util/common/ring.cpp

/*
 * Copyright (C) 2015 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 "ring.h"

#include <stdlib.h>
#include <string.h>
#include <atomic>

namespace android {

RingBuffer::RingBuffer(size_t size)
    : mSize(size),
      mData((sensors_event_t *)malloc(sizeof(sensors_event_t) * mSize)),
      mReadPos(0),
      mWritePos(0) {
}

RingBuffer::~RingBuffer() {
    free(mData);
    mData = NULL;
}

ssize_t RingBuffer::write(const sensors_event_t *ev, size_t size) {
    Mutex::Autolock autoLock(mLock);

    size_t numAvailableToRead = mWritePos - mReadPos;
    size_t numAvailableToWrite = mSize - numAvailableToRead;

    if (size > numAvailableToWrite) {
        size = numAvailableToWrite;
    }

    size_t writePos = (mWritePos % mSize);
    size_t copy = mSize - writePos;

    if (copy > size) {
        copy = size;
    }

    memcpy(&mData[writePos], ev, copy * sizeof(sensors_event_t));

    if (size > copy) {
        memcpy(mData, &ev[copy], (size - copy) * sizeof(sensors_event_t));
    }

    mWritePos += size;

    if (numAvailableToRead == 0 && size > 0) {
        mNotEmptyCondition.broadcast();
    }

    return size;
}

ssize_t RingBuffer::read(sensors_event_t *ev, size_t size) {
    Mutex::Autolock autoLock(mLock);

    size_t numAvailableToRead;
    for (;;) {
        numAvailableToRead = mWritePos - mReadPos;
        if (numAvailableToRead > 0) {
            break;
        }

        mNotEmptyCondition.wait(mLock);
    }

    if (size > numAvailableToRead) {
        size = numAvailableToRead;
    }

    size_t readPos = (mReadPos % mSize);
    size_t copy = mSize - readPos;

    if (copy > size) {
        copy = size;
    }

    memcpy(ev, &mData[readPos], copy * sizeof(sensors_event_t));

    if (size > copy) {
        memcpy(&ev[copy], mData, (size - copy) * sizeof(sensors_event_t));
    }

    mReadPos += size;

    return size;
}

LockfreeBuffer::LockfreeBuffer(void* buf, size_t size)
        : mData((sensors_event_t *)buf), mSize(size/sizeof(sensors_event_t)),
        mWritePos(0), mCounter(1) {
    memset(mData, 0, size);
}

LockfreeBuffer::~LockfreeBuffer() {
    memset(mData, 0, mSize*sizeof(sensors_event_t));
}

void LockfreeBuffer::write(const sensors_event_t *ev, size_t size) {
    if (!mSize) {
        return;
    }

    while(size--) {
        // part before reserved0 field
        memcpy(&mData[mWritePos], ev, offsetof(sensors_event_t, reserved0));
        // part after reserved0 field
        memcpy(reinterpret_cast<char *>(&mData[mWritePos]) + offsetof(sensors_event_t, timestamp),
               reinterpret_cast<const char *>(ev) + offsetof(sensors_event_t, timestamp),
               sizeof(sensors_event_t) - offsetof(sensors_event_t, timestamp));
        // barrier before writing the atomic counter
        std::atomic_thread_fence(std::memory_order_release);
        mData[mWritePos].reserved0 = mCounter++;
        // barrier after writing the atomic counter
        std::atomic_thread_fence(std::memory_order_release);
        ++ev;

        if (++mWritePos >= mSize) {
            mWritePos = 0;
        }
    }
}

}  // namespace android