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      1 /*
      2  * Copyright (C) 2012 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #define LOG_TAG "PipeReader"
     18 //#define LOG_NDEBUG 0
     19 
     20 #include <cutils/compiler.h>
     21 #include <utils/Log.h>
     22 #include <media/nbaio/PipeReader.h>
     23 
     24 namespace android {
     25 
     26 PipeReader::PipeReader(Pipe& pipe) :
     27         NBAIO_Source(pipe.mFormat),
     28         mPipe(pipe),
     29         // any data already in the pipe is not visible to this PipeReader
     30         mFront(android_atomic_acquire_load(&pipe.mRear)),
     31         mFramesOverrun(0),
     32         mOverruns(0)
     33 {
     34     android_atomic_inc(&pipe.mReaders);
     35 }
     36 
     37 PipeReader::~PipeReader()
     38 {
     39 #if !LOG_NDEBUG
     40     int32_t readers =
     41 #else
     42     (void)
     43 #endif
     44             android_atomic_dec(&mPipe.mReaders);
     45     ALOG_ASSERT(readers > 0);
     46 }
     47 
     48 ssize_t PipeReader::availableToRead()
     49 {
     50     if (CC_UNLIKELY(!mNegotiated)) {
     51         return NEGOTIATE;
     52     }
     53     int32_t rear = android_atomic_acquire_load(&mPipe.mRear);
     54     // read() is not multi-thread safe w.r.t. itself, so no mutex or atomic op needed to read mFront
     55     size_t avail = rear - mFront;
     56     if (CC_UNLIKELY(avail > mPipe.mMaxFrames)) {
     57         // Discard 1/16 of the most recent data in pipe to avoid another overrun immediately
     58         int32_t oldFront = mFront;
     59         mFront = rear - mPipe.mMaxFrames + (mPipe.mMaxFrames >> 4);
     60         mFramesOverrun += (size_t) (mFront - oldFront);
     61         ++mOverruns;
     62         return OVERRUN;
     63     }
     64     return avail;
     65 }
     66 
     67 ssize_t PipeReader::read(void *buffer, size_t count)
     68 {
     69     ssize_t avail = availableToRead();
     70     if (CC_UNLIKELY(avail <= 0)) {
     71         return avail;
     72     }
     73     // An overrun can occur from here on and be silently ignored,
     74     // but it will be caught at next read()
     75     if (CC_LIKELY(count > (size_t) avail)) {
     76         count = avail;
     77     }
     78     size_t front = mFront & (mPipe.mMaxFrames - 1);
     79     size_t red = mPipe.mMaxFrames - front;
     80     if (CC_LIKELY(red > count)) {
     81         red = count;
     82     }
     83     // In particular, an overrun during the memcpy will result in reading corrupt data
     84     memcpy(buffer, (char *) mPipe.mBuffer + (front * mFrameSize), red * mFrameSize);
     85     // We could re-read the rear pointer here to detect the corruption, but why bother?
     86     if (CC_UNLIKELY(front + red == mPipe.mMaxFrames)) {
     87         if (CC_UNLIKELY((count -= red) > front)) {
     88             count = front;
     89         }
     90         if (CC_LIKELY(count > 0)) {
     91             memcpy((char *) buffer + (red * mFrameSize), mPipe.mBuffer, count * mFrameSize);
     92             red += count;
     93         }
     94     }
     95     mFront += red;
     96     mFramesRead += red;
     97     return red;
     98 }
     99 
    100 }   // namespace android
    101