必讀：
Android 12(S) 影像顯示系統 - 開篇
一、前言
SurfaceFlinger中的圖層選擇GPU合成(CLIENT合成方式)時，會把待合成的圖層Layers通過renderengine(SkiaGLRenderEngine)繪製到一塊GraphicBuffer中，然後把這塊GraphicBuffer圖形快取通過呼叫setClientTarget傳遞給HWC模組，HWC進一步處理後把這個GraphicBuffer中的影像呈現到螢幕上。
本篇文章，我們先聚焦一點做介紹：用於儲存GPU合成後的圖形資料的GraphicBuffer是從哪裡來的？下面的講解會圍繞這個問題展開。
二、從dumpsys SurfaceFlinger中的資訊談起
如果你檢視過dumpsys SurfaceFlinger的資訊，也許你注意過一些GraphicBufferAllocator/GraphicBufferMapper列印出的一些資訊，這些資訊記錄了所有通過Gralloc模組allocate和import的圖形快取的資訊。
如下是在我的平臺下擷取的dumpsys SurfaceFlinger部分資訊：
GraphicBufferAllocator buffers:
Handle | Size | W (Stride) x H | Layers | Format | Usage | Requestor
0xf3042b90 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf3042f30 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf3046020 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
Total allocated by GraphicBufferAllocator (estimate): 24300.00 KB
Imported gralloc buffers:
+ name:FramebufferSurface, id:e100000000, size:8.3e+03KiB, w/h:780x438, usage: 0x40001b00, req fmt:5, fourcc/mod:875713089/576460752303423505, dataspace: 0x0, compressed: true
planes: B/G/R/A: w/h:780x440, stride:1e00 bytes, size:818000
+ name:FramebufferSurface, id:e100000001, size:8.3e+03KiB, w/h:780x438, usage: 0x40001b00, req fmt:5, fourcc/mod:875713089/576460752303423505, dataspace: 0x0, compressed: true
planes: B/G/R/A: w/h:780x440, stride:1e00 bytes, size:818000
+ name:FramebufferSurface, id:e100000002, size:8.3e+03KiB, w/h:780x438, usage: 0x40001b00, req fmt:5, fourcc/mod:875713089/576460752303423505, dataspace: 0x0, compressed: true
planes: B/G/R/A: w/h:780x440, stride:1e00 bytes, size:818000
Total imported by gralloc: 5e+04KiB上面的資訊中可以看到一些兒冥冥之中貌似、似乎、好像很有意思的字眼：FramebufferSurface。
作為Requestor的FramebufferSurface去請求分配了三塊圖形快取，還規定了width、height、format、usage等資訊。
如上你看到的這3塊GraphicBuffer，就是用來儲存CPU合成後的圖形資料的。
三、建立與初始化FramebufferSurface的流程
FramebufferSurface的初始化邏輯需要從SurfaceFlinger的初始化談起，在文章Android 12(S) 影像顯示系統 - SurfaceFlinger的啟動和訊息佇列處理機制（四）
中，曾分析過，SurfaceFlinger::init()中會去註冊HWC的回撥函式mCompositionEngine->getHwComposer().setCallback(this)，當第一次註冊callback時，onComposerHalHotplug()會立即在呼叫registerCallback()的執行緒中被呼叫，並跨程式回撥到SurfaceFlinger::onComposerHalHotplug。然後一路飛奔：
在SurfaceFlinger::processDisplayAdded這個方法中去建立了BufferQueue和FramebufferSurface，簡單理解為連線上了顯示螢幕(Display)，那就要給準備一個BufferQueue，以便GPU合成UI等圖層時，可以向這個BufferQueue索要GraphicBuffer來儲存合成後的圖形資料，再呈現到螢幕上去（我的傻瓜式理解）
摘取關鍵程式碼如下：
[/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp]
void SurfaceFlinger::processDisplayAdded(const wp<IBinder>& displayToken,
const DisplayDeviceState& state) {
......
sp<compositionengine::DisplaySurface> displaySurface;
sp<IGraphicBufferProducer> producer;
// 建立BufferQueue,獲取到生產者和消費者，而且消費者不是SurfaceFlinger哦
sp<IGraphicBufferProducer> bqProducer;
sp<IGraphicBufferConsumer> bqConsumer;
getFactory().createBufferQueue(&bqProducer, &bqConsumer, /*consumerIsSurfaceFlinger =*/false);
if (state.isVirtual()) { // 虛擬螢幕，不管它
const auto displayId = VirtualDisplayId::tryCast(compositionDisplay->getId());
LOG_FATAL_IF(!displayId);
auto surface = sp<VirtualDisplaySurface>::make(getHwComposer(), *displayId, state.surface,
bqProducer, bqConsumer, state.displayName);
displaySurface = surface;
producer = std::move(surface);
} else { // 看這個case
ALOGE_IF(state.surface != nullptr,
"adding a supported display, but rendering "
"surface is provided (%p), ignoring it",
state.surface.get());
const auto displayId = PhysicalDisplayId::tryCast(compositionDisplay->getId());
LOG_FATAL_IF(!displayId);
// 建立了FramebufferSurface物件，FramebufferSurface繼承自compositionengine::DisplaySurface
// FramebufferSurface是作為消費者的角色工作的，消費SF GPU合成後的圖形資料
displaySurface =
sp<FramebufferSurface>::make(getHwComposer(), *displayId, bqConsumer,
state.physical->activeMode->getSize(),
ui::Size(maxGraphicsWidth, maxGraphicsHeight));
producer = bqProducer;
}
LOG_FATAL_IF(!displaySurface);
// 建立DisplayDevice，其又去建立RenderSurface，作為生產者角色工作，displaySurface就是FramebufferSurface物件
const auto display = setupNewDisplayDeviceInternal(displayToken, std::move(compositionDisplay),
state, displaySurface, producer);
mDisplays.emplace(displayToken, display);
......
}瞅一瞅 FramebufferSuraface的建構函式，沒啥複雜的，就是一些設定，初始化一些成員
FramebufferSurface::FramebufferSurface(HWComposer& hwc, PhysicalDisplayId displayId,
const sp<IGraphicBufferConsumer>& consumer,
const ui::Size& size, const ui::Size& maxSize)
: ConsumerBase(consumer),
mDisplayId(displayId),
mMaxSize(maxSize),
mCurrentBufferSlot(-1),
mCurrentBuffer(),
mCurrentFence(Fence::NO_FENCE),
mHwc(hwc),
mHasPendingRelease(false),
mPreviousBufferSlot(BufferQueue::INVALID_BUFFER_SLOT),
mPreviousBuffer() {
ALOGV("Creating for display %s", to_string(displayId).c_str());
mName = "FramebufferSurface";
mConsumer->setConsumerName(mName); // 設定消費者的名字是 "FramebufferSurface"
mConsumer->setConsumerUsageBits(GRALLOC_USAGE_HW_FB | // 設定usage
GRALLOC_USAGE_HW_RENDER |
GRALLOC_USAGE_HW_COMPOSER);
const auto limitedSize = limitSize(size);
mConsumer->setDefaultBufferSize(limitedSize.width, limitedSize.height); // 設定buffer 大小
mConsumer->setMaxAcquiredBufferCount(
SurfaceFlinger::maxFrameBufferAcquiredBuffers - 1);
}
再進到SurfaceFlinger::setupNewDisplayDeviceInternal中看看相關的邏輯：
[/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp]
sp<DisplayDevice> SurfaceFlinger::setupNewDisplayDeviceInternal(
const wp<IBinder>& displayToken,
std::shared_ptr<compositionengine::Display> compositionDisplay,
const DisplayDeviceState& state,
const sp<compositionengine::DisplaySurface>& displaySurface,
const sp<IGraphicBufferProducer>& producer) {
......
creationArgs.displaySurface = displaySurface; // displaySurface就是FramebufferSurface物件
// producer是前面processDisplayAdded中建立的
auto nativeWindowSurface = getFactory().createNativeWindowSurface(producer);
auto nativeWindow = nativeWindowSurface->getNativeWindow();
creationArgs.nativeWindow = nativeWindow;
....
// 前面一大坨程式碼是在初始話creationArgs，這些引數用來建立DisplayDevice
// creationArgs.nativeWindow會把前面建立的producer關聯到了DisplayDevice
sp<DisplayDevice> display = getFactory().createDisplayDevice(creationArgs);
// 後面一大坨，對display進行了些設定
if (!state.isVirtual()) {
display->setActiveMode(state.physical->activeMode->getId());
display->setDeviceProductInfo(state.physical->deviceProductInfo);
}
....
}
接下來就是 DisplayDevice 的建構函式了，裡面主要是建立了RenderSurface物件，然後對其進行初始化
[/frameworks/native/services/surfaceflinger/DisplayDevice.cpp]
DisplayDevice::DisplayDevice(DisplayDeviceCreationArgs& args)
: mFlinger(args.flinger),
mHwComposer(args.hwComposer),
mDisplayToken(args.displayToken),
mSequenceId(args.sequenceId),
mConnectionType(args.connectionType),
mCompositionDisplay{args.compositionDisplay},
mPhysicalOrientation(args.physicalOrientation),
mSupportedModes(std::move(args.supportedModes)),
mIsPrimary(args.isPrimary) {
mCompositionDisplay->editState().isSecure = args.isSecure;
// 建立RenderSurface，args.nativeWindow 即為producer,指向生產者
mCompositionDisplay->createRenderSurface(
compositionengine::RenderSurfaceCreationArgsBuilder()
.setDisplayWidth(ANativeWindow_getWidth(args.nativeWindow.get()))
.setDisplayHeight(ANativeWindow_getHeight(args.nativeWindow.get()))
.setNativeWindow(std::move(args.nativeWindow))
.setDisplaySurface(std::move(args.displaySurface)) // displaySurface就是FramebufferSurface物件
.setMaxTextureCacheSize(
static_cast<size_t>(SurfaceFlinger::maxFrameBufferAcquiredBuffers))
.build());
if (!mFlinger->mDisableClientCompositionCache &&
SurfaceFlinger::maxFrameBufferAcquiredBuffers > 0) {
mCompositionDisplay->createClientCompositionCache(
static_cast<uint32_t>(SurfaceFlinger::maxFrameBufferAcquiredBuffers));
}
mCompositionDisplay->createDisplayColorProfile(
compositionengine::DisplayColorProfileCreationArgs{args.hasWideColorGamut,
std::move(args.hdrCapabilities),
args.supportedPerFrameMetadata,
args.hwcColorModes});
if (!mCompositionDisplay->isValid()) {
ALOGE("Composition Display did not validate!");
}
// 初始化RenderSurface
mCompositionDisplay->getRenderSurface()->initialize();
setPowerMode(args.initialPowerMode);
// initialize the display orientation transform.
setProjection(ui::ROTATION_0, Rect::INVALID_RECT, Rect::INVALID_RECT);
}本文作者@二的次方 2022-05-10 釋出於部落格園
RenderSurface作為生產者的角色工作，建構函式如下，留意啟成員displaySurface就是SurfaceFlinger中建立的FramebufferSurface物件
也就是 作為生產者的RenderSurface中持有 消費者的引用 displaySurface，可以呼叫FramebufferSurface的方法
[ /frameworks/native/services/surfaceflinger/CompositionEngine/src/RenderSurface.cpp]
RenderSurface::RenderSurface(const CompositionEngine& compositionEngine, Display& display,
const RenderSurfaceCreationArgs& args)
: mCompositionEngine(compositionEngine),
mDisplay(display),
mNativeWindow(args.nativeWindow),
mDisplaySurface(args.displaySurface), // displaySurface就是FramebufferSurface物件
mSize(args.displayWidth, args.displayHeight),
mMaxTextureCacheSize(args.maxTextureCacheSize) {
LOG_ALWAYS_FATAL_IF(!mNativeWindow);
}
我們看看他的RenderSurface::initialize()方法
[/frameworks/native/services/surfaceflinger/CompositionEngine/src/RenderSurface.cpp]
void RenderSurface::initialize() {
ANativeWindow* const window = mNativeWindow.get();
int status = native_window_api_connect(window, NATIVE_WINDOW_API_EGL);
ALOGE_IF(status != NO_ERROR, "Unable to connect BQ producer: %d", status);
status = native_window_set_buffers_format(window, HAL_PIXEL_FORMAT_RGBA_8888);
ALOGE_IF(status != NO_ERROR, "Unable to set BQ format to RGBA888: %d", status);
status = native_window_set_usage(window, DEFAULT_USAGE);
ALOGE_IF(status != NO_ERROR, "Unable to set BQ usage bits for GPU rendering: %d", status);
}上述方法也很簡單，就是作為producer去和BufferQueue建立connect，並設定format為RGBA_8888，設定usage為GRALLOC_USAGE_HW_RENDER | GRALLOC_USAGE_HW_TEXTURE
為了驗證上述分析的流程是正確的，我在BufferQueueProducer::connect中加log來列印呼叫棧的資訊，如下，是不是和分析的一樣啊
11-13 00:52:58.497 227 227 D BufferQueueProducer: connect[1303] /vendor/bin/hw/android.hardware.graphics.composer@2.4-service start
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#00 pc 0005e77f /system/lib/libgui.so (android::BufferQueueProducer::connect(android::sp<android::IProducerListener> const&, int, bool, android::IGraphicBufferProducer::QueueBufferOutput*)+1282)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#01 pc 000a276b /system/lib/libgui.so (android::Surface::connect(int, android::sp<android::IProducerListener> const&, bool)+138)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#02 pc 0009de41 /system/lib/libgui.so (android::Surface::hook_perform(ANativeWindow*, int, ...)+128)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#03 pc 00121b1d /system/bin/surfaceflinger (android::compositionengine::impl::RenderSurface::initialize()+12)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#04 pc 00083cc5 /system/bin/surfaceflinger (android::DisplayDevice::DisplayDevice(android::DisplayDeviceCreationArgs&)+1168)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#05 pc 000d8bed /system/bin/surfaceflinger (android::SurfaceFlinger::processDisplayAdded(android::wp<android::IBinder> const&, android::DisplayDeviceState const&)+4440)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#06 pc 000d0db5 /system/bin/surfaceflinger (android::SurfaceFlinger::processDisplayChangesLocked()+2436)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#07 pc 000cef6b /system/bin/surfaceflinger (android::SurfaceFlinger::processDisplayHotplugEventsLocked()+6422)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#08 pc 000d2c7f /system/bin/surfaceflinger (android::SurfaceFlinger::onComposerHalHotplug(unsigned long long, android::hardware::graphics::composer::V2_1::IComposerCallback::Connection)+334)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#09 pc 0009afab /system/bin/surfaceflinger (_ZN7android12_GLOBAL__N_122ComposerCallbackBridge9onHotplugEyNS_8hardware8graphics8composer4V2_117IComposerCallback10ConnectionE$d689f7ac1c60e4abeed02ca92a51bdcd+20)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#10 pc 0001bb97 /system/lib/android.hardware.graphics.composer@2.1.so (android::hardware::graphics::composer::V2_1::BnHwComposerCallback::_hidl_onHotplug(android::hidl::base::V1_0::BnHwBase*, android::hardware::Parcel const&, android::hardware::Parcel*, std::__1::function<void (android::hardware::Parcel&)>)+166)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#11 pc 000275e9 /system/lib/android.hardware.graphics.composer@2.4.so (android::hardware::graphics::composer::V2_4::BnHwComposerCallback::onTransact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+228)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#12 pc 00054779 /system/lib/libhidlbase.so (android::hardware::BHwBinder::transact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+96)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#13 pc 0004fc67 /system/lib/libhidlbase.so (android::hardware::IPCThreadState::transact(int, unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int)+2174)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#14 pc 0004f2e5 /system/lib/libhidlbase.so (android::hardware::BpHwBinder::transact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+36)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#15 pc 0002bdf1 /system/lib/android.hardware.graphics.composer@2.4.so (android::hardware::graphics::composer::V2_4::BpHwComposerClient::_hidl_registerCallback_2_4(android::hardware::IInterface*, android::hardware::details::HidlInstrumentor*, android::sp<android::hardware::graphics::composer::V2_4::IComposerCallback> const&)+296)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#16 pc 0002ed8d /system/lib/android.hardware.graphics.composer@2.4.so (android::hardware::graphics::composer::V2_4::BpHwComposerClient::registerCallback_2_4(android::sp<android::hardware::graphics::composer::V2_4::IComposerCallback> const&)+34)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#17 pc 00085627 /system/bin/surfaceflinger (android::Hwc2::impl::Composer::registerCallback(android::sp<android::hardware::graphics::composer::V2_4::IComposerCallback> const&)+98)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#18 pc 00092d63 /system/bin/surfaceflinger (android::impl::HWComposer::setCallback(android::HWC2::ComposerCallback*)+2206)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#19 pc 000cd35b /system/bin/surfaceflinger (android::SurfaceFlinger::init()+438)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#20 pc 000feb03 /system/bin/surfaceflinger (main+862)
11-13 00:52:58.581 227 227 E BufferQueueProducer: stackdump:#21 pc 0003253b /apex/com.android.runtime/lib/bionic/libc.so (__libc_init+54)
11-13 00:52:58.582 227 227 D BufferQueueProducer: connect[1307] /vendor/bin/hw/android.hardware.graphics.composer@2.4-service end注意 本文作者@二的次方 2022-05-10 釋出於部落格園
注意 本文作者@二的次方 2022-05-10 釋出於部落格園
注意 本文作者@二的次方 2022-05-10 釋出於部落格園
這裡有一個小細節要留意下，因為SurfaceFlinger::onComposerHalHotplug是HWC回撥過來的，所以程式碼執行是在android.hardware.graphics.composer@2.4-service這個程式中的。
BufferQueueProducer::connect中記錄的mConnectedPid就是composer service的PID
[ /frameworks/native/libs/gui/BufferQueueProducer.cpp]
mCore->mConnectedPid = BufferQueueThreadState::getCallingPid();在dump BufferQueue的資訊時，根據PID獲取的 producer name 也就是 android.hardware.graphics.composer@2.4-service
[/frameworks/native/libs/gui/BufferQueueCore.cpp]
void BufferQueueCore::dumpState(const String8& prefix, String8* outResult) const {
...
getProcessName(mConnectedPid, producerProcName);
getProcessName(pid, consumerProcName);
....
}如下是我的平臺dumpsys SurfaceFlinger的資訊列印出來的Composition RenderSurface State的資訊，看看是不是和程式碼的設定都有對應起來：
? mConsumerName=FramebufferSurface
? producer=[342:/vendor/bin/hw/android.hardware.graphics.composer@2.4-service]
? consumer=[223:/system/bin/surfaceflinger])
? format/size/usage也都可以對應到程式碼的設定
Composition RenderSurface State:
size=[1920 1080] ANativeWindow=0xef2c3278 (format 1) flips=605
FramebufferSurface: dataspace: Default(0)
mAbandoned=0
- BufferQueue mMaxAcquiredBufferCount=2 mMaxDequeuedBufferCount=1
mDequeueBufferCannotBlock=0 mAsyncMode=0
mQueueBufferCanDrop=0 mLegacyBufferDrop=1
default-size=[1920x1080] default-format=1 transform-hint=00 frame-counter=580
mTransformHintInUse=00 mAutoPrerotation=0
FIFO(0):
(mConsumerName=FramebufferSurface, mConnectedApi=1, mConsumerUsageBits=6656, mId=df00000000, producer=[342:/vendor/bin/hw/android.hardware.graphics.composer@2.4-service], consumer=[223:/system/bin/surfaceflinger])
Slots:
>[01:0xeec82110] state=ACQUIRED 0xef4429c0 frame=2 [1920x1080:1920, 1]
>[02:0xeec806f0] state=ACQUIRED 0xef443100 frame=580 [1920x1080:1920, 1]
[00:0xeec81f00] state=FREE 0xef440580 frame=579 [1920x1080:1920, 1]
四、對上述分析的一個小結和猜想
上述內容中出現的一些字眼，不禁令人”瞎想連篇“
SurfaceFlinger建立了BufferQueue ==> Producer & Consumer
建立了RenderSurface作為生產者，它持有Producer
建立了FramebufferSurface作為消費者，它持有Consumer
前面分析BufferQueue的工作原理時，有講過：
生產者不斷的dequeueBuffer & queueBuffer ; 而消費者不斷的acquireBuffer & releaseBuffer ,這樣影像快取就在 生產者 -- BufferQueue -- 消費者 間流轉起來了。
看看作為生產者的RenderSurface中方法：
[/frameworks/native/services/surfaceflinger/CompositionEngine/include/compositionengine/RenderSurface.h]
/**
* Encapsulates everything for composing to a render surface with RenderEngine
*/
class RenderSurface {
....
// Allocates a buffer as scratch space for GPU composition
virtual std::shared_ptr<renderengine::ExternalTexture> dequeueBuffer(
base::unique_fd* bufferFence) = 0;
// Queues the drawn buffer for consumption by HWC. readyFence is the fence
// which will fire when the buffer is ready for consumption.
virtual void queueBuffer(base::unique_fd readyFence) = 0;
...
};熟悉的味道：
dequeueBuffer : 分配一個緩衝區作為GPU合成的暫存空間
queueBuffer : 入佇列已繪製好的圖形快取供HWC使用
同樣如果去檢視作為消費者的FramebufferSurface也會看到acquireBuffer & releaseBuffer的呼叫，如下：
[/frameworks/native/services/surfaceflinger/DisplayHardware/FramebufferSurface.cpp]
status_t FramebufferSurface::nextBuffer(uint32_t& outSlot,
sp<GraphicBuffer>& outBuffer, sp<Fence>& outFence,
Dataspace& outDataspace) {
Mutex::Autolock lock(mMutex);
BufferItem item;
status_t err = acquireBufferLocked(&item, 0); // 獲取待顯示的buffer
...
status_t result = mHwc.setClientTarget(mDisplayId, outSlot, outFence, outBuffer, outDataspace); // 傳遞給HWC進一步處理顯示
return NO_ERROR;
}
大概會有這樣一種邏輯處理流程：
當需要GPU合成時，會通過生產者RenderSurface::dequeueBuffer請求一塊圖形快取，然後GPU就合成/繪圖，把資料儲存到這塊圖形快取中，通過RenderSurface::queueBuffer提交這塊快取
呼叫mDisplaySurface->advanceFrame()通知消費者來消費:
FramebufferSurface::advanceFrame ==>FramebufferSurface::nextBuffer ==> acquireBufferLocked
去請求可用的圖形快取，這個buffer中儲存有GPU合成的結果，然後通過setClientTarget把這個buffer傳遞給HWC做處理顯示。
最開始我們提出的問題：用於儲存GPU合成後的圖形資料的GraphicBuffer是從哪裡來的？分析到這裡大概應該有講明白吧
本文作者@二的次方 2022-05-10 釋出於部落格園
五、補充知識點
5.1 用於GPU合成的GraphicBuffer的數量控制
通過屬性值來控制數量 ro.surface_flinger.max_frame_buffer_acquired_buffers，如何控制的可以看SF的程式碼
[/frameworks/native/services/surfaceflinger/sysprop/SurfaceFlingerProperties.sysprop]
# Controls the number of buffers SurfaceFlinger will allocate for use in FramebufferSurface.
prop {
api_name: "max_frame_buffer_acquired_buffers"
type: Long
scope: Public
access: Readonly
prop_name: "ro.surface_flinger.max_frame_buffer_acquired_buffers"
}這個屬性值。控制用於GPU合成的FramebufferSurface分配幾個GraphicBuffer,，一般是 3 個
5.2 一個疑問
為什麼在allocator出列印出來的Framebuffer的format=1 (PIXEL_FORMAT_RGBA_8888) , 而importer處列印出來的Framebuffer的format=5 (PIXEL_FORMAT_BGRA_8888) ?
由於對整個體系架構瞭解還不是很深入，為了找到這個問題的原因，確實費了不少功夫。其實答案在 Gralloc HAL 中。
我的平臺採用的是 Mali GPU，Gralloc HAL使用的是 Open Source Mali GPUs Gralloc Module 為基礎的Source code。
在 Gralloc 的程式碼實現中有定義一個巨集開關 GRALLOC_HWC_FORCE_BGRA_8888 ，開啟這個巨集的時候，SurfaceFlinger中的 RenderSurface::initialize() 設定的RGBA_8888 會在Gralloc allocate/map時強轉為了BGRA_8888來處理。具體的可以開啟和關閉這個巨集看dumpsys SurfaceFlinger對比
# When enabled, forces format to BGRA_8888 for FB usage when HWC is in use
GRALLOC_HWC_FORCE_BGRA_8888?=0我的平臺對比的資訊：
// 關閉GRALLOC_HWC_FORCE_BGRA_8888這個巨集定義，仍是framework設定的RGBA8888
+ name:FramebufferSurface, id:e400000000, size:8.1e+03KiB, w/h:780x438, usage: 0x1b00, req fmt:1, fourcc/mod:875708993/0, dataspace: 0x0, compressed: false
planes: R/G/B/A: w/h:780x438, stride:1e00 bytes, size:7e9000
// 開啟GRALLOC_HWC_FORCE_BGRA_8888這個巨集定義，強制轉為了BGRA8888
+ name:FramebufferSurface, id:e200000000, size:8.1e+03KiB, w/h:780x438, usage: 0x1b00, req fmt:1, fourcc/mod:875713089/0, dataspace: 0x0, compressed: false
planes: B/G/R/A: w/h:780x438, stride:1e00 bytes, size:7e9000
5.3 用於GPU合成的GraphicBuffer的size（width & height）是由什麼決定的？
答案是當前螢幕的解析度，也就是 Display的 active mode
比如我的Android TV平臺，設定不同的電視解析度
// 720 TV
GraphicBufferAllocator buffers:
Handle | Size | W (Stride) x H | Layers | Format | Usage | Requestor
0xf2d8e0d0 | 3600.00 KiB | 1280 (1280) x 720 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf2d937d0 | 3600.00 KiB | 1280 (1280) x 720 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf2d98050 | 3600.00 KiB | 1280 (1280) x 720 | 1 | 1 | 0x 1b00 | FramebufferSurface
// 1080 TV
GraphicBufferAllocator buffers:
Handle | Size | W (Stride) x H | Layers | Format | Usage | Requestor
0xf2d81d10 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf2d83840 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xf2d85ab0 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
// 4K TV(限制ro.surface_flinger.max_graphics_height/width)
GraphicBufferAllocator buffers:
Handle | Size | W (Stride) x H | Layers | Format | Usage | Requestor
0xe8041b40 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xe8045c80 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface
0xe804fa30 | 8100.00 KiB | 1920 (1920) x 1080 | 1 | 1 | 0x 1b00 | FramebufferSurface下面的屬性是受平臺限制，GPU最大可以支援的合成的size
console:/ $ getprop | grep max_graphics
[ro.surface_flinger.max_graphics_height]: [1080]
[ro.surface_flinger.max_graphics_width]: [1920]
本文作者@二的次方 2022-05-10 釋出於部落格園
大概就講這些吧，之後想到什麼問題再補充。