graphics/composer/2.1/default/IComposerCommandBuffer.h

/*
 * Copyright 2016 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.
 */

#ifndef ANDROID_HARDWARE_GRAPHICS_COMPOSER_COMMAND_BUFFER_H
#define ANDROID_HARDWARE_GRAPHICS_COMPOSER_COMMAND_BUFFER_H

#ifndef LOG_TAG
#warn "IComposerCommandBuffer.h included without LOG_TAG"
#endif

#undef LOG_NDEBUG
#define LOG_NDEBUG 0

#include <algorithm>
#include <limits>
#include <memory>
#include <vector>

#include <inttypes.h>
#include <string.h>

#include <android/hardware/graphics/composer/2.1/IComposer.h>
#include <log/log.h>
#include <sync/sync.h>
#include <fmq/MessageQueue.h>

namespace android {
namespace hardware {
namespace graphics {
namespace composer {
namespace V2_1 {

using android::hardware::graphics::common::V1_0::ColorTransform;
using android::hardware::graphics::common::V1_0::Dataspace;
using android::hardware::graphics::common::V1_0::Transform;
using android::hardware::MessageQueue;

using CommandQueueType = MessageQueue<uint32_t, kSynchronizedReadWrite>;

// This class helps build a command queue.  Note that all sizes/lengths are in
// units of uint32_t's.
class CommandWriterBase {
public:
    CommandWriterBase(uint32_t initialMaxSize)
        : mDataMaxSize(initialMaxSize)
    {
        mData = std::make_unique<uint32_t[]>(mDataMaxSize);
        reset();
    }

    virtual ~CommandWriterBase()
    {
        reset();
    }

    void reset()
    {
        mDataWritten = 0;
        mCommandEnd = 0;

        // handles in mDataHandles are owned by the caller
        mDataHandles.clear();

        // handles in mTemporaryHandles are owned by the writer
        for (auto handle : mTemporaryHandles) {
            native_handle_close(handle);
            native_handle_delete(handle);
        }
        mTemporaryHandles.clear();
    }

    IComposerClient::Command getCommand(uint32_t offset)
    {
        uint32_t val = (offset < mDataWritten) ? mData[offset] : 0;
        return static_cast<IComposerClient::Command>(val &
                static_cast<uint32_t>(IComposerClient::Command::OPCODE_MASK));
    }

    bool writeQueue(bool* outQueueChanged, uint32_t* outCommandLength,
            hidl_vec<hidl_handle>* outCommandHandles)
    {
        // After data are written to the queue, it may not be read by the
        // remote reader when
        //
        //  - the writer does not send them (because of other errors)
        //  - the hwbinder transaction fails
        //  - the reader does not read them (because of other errors)
        //
        // Discard the stale data here.
        size_t staleDataSize = mQueue ? mQueue->availableToRead() : 0;
        if (staleDataSize > 0) {
            ALOGW("discarding stale data from message queue");
            CommandQueueType::MemTransaction tx;
            if (mQueue->beginRead(staleDataSize, &tx)) {
                mQueue->commitRead(staleDataSize);
            }
        }

        // write data to queue, optionally resizing it
        if (mQueue && (mDataMaxSize <= mQueue->getQuantumCount())) {
            if (!mQueue->write(mData.get(), mDataWritten)) {
                ALOGE("failed to write commands to message queue");
                return false;
            }

            *outQueueChanged = false;
        } else {
            auto newQueue = std::make_unique<CommandQueueType>(mDataMaxSize);
            if (!newQueue->isValid() ||
                    !newQueue->write(mData.get(), mDataWritten)) {
                ALOGE("failed to prepare a new message queue ");
                return false;
            }

            mQueue = std::move(newQueue);
            *outQueueChanged = true;
        }

        *outCommandLength = mDataWritten;
        outCommandHandles->setToExternal(
                const_cast<hidl_handle*>(mDataHandles.data()),
                mDataHandles.size());

        return true;
    }

    const MQDescriptorSync<uint32_t>* getMQDescriptor() const
    {
        return (mQueue) ? mQueue->getDesc() : nullptr;
    }

    static constexpr uint16_t kSelectDisplayLength = 2;
    void selectDisplay(Display display)
    {
        beginCommand(IComposerClient::Command::SELECT_DISPLAY,
                kSelectDisplayLength);
        write64(display);
        endCommand();
    }

    static constexpr uint16_t kSelectLayerLength = 2;
    void selectLayer(Layer layer)
    {
        beginCommand(IComposerClient::Command::SELECT_LAYER,
                kSelectLayerLength);
        write64(layer);
        endCommand();
    }

    static constexpr uint16_t kSetErrorLength = 2;
    void setError(uint32_t location, Error error)
    {
        beginCommand(IComposerClient::Command::SET_ERROR, kSetErrorLength);
        write(location);
        writeSigned(static_cast<int32_t>(error));
        endCommand();
    }

    static constexpr uint32_t kPresentOrValidateDisplayResultLength = 1;
    void setPresentOrValidateResult(uint32_t  state) {
       beginCommand(IComposerClient::Command::SET_PRESENT_OR_VALIDATE_DISPLAY_RESULT, kPresentOrValidateDisplayResultLength);
       write(state);
       endCommand();
    }

    void setChangedCompositionTypes(const std::vector<Layer>& layers,
            const std::vector<IComposerClient::Composition>& types)
    {
        size_t totalLayers = std::min(layers.size(), types.size());
        size_t currentLayer = 0;

        while (currentLayer < totalLayers) {
            size_t count = std::min(totalLayers - currentLayer,
                    static_cast<size_t>(kMaxLength) / 3);

            beginCommand(
                    IComposerClient::Command::SET_CHANGED_COMPOSITION_TYPES,
                    count * 3);
            for (size_t i = 0; i < count; i++) {
                write64(layers[currentLayer + i]);
                writeSigned(static_cast<int32_t>(types[currentLayer + i]));
            }
            endCommand();

            currentLayer += count;
        }
    }

    void setDisplayRequests(uint32_t displayRequestMask,
            const std::vector<Layer>& layers,
            const std::vector<uint32_t>& layerRequestMasks)
    {
        size_t totalLayers = std::min(layers.size(),
                layerRequestMasks.size());
        size_t currentLayer = 0;

        while (currentLayer < totalLayers) {
            size_t count = std::min(totalLayers - currentLayer,
                    static_cast<size_t>(kMaxLength - 1) / 3);

            beginCommand(IComposerClient::Command::SET_DISPLAY_REQUESTS,
                    1 + count * 3);
            write(displayRequestMask);
            for (size_t i = 0; i < count; i++) {
                write64(layers[currentLayer + i]);
                write(static_cast<int32_t>(layerRequestMasks[currentLayer + i]));
            }
            endCommand();

            currentLayer += count;
        }
    }

    static constexpr uint16_t kSetPresentFenceLength = 1;
    void setPresentFence(int presentFence)
    {
        beginCommand(IComposerClient::Command::SET_PRESENT_FENCE,
                kSetPresentFenceLength);
        writeFence(presentFence);
        endCommand();
    }

    void setReleaseFences(const std::vector<Layer>& layers,
            const std::vector<int>& releaseFences)
    {
        size_t totalLayers = std::min(layers.size(), releaseFences.size());
        size_t currentLayer = 0;

        while (currentLayer < totalLayers) {
            size_t count = std::min(totalLayers - currentLayer,
                    static_cast<size_t>(kMaxLength) / 3);

            beginCommand(IComposerClient::Command::SET_RELEASE_FENCES,
                    count * 3);
            for (size_t i = 0; i < count; i++) {
                write64(layers[currentLayer + i]);
                writeFence(releaseFences[currentLayer + i]);
            }
            endCommand();

            currentLayer += count;
        }
    }

    static constexpr uint16_t kSetColorTransformLength = 17;
    void setColorTransform(const float* matrix, ColorTransform hint)
    {
        beginCommand(IComposerClient::Command::SET_COLOR_TRANSFORM,
                kSetColorTransformLength);
        for (int i = 0; i < 16; i++) {
            writeFloat(matrix[i]);
        }
        writeSigned(static_cast<int32_t>(hint));
        endCommand();
    }

    void setClientTarget(uint32_t slot, const native_handle_t* target,
            int acquireFence, Dataspace dataspace,
            const std::vector<IComposerClient::Rect>& damage)
    {
        bool doWrite = (damage.size() <= (kMaxLength - 4) / 4);
        size_t length = 4 + ((doWrite) ? damage.size() * 4 : 0);

        beginCommand(IComposerClient::Command::SET_CLIENT_TARGET, length);
        write(slot);
        writeHandle(target, true);
        writeFence(acquireFence);
        writeSigned(static_cast<int32_t>(dataspace));
        // When there are too many rectangles in the damage region and doWrite
        // is false, we write no rectangle at all which means the entire
        // client target is damaged.
        if (doWrite) {
            writeRegion(damage);
        }
        endCommand();
    }

    static constexpr uint16_t kSetOutputBufferLength = 3;
    void setOutputBuffer(uint32_t slot, const native_handle_t* buffer,
            int releaseFence)
    {
        beginCommand(IComposerClient::Command::SET_OUTPUT_BUFFER,
                kSetOutputBufferLength);
        write(slot);
        writeHandle(buffer, true);
        writeFence(releaseFence);
        endCommand();
    }

    static constexpr uint16_t kValidateDisplayLength = 0;
    void validateDisplay()
    {
        beginCommand(IComposerClient::Command::VALIDATE_DISPLAY,
                kValidateDisplayLength);
        endCommand();
    }

    static constexpr uint16_t kPresentOrValidateDisplayLength = 0;
    void presentOrvalidateDisplay()
    {
        beginCommand(IComposerClient::Command::PRESENT_OR_VALIDATE_DISPLAY,
                     kPresentOrValidateDisplayLength);
        endCommand();
    }

    static constexpr uint16_t kAcceptDisplayChangesLength = 0;
    void acceptDisplayChanges()
    {
        beginCommand(IComposerClient::Command::ACCEPT_DISPLAY_CHANGES,
                kAcceptDisplayChangesLength);
        endCommand();
    }

    static constexpr uint16_t kPresentDisplayLength = 0;
    void presentDisplay()
    {
        beginCommand(IComposerClient::Command::PRESENT_DISPLAY,
                kPresentDisplayLength);
        endCommand();
    }

    static constexpr uint16_t kSetLayerCursorPositionLength = 2;
    void setLayerCursorPosition(int32_t x, int32_t y)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_CURSOR_POSITION,
                kSetLayerCursorPositionLength);
        writeSigned(x);
        writeSigned(y);
        endCommand();
    }

    static constexpr uint16_t kSetLayerBufferLength = 3;
    void setLayerBuffer(uint32_t slot, const native_handle_t* buffer,
            int acquireFence)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_BUFFER,
                kSetLayerBufferLength);
        write(slot);
        writeHandle(buffer, true);
        writeFence(acquireFence);
        endCommand();
    }

    void setLayerSurfaceDamage(
            const std::vector<IComposerClient::Rect>& damage)
    {
        bool doWrite = (damage.size() <= kMaxLength / 4);
        size_t length = (doWrite) ? damage.size() * 4 : 0;

        beginCommand(IComposerClient::Command::SET_LAYER_SURFACE_DAMAGE,
                length);
        // When there are too many rectangles in the damage region and doWrite
        // is false, we write no rectangle at all which means the entire
        // layer is damaged.
        if (doWrite) {
            writeRegion(damage);
        }
        endCommand();
    }

    static constexpr uint16_t kSetLayerBlendModeLength = 1;
    void setLayerBlendMode(IComposerClient::BlendMode mode)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_BLEND_MODE,
                kSetLayerBlendModeLength);
        writeSigned(static_cast<int32_t>(mode));
        endCommand();
    }

    static constexpr uint16_t kSetLayerColorLength = 1;
    void setLayerColor(IComposerClient::Color color)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_COLOR,
                kSetLayerColorLength);
        writeColor(color);
        endCommand();
    }

    static constexpr uint16_t kSetLayerCompositionTypeLength = 1;
    void setLayerCompositionType(IComposerClient::Composition type)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_COMPOSITION_TYPE,
                kSetLayerCompositionTypeLength);
        writeSigned(static_cast<int32_t>(type));
        endCommand();
    }

    static constexpr uint16_t kSetLayerDataspaceLength = 1;
    void setLayerDataspace(Dataspace dataspace)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_DATASPACE,
                kSetLayerDataspaceLength);
        writeSigned(static_cast<int32_t>(dataspace));
        endCommand();
    }

    static constexpr uint16_t kSetLayerDisplayFrameLength = 4;
    void setLayerDisplayFrame(const IComposerClient::Rect& frame)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_DISPLAY_FRAME,
                kSetLayerDisplayFrameLength);
        writeRect(frame);
        endCommand();
    }

    static constexpr uint16_t kSetLayerPlaneAlphaLength = 1;
    void setLayerPlaneAlpha(float alpha)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_PLANE_ALPHA,
                kSetLayerPlaneAlphaLength);
        writeFloat(alpha);
        endCommand();
    }

    static constexpr uint16_t kSetLayerSidebandStreamLength = 1;
    void setLayerSidebandStream(const native_handle_t* stream)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_SIDEBAND_STREAM,
                kSetLayerSidebandStreamLength);
        writeHandle(stream);
        endCommand();
    }

    static constexpr uint16_t kSetLayerSourceCropLength = 4;
    void setLayerSourceCrop(const IComposerClient::FRect& crop)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_SOURCE_CROP,
                kSetLayerSourceCropLength);
        writeFRect(crop);
        endCommand();
    }

    static constexpr uint16_t kSetLayerTransformLength = 1;
    void setLayerTransform(Transform transform)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_TRANSFORM,
                kSetLayerTransformLength);
        writeSigned(static_cast<int32_t>(transform));
        endCommand();
    }

    void setLayerVisibleRegion(
            const std::vector<IComposerClient::Rect>& visible)
    {
        bool doWrite = (visible.size() <= kMaxLength / 4);
        size_t length = (doWrite) ? visible.size() * 4 : 0;

        beginCommand(IComposerClient::Command::SET_LAYER_VISIBLE_REGION,
                length);
        // When there are too many rectangles in the visible region and
        // doWrite is false, we write no rectangle at all which means the
        // entire layer is visible.
        if (doWrite) {
            writeRegion(visible);
        }
        endCommand();
    }

    static constexpr uint16_t kSetLayerZOrderLength = 1;
    void setLayerZOrder(uint32_t z)
    {
        beginCommand(IComposerClient::Command::SET_LAYER_Z_ORDER,
                kSetLayerZOrderLength);
        write(z);
        endCommand();
    }

protected:
    void beginCommand(IComposerClient::Command command, uint16_t length)
    {
        if (mCommandEnd) {
            LOG_FATAL("endCommand was not called before command 0x%x",
                    command);
        }

        growData(1 + length);
        write(static_cast<uint32_t>(command) | length);

        mCommandEnd = mDataWritten + length;
    }

    void endCommand()
    {
        if (!mCommandEnd) {
            LOG_FATAL("beginCommand was not called");
        } else if (mDataWritten > mCommandEnd) {
            LOG_FATAL("too much data written");
            mDataWritten = mCommandEnd;
        } else if (mDataWritten < mCommandEnd) {
            LOG_FATAL("too little data written");
            while (mDataWritten < mCommandEnd) {
                write(0);
            }
        }

        mCommandEnd = 0;
    }

    void write(uint32_t val)
    {
        mData[mDataWritten++] = val;
    }

    void writeSigned(int32_t val)
    {
        memcpy(&mData[mDataWritten++], &val, sizeof(val));
    }

    void writeFloat(float val)
    {
        memcpy(&mData[mDataWritten++], &val, sizeof(val));
    }

    void write64(uint64_t val)
    {
        uint32_t lo = static_cast<uint32_t>(val & 0xffffffff);
        uint32_t hi = static_cast<uint32_t>(val >> 32);
        write(lo);
        write(hi);
    }

    void writeRect(const IComposerClient::Rect& rect)
    {
        writeSigned(rect.left);
        writeSigned(rect.top);
        writeSigned(rect.right);
        writeSigned(rect.bottom);
    }

    void writeRegion(const std::vector<IComposerClient::Rect>& region)
    {
        for (const auto& rect : region) {
            writeRect(rect);
        }
    }

    void writeFRect(const IComposerClient::FRect& rect)
    {
        writeFloat(rect.left);
        writeFloat(rect.top);
        writeFloat(rect.right);
        writeFloat(rect.bottom);
    }

    void writeColor(const IComposerClient::Color& color)
    {
        write((color.r <<  0) |
              (color.g <<  8) |
              (color.b << 16) |
              (color.a << 24));
    }

    // ownership of handle is not transferred
    void writeHandle(const native_handle_t* handle, bool useCache)
    {
        if (!handle) {
            writeSigned(static_cast<int32_t>((useCache) ?
                        IComposerClient::HandleIndex::CACHED :
                        IComposerClient::HandleIndex::EMPTY));
            return;
        }

        mDataHandles.push_back(handle);
        writeSigned(mDataHandles.size() - 1);
    }

    void writeHandle(const native_handle_t* handle)
    {
        writeHandle(handle, false);
    }

    // ownership of fence is transferred
    void writeFence(int fence)
    {
        native_handle_t* handle = nullptr;
        if (fence >= 0) {
            handle = getTemporaryHandle(1, 0);
            if (handle) {
                handle->data[0] = fence;
            } else {
                ALOGW("failed to get temporary handle for fence %d", fence);
                sync_wait(fence, -1);
                close(fence);
            }
        }

        writeHandle(handle);
    }

    native_handle_t* getTemporaryHandle(int numFds, int numInts)
    {
        native_handle_t* handle = native_handle_create(numFds, numInts);
        if (handle) {
            mTemporaryHandles.push_back(handle);
        }
        return handle;
    }

    static constexpr uint16_t kMaxLength =
        std::numeric_limits<uint16_t>::max();

private:
    void growData(uint32_t grow)
    {
        uint32_t newWritten = mDataWritten + grow;
        if (newWritten < mDataWritten) {
            LOG_ALWAYS_FATAL("buffer overflowed; data written %" PRIu32
                    ", growing by %" PRIu32, mDataWritten, grow);
        }

        if (newWritten <= mDataMaxSize) {
            return;
        }

        uint32_t newMaxSize = mDataMaxSize << 1;
        if (newMaxSize < newWritten) {
            newMaxSize = newWritten;
        }

        auto newData = std::make_unique<uint32_t[]>(newMaxSize);
        std::copy_n(mData.get(), mDataWritten, newData.get());
        mDataMaxSize = newMaxSize;
        mData = std::move(newData);
    }

    uint32_t mDataMaxSize;
    std::unique_ptr<uint32_t[]> mData;

    uint32_t mDataWritten;
    // end offset of the current command
    uint32_t mCommandEnd;

    std::vector<hidl_handle> mDataHandles;
    std::vector<native_handle_t *> mTemporaryHandles;

    std::unique_ptr<CommandQueueType> mQueue;
};

// This class helps parse a command queue.  Note that all sizes/lengths are in
// units of uint32_t's.
class CommandReaderBase {
public:
    CommandReaderBase() : mDataMaxSize(0)
    {
        reset();
    }

    bool setMQDescriptor(const MQDescriptorSync<uint32_t>& descriptor)
    {
        mQueue = std::make_unique<CommandQueueType>(descriptor, false);
        if (mQueue->isValid()) {
            return true;
        } else {
            mQueue = nullptr;
            return false;
        }
    }

    bool readQueue(uint32_t commandLength,
            const hidl_vec<hidl_handle>& commandHandles)
    {
        if (!mQueue) {
            return false;
        }

        auto quantumCount = mQueue->getQuantumCount();
        if (mDataMaxSize < quantumCount) {
            mDataMaxSize = quantumCount;
            mData = std::make_unique<uint32_t[]>(mDataMaxSize);
        }

        if (commandLength > mDataMaxSize ||
                !mQueue->read(mData.get(), commandLength)) {
            ALOGE("failed to read commands from message queue");
            return false;
        }

        mDataSize = commandLength;
        mDataRead = 0;
        mCommandBegin = 0;
        mCommandEnd = 0;
        mDataHandles.setToExternal(
                const_cast<hidl_handle*>(commandHandles.data()),
                commandHandles.size());

        return true;
    }

    void reset()
    {
        mDataSize = 0;
        mDataRead = 0;
        mCommandBegin = 0;
        mCommandEnd = 0;
        mDataHandles.setToExternal(nullptr, 0);
    }

protected:
    bool isEmpty() const
    {
        return (mDataRead >= mDataSize);
    }

    bool beginCommand(IComposerClient::Command* outCommand,
            uint16_t* outLength)
    {
        if (mCommandEnd) {
            LOG_FATAL("endCommand was not called for last command");
        }

        constexpr uint32_t opcode_mask =
            static_cast<uint32_t>(IComposerClient::Command::OPCODE_MASK);
        constexpr uint32_t length_mask =
            static_cast<uint32_t>(IComposerClient::Command::LENGTH_MASK);

        uint32_t val = read();
        *outCommand = static_cast<IComposerClient::Command>(
                val & opcode_mask);
        *outLength = static_cast<uint16_t>(val & length_mask);

        if (mDataRead + *outLength > mDataSize) {
            ALOGE("command 0x%x has invalid command length %" PRIu16,
                    *outCommand, *outLength);
            // undo the read() above
            mDataRead--;
            return false;
        }

        mCommandEnd = mDataRead + *outLength;

        return true;
    }

    void endCommand()
    {
        if (!mCommandEnd) {
            LOG_FATAL("beginCommand was not called");
        } else if (mDataRead > mCommandEnd) {
            LOG_FATAL("too much data read");
            mDataRead = mCommandEnd;
        } else if (mDataRead < mCommandEnd) {
            LOG_FATAL("too little data read");
            mDataRead = mCommandEnd;
        }

        mCommandBegin = mCommandEnd;
        mCommandEnd = 0;
    }

    uint32_t getCommandLoc() const
    {
        return mCommandBegin;
    }

    uint32_t read()
    {
        return mData[mDataRead++];
    }

    int32_t readSigned()
    {
        int32_t val;
        memcpy(&val, &mData[mDataRead++], sizeof(val));
        return val;
    }

    float readFloat()
    {
        float val;
        memcpy(&val, &mData[mDataRead++], sizeof(val));
        return val;
    }

    uint64_t read64()
    {
        uint32_t lo = read();
        uint32_t hi = read();
        return (static_cast<uint64_t>(hi) << 32) | lo;
    }

    IComposerClient::Color readColor()
    {
        uint32_t val = read();
        return IComposerClient::Color{
            static_cast<uint8_t>((val >>  0) & 0xff),
            static_cast<uint8_t>((val >>  8) & 0xff),
            static_cast<uint8_t>((val >> 16) & 0xff),
            static_cast<uint8_t>((val >> 24) & 0xff),
        };
    }

    // ownership of handle is not transferred
    const native_handle_t* readHandle(bool* outUseCache)
    {
        const native_handle_t* handle = nullptr;

        int32_t index = readSigned();
        switch (index) {
        case static_cast<int32_t>(IComposerClient::HandleIndex::EMPTY):
            *outUseCache = false;
            break;
        case static_cast<int32_t>(IComposerClient::HandleIndex::CACHED):
            *outUseCache = true;
            break;
        default:
            if (static_cast<size_t>(index) < mDataHandles.size()) {
                handle = mDataHandles[index].getNativeHandle();
            } else {
                ALOGE("invalid handle index %zu", static_cast<size_t>(index));
            }
            *outUseCache = false;
            break;
        }

        return handle;
    }

    const native_handle_t* readHandle()
    {
        bool useCache;
        return readHandle(&useCache);
    }

    // ownership of fence is transferred
    int readFence()
    {
        auto handle = readHandle();
        if (!handle || handle->numFds == 0) {
            return -1;
        }

        if (handle->numFds != 1) {
            ALOGE("invalid fence handle with %d fds", handle->numFds);
            return -1;
        }

        int fd = dup(handle->data[0]);
        if (fd < 0) {
            ALOGW("failed to dup fence %d", handle->data[0]);
            sync_wait(handle->data[0], -1);
            fd = -1;
        }

        return fd;
    }

private:
    std::unique_ptr<CommandQueueType> mQueue;
    uint32_t mDataMaxSize;
    std::unique_ptr<uint32_t[]> mData;

    uint32_t mDataSize;
    uint32_t mDataRead;

    // begin/end offsets of the current command
    uint32_t mCommandBegin;
    uint32_t mCommandEnd;

    hidl_vec<hidl_handle> mDataHandles;
};

} // namespace V2_1
} // namespace composer
} // namespace graphics
} // namespace hardware
} // namespace android

#endif // ANDROID_HARDWARE_GRAPHICS_COMPOSER_COMMAND_BUFFER_H