

s/(c) 2015/2016/

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/sys_info.h"
+#include "content/common/gpu/media/shared_memory_region.h"
+
+namespace content {
+
+SharedMemoryRegion::SharedMemoryRegion(const base::SharedMemoryHandle& handle,
+                                       off_t offset,
+                                       size_t size,
+                                       bool read_only)
+    : shm_(handle, read_only),
+      offset_(offset),
+      size_(size),
+      alignment_size_(offset % base::SysInfo::VMAllocationGranularity()) {

[kcwu, 2016/03/16 06:20:53]

How about name it |offset_to_alignment| or |padding_size|? My first impression
for "size" of |alignment_size_| is something like 4096, so I feel it a little
confusing.

+  DCHECK_GE(offset_, 0) << "Invalid offset: " << offset_;
+}
+
+SharedMemoryRegion::SharedMemoryRegion(
+    const media::BitstreamBuffer& bitstream_buffer,
+    bool read_only)
+    : SharedMemoryRegion(bitstream_buffer.handle(),
+                         bitstream_buffer.offset(),
+                         bitstream_buffer.size(),
+                         read_only) {}
+
+bool SharedMemoryRegion::Map() {
+  if (offset_ < 0) {
+    DVLOG(1) << "Invalid offset: " << offset_;
+    return false;
+  }
+  return shm_.MapAt(offset_ - alignment_size_, size_ + alignment_size_);
+}
+
+void* SharedMemoryRegion::memory() {
+  int8_t* addr = reinterpret_cast<int8_t*>(shm_.memory());
+  return addr ? addr + alignment_size_ : nullptr;
+}
+
+}  // namespace content