必读：
Android 12(S) 图像显示系统 - 开篇
合成方式
合成类型的定义：/hardware/interfaces/graphics/composer/2.1/IComposerClient.hal
/** Possible composition types for a given layer. */
/** 建议去看源码中的注释，可以理解每一个type的含义 */
enum Composition : int32_t {
INVALID = 0,
CLIENT = 1,
DEVICE = 2,
SOLID_COLOR = 3,
CURSOR = 4,
SIDEBAND = 5,
};后端的设计逻辑
有三个类定义
1. Backend == 一个后端的实现，注册为"generic"，主要是定义了ValidateDisplay方法，这个方法用来设置可见的HwcLayer应该采用什么合成方式
2. BackendClient == 一个后端的实现，注册为"client"，主要是定义了ValidateDisplay方法，它把所有HwcLayer都设置成立Client合成方式
3. BackendManager == 后端的管理器，用来根据Device name从已注册的backend列表中选择一个，设置给HwcDisplay；GetBackendByName就是通过Device name来从available_backends_中选择一个匹配的Backend构造函数来构建后端对象。
HWC 中如何为每一个Layer选择合成方式
[drm-hwcomposer/hwc2_device/HwcDisplay.cpp]
HWC2::Error HwcDisplay::ValidateDisplay(uint32_t *num_types,
uint32_t *num_requests) {
if (IsInHeadlessMode()) {
*num_types = *num_requests = 0;
return HWC2::Error::None;
}
return backend_->ValidateDisplay(this, num_types, num_requests); //调用backend的方法
}去调用到后端的具体validate方法，我的平台就是走到Backend::ValidateDisplay
[drm-hwcomposer/backend/Backend.cpp] HWC2::Error Backend::ValidateDisplay(HwcDisplay *display, uint32_t *num_types, uint32_t *num_requests) { *num_types = 0; *num_requests = 0; auto layers = display->GetOrderLayersByZPos(); // 按Z-order顺序排列的HwcLayer的集合 int client_start = -1; // layers中，需要Client合成的layer的起始位置 size_t client_size = 0; // layers中，需要Client合成的layer的个数 if (display->ProcessClientFlatteningState(layers.size() <= 1)) { display->total_stats().frames_flattened_++; client_start = 0; client_size = layers.size(); //设置合成类型，client_start到client_start+client_size之间的设置为Client,其它的设置为Device MarkValidated(layers, client_start, client_size); } else { std::tie(client_start, client_size) = GetClientLayers(display, layers);// 刷选哪些layer需要Client合成 //设置合成类型，client_start到client_start+client_size之间的设置为Client,其它的设置为Device 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(); //设置合成类型，client_start到client_start+client_size之间的设置为Client,其它的设置为Device 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; }折叠
Backend中还有几个辅助方法，简单介绍下
GetClientLayers：刷选出哪些layer需要Client合成，筛选是会经过两层考核 IsClientLayer & GetExtraClientRange
IsClientLayer：判断指定的Layer是否要Client合成，有几个条件：1. HardwareSupportsLayerType硬件不支持的合成方式
2. IsHandleUsable buffer handle无法转为DRM要求的buffer object
3. color_transform_hint !=HAL_COLOR_TRANSFORM_IDENTITY
4. 需要scale or phase,但hwc强制GPU来处理
GetExtraClientRange: 进一步筛选client layer, 当layer的数量多于hwc支持的planes时，需要留出一个给 client target
合成显示
PresentDisplay方法的作用就是把内容呈现到屏幕上去
[drm-hwcomposer/hwc2_device/HwcDisplay.cpp]
HWC2::Error HwcDisplay::PresentDisplay(int32_t *present_fence) {
...
AtomicCommitArgs a_args{};
ret = CreateComposition(a_args);// 调用
...
}主要是去调用了CreateComposition这个方法
[drm-hwcomposer/hwc2_device/HwcDisplay.cpp] HWC2::Error HwcDisplay::CreateComposition(AtomicCommitArgs &a_args) { if (IsInHeadlessMode()) { // 无头模式，不做处理，返回 ALOGE("%s: Display is in headless mode, should never reach here", __func__); return HWC2::Error::None; } int PrevModeVsyncPeriodNs = static_cast<int>( 1E9 / GetPipe().connector->Get()->GetActiveMode().v_refresh()); auto mode_update_commited_ = false; // 是否需要更新/提交 if (staged_mode_ && // staged_mode_ 当前所处的显示模式 staged_mode_change_time_ <= ResourceManager::GetTimeMonotonicNs()) { client_layer_.SetLayerDisplayFrame( // 设置显示的位置大小 (hwc_rect_t){.left = 0, .top = 0, .right = static_cast<int>(staged_mode_->h_display()), .bottom = static_cast<int>(staged_mode_->v_display())}); configs_.active_config_id = staged_mode_config_id_; a_args.display_mode = *staged_mode_; if (!a_args.test_only) { mode_update_commited_ = true; } } // order the layers by z-order bool use_client_layer = false; // 是否有GPU合成的图层 uint32_t client_z_order = UINT32_MAX; std::map<uint32_t, HwcLayer *> z_map; for (std::pair<const hwc2_layer_t, HwcLayer> &l : layers_) { switch (l.second.GetValidatedType()) { case HWC2::Composition::Device: z_map.emplace(std::make_pair(l.second.GetZOrder(), &l.second)); // z_map中是按照z-order排序的，Device合成的图层 break; case HWC2::Composition::Client: // Place it at the z_order of the lowest client layer use_client_layer = true; client_z_order = std::min(client_z_order, l.second.GetZOrder()); // 找到GPU合成图层中最小的z-order break; default: continue; } } if (use_client_layer) z_map.emplace(std::make_pair(client_z_order, &client_layer_)); // GPU合成的Client图层加入z_map集合 if (z_map.empty()) // 空集合，没有要合成的图层 return HWC2::Error::BadLayer; std::vector<DrmHwcLayer> composition_layers; // now that they're ordered by z, add them to the composition for (std::pair<const uint32_t, HwcLayer *> &l : z_map) { DrmHwcLayer layer; l.second->PopulateDrmLayer(&layer); // 把HwcLayer转为DrmHwcLayer，主要是一些信息 int ret = layer.ImportBuffer(GetPipe().device); // 1. 把buffer_handle_t转为drm buffer object // 2. 做drmPrimeFDToHandle处理 if (ret) { ALOGE("Failed to import layer, ret=%d", ret); return HWC2::Error::NoResources; } composition_layers.emplace_back(std::move(layer)); } /* Store plan to ensure shared planes won't be stolen by other display * in between of ValidateDisplay() and PresentDisplay() calls */ current_plan_ = DrmKmsPlan::CreateDrmKmsPlan(GetPipe(), // 创建一个计划：合成显示 std::move(composition_layers)); if (!current_plan_) { if (!a_args.test_only) { ALOGE("Failed to create DrmKmsPlan"); } return HWC2::Error::BadConfig; } a_args.composition = current_plan_; // 提交/合成/显示到屏幕 == >DrmAtomicStateManager::ExecuteAtomicCommit int ret = GetPipe().atomic_state_manager->ExecuteAtomicCommit(a_args); if (ret) { if (!a_args.test_only) ALOGE("Failed to apply the frame composition ret=%d", ret); return HWC2::Error::BadParameter; } if (mode_update_commited_) { staged_mode_.reset(); vsync_tracking_en_ = false; if (last_vsync_ts_ != 0) { hwc2_->SendVsyncPeriodTimingChangedEventToClient( handle_, last_vsync_ts_ + PrevModeVsyncPeriodNs); } } return HWC2::Error::None; }折叠
上面出现了一个新的类型
struct DrmHwcLayer {
buffer_handle_t sf_handle = nullptr;
hwc_drm_bo_t buffer_info{};
std::shared_ptr<DrmFbIdHandle> fb_id_handle;
int gralloc_buffer_usage = 0;
DrmHwcTransform transform{};
DrmHwcBlending blending = DrmHwcBlending::kNone;
uint16_t alpha = UINT16_MAX;
hwc_frect_t source_crop;
hwc_rect_t display_frame;
DrmHwcColorSpace color_space;
DrmHwcSampleRange sample_range;
UniqueFd acquire_fence;
int ImportBuffer(DrmDevice *drm_device);
bool IsProtected() const {
return (gralloc_buffer_usage & GRALLOC_USAGE_PROTECTED) ==
GRALLOC_USAGE_PROTECTED;
}
};ImportBuffer调用的流程：
int DrmHwcLayer::ImportBuffer(DrmDevice *drm_device) {
buffer_info = hwc_drm_bo_t{};
int ret = BufferInfoGetter::GetInstance()->ConvertBoInfo(sf_handle,
&buffer_info);
if (ret != 0) {
ALOGE("Failed to convert buffer info %d", ret);
return ret;
}
fb_id_handle = drm_device->GetDrmFbImporter().GetOrCreateFbId(&buffer_info);
if (!fb_id_handle) {
ALOGE("Failed to import buffer");
return -EINVAL;
}
return 0;
}进而调用到相关方法
BufferInfoMapperMetadata::ConvertBoInfo
DrmFbImporter::GetOrCreateFbId
DrmAtomicStateManager::CommitFrame方法中应该是最终去显示内容的逻辑。
看一下CommitFrame调用栈信息：
PresentDisplay调用栈信息
ValidateDisplay调用栈信息
