[096]图解HWC的合成策略

//关键代码，返回client layer的start index和size。
std::tie(client_start, client_size) = GetClientLayers(display, layers);

/*
 * Copyright (C) 2020 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "Backend.h"

#include <climits>

#include "BackendManager.h"
#include "bufferinfo/BufferInfoGetter.h"

namespace android {

HWC2::Error Backend::ValidateDisplay(HwcDisplay *display, uint32_t *num_types,
                                     uint32_t *num_requests) {
  *num_types = 0;
  *num_requests = 0;

  auto layers = display->GetOrderLayersByZPos();

  int client_start = -1;
  size_t client_size = 0;

  if (display->ProcessClientFlatteningState(layers.size() <= 1)) {
    display->total_stats().frames_flattened_++;
    client_start = 0;
    client_size = layers.size();
    MarkValidated(layers, client_start, client_size);
  } else {
    std::tie(client_start, client_size) = GetClientLayers(display, layers);//关键代码，返回client layer的start id和size。

    MarkValidated(layers, client_start, client_size);

    bool testing_needed = !(client_start == 0 && client_size == layers.size());

    AtomicCommitArgs a_args = {.test_only = true};

    if (testing_needed &&
        display->CreateComposition(a_args) != HWC2::Error::None) {
      ++display->total_stats().failed_kms_validate_;
      client_start = 0;
      client_size = layers.size();
      MarkValidated(layers, 0, client_size);
    }
  }

  *num_types = client_size;

  display->total_stats().gpu_pixops_ += CalcPixOps(layers, client_start,
                                                   client_size);
  display->total_stats().total_pixops_ += CalcPixOps(layers, 0, layers.size());

  return *num_types != 0 ? HWC2::Error::HasChanges : HWC2::Error::None;
}

std::tuple<int, size_t> Backend::GetClientLayers(
    HwcDisplay *display, const std::vector<HwcLayer *> &layers) {
  int client_start = -1;
  size_t client_size = 0;

  for (size_t z_order = 0; z_order < layers.size(); ++z_order) {
    if (IsClientLayer(display, layers[z_order])) {
      if (client_start < 0)
        client_start = (int)z_order;
      client_size = (z_order - client_start) + 1;
    }
  }//代码分析3_1

  return GetExtraClientRange(display, layers, client_start, client_size);//扩大额外的需要为client的layers
}

//代码分析3_0
//判断layer是不是必须为client合成或者已经指定了client合成
bool Backend::IsClientLayer(HwcDisplay *display, HwcLayer *layer) {
  return !HardwareSupportsLayerType(layer->GetSfType()) ||
         !BufferInfoGetter::GetInstance()->IsHandleUsable(layer->GetBuffer()) ||
         display->color_transform_hint() != HAL_COLOR_TRANSFORM_IDENTITY ||
         (layer->RequireScalingOrPhasing() &&
          display->GetHwc2()->GetResMan().ForcedScalingWithGpu());
}

bool Backend::HardwareSupportsLayerType(HWC2::Composition comp_type) {
  return comp_type == HWC2::Composition::Device ||
         comp_type == HWC2::Composition::Cursor;
}

//计算连续几个layer的像素点之和
uint32_t Backend::CalcPixOps(const std::vector<HwcLayer *> &layers,
                             size_t first_z, size_t size) {
  uint32_t pixops = 0;
  for (size_t z_order = 0; z_order < layers.size(); ++z_order) {
    if (z_order >= first_z && z_order < first_z + size) {
      hwc_rect_t df = layers[z_order]->GetDisplayFrame();
      pixops += (df.right - df.left) * (df.bottom - df.top);
    }
  }
  return pixops;
}

//标记每个layer的合成策略，最后sf拿到的就是这个标记
void Backend::MarkValidated(std::vector<HwcLayer *> &layers,
                            size_t client_first_z, size_t client_size) {
  for (size_t z_order = 0; z_order < layers.size(); ++z_order) {
    if (z_order >= client_first_z && z_order < client_first_z + client_size)
      layers[z_order]->SetValidatedType(HWC2::Composition::Client);
    else
      layers[z_order]->SetValidatedType(HWC2::Composition::Device);
  }
}

std::tuple<int, int> Backend::GetExtraClientRange(
    HwcDisplay *display, const std::vector<HwcLayer *> &layers,
    int client_start, size_t client_size) {
  auto planes = display->GetPipe().GetUsablePlanes();
  size_t avail_planes = planes.size();//获取整个drm支持的planes的数量

  /*
   * If more layers then planes, save one plane
   * for client composited layers
   */
  if (avail_planes < display->layers().size())
    avail_planes--;//保留一个用于存放所有client合成的layer

  int extra_client = int(layers.size() - client_size) - int(avail_planes);//计算剩余需要成为client的数量
  //上面是代码分析2_1

  if (extra_client > 0) {
    int start = 0;
    size_t steps = 0;
    if (client_size != 0) {//有layer指定了client合成
      int prepend = std::min(client_start, extra_client);
      int append = std::min(int(layers.size()) -
                                int(client_start + client_size),
                            extra_client);
      start = client_start - (int)prepend;
      client_size += extra_client;
      steps = 1 + std::min(std::min(append, prepend),
                           int(layers.size()) - int(start + client_size));
    } else {//没有layer指定了client合成。代码分析
      client_size = extra_client;//剩余的都是client
      steps = 1 + layers.size() - extra_client;//计算硬件合成步伐
    }

    //选择像素之和最少的连续layers
    uint32_t gpu_pixops = UINT32_MAX;
    for (size_t i = 0; i < steps; i++) {
      uint32_t po = CalcPixOps(layers, start + i, client_size);
      if (po < gpu_pixops) {
        gpu_pixops = po;
        client_start = start + int(i);
      }
    } //代码分析2_2
  }

  return std::make_tuple(client_start, client_size);
}

// clang-format off
// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables, cert-err58-cpp)
REGISTER_BACKEND("generic", Backend);
// clang-format on

}  // namespace android