必讀：
Android 12(S) 影像顯示系統 - 開篇
合成方式
合成型別的定義：/hardware/interfaces/graphics/composer/2.1/IComposerClient.hal
/** Possible composition types for a given layer. */
/** 建議去看原始碼中的註釋，可以理解每一個type的含義 */
enum Composition : int32_t {
INVALID = 0,
CLIENT = 1,
DEVICE = 2,
SOLID_COLOR = 3,
CURSOR = 4,
SIDEBAND = 5,
};後端的設計邏輯
有三個類定義
1. Backend == 一個後端的實現，註冊為"generic"，主要是定義了ValidateDisplay方法，這個方法用來設定可見的HwcLayer應該採用什麼合成方式
2. BackendClient == 一個後端的實現，註冊為"client"，主要是定義了ValidateDisplay方法，它把所有HwcLayer都設定成立Client合成方式
3. BackendManager == 後端的管理器，用來根據Device name從已註冊的backend列表中選擇一個，設定給HwcDisplay；GetBackendByName就是通過Device name來從available_backends_中選擇一個匹配的Backend建構函式來構建後端物件。
HWC 中如何為每一個Layer選擇合成方式
[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); //呼叫backend的方法
}去呼叫到後端的具體validate方法，我的平臺就是走到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順序排列的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的個數
if (display->ProcessClientFlatteningState(layers.size() <= 1)) {
display->total_stats().frames_flattened_++;
client_start = 0;
client_size = layers.size();
//設定合成型別，client_start到client_start+client_size之間的設定為Client,其它的設定為Device
MarkValidated(layers, client_start, client_size);
} else {
std::tie(client_start, client_size) = GetClientLayers(display, layers);// 刷選哪些layer需要Client合成
//設定合成型別，client_start到client_start+client_size之間的設定為Client,其它的設定為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();
//設定合成型別，client_start到client_start+client_size之間的設定為Client,其它的設定為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;
}Backend中還有幾個輔助方法，簡單介紹下
GetClientLayers：刷選出哪些layer需要Client合成，篩選是會經過兩層考核 IsClientLayer & GetExtraClientRange
IsClientLayer：判斷指定的Layer是否要Client合成，有幾個條件：1. HardwareSupportsLayerType硬體不支援的合成方式
2. IsHandleUsable buffer handle無法轉為DRM要求的buffer object
3. color_transform_hint !=HAL_COLOR_TRANSFORM_IDENTITY
4. 需要scale or phase,但hwc強制GPU來處理
GetExtraClientRange: 進一步篩選client layer, 當layer的數量多於hwc支援的planes時，需要留出一個給 client target
合成顯示
PresentDisplay方法的作用就是把內容呈現到螢幕上去
[drm-hwcomposer/hwc2_device/HwcDisplay.cpp]
HWC2::Error HwcDisplay::PresentDisplay(int32_t *present_fence) {
...
AtomicCommitArgs a_args{};
ret = CreateComposition(a_args);// 呼叫
...
}主要是去呼叫了CreateComposition這個方法
[drm-hwcomposer/hwc2_device/HwcDisplay.cpp]
HWC2::Error HwcDisplay::CreateComposition(AtomicCommitArgs &a_args) {
if (IsInHeadlessMode()) { // 無頭模式，不做處理，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_ 當前所處的顯示模式
staged_mode_change_time_ <= ResourceManager::GetTimeMonotonicNs()) {
client_layer_.SetLayerDisplayFrame( // 設定顯示的位置大小
(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合成的圖層
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合成的圖層
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合成圖層中最小的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圖層加入z_map集合
if (z_map.empty()) // 空集合，沒有要合成的圖層
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轉為DrmHwcLayer，主要是一些資訊
int ret = layer.ImportBuffer(GetPipe().device); // 1. 把buffer_handle_t轉為drm buffer object
// 2. 做drmPrimeFDToHandle處理
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(), // 建立一個計劃：合成顯示
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/顯示到螢幕 == >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;
}上面出現了一個新的型別
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呼叫的流程：
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;
}進而呼叫到相關方法
BufferInfoMapperMetadata::ConvertBoInfo
DrmFbImporter::GetOrCreateFbId
DrmAtomicStateManager::CommitFrame方法中應該是最終去顯示內容的邏輯。
看一下CommitFrame呼叫棧資訊：
PresentDisplay呼叫棧資訊
ValidateDisplay呼叫棧資訊
