Audiosink design

Audiosink的设计，需要满足下列一些需求：

•  良好的chain_based 支持。绝大多数简单playback pipelines都是将音频数据从decoder直接push给audiosink;

•  良好 getrange_based支持。大部分专业的应用都是audio sink从pipeline拉取数据。典型的作法是在一个回调函数中pull N Samples，回调函数一般在一个单独的线程或者是硬件设备来调度。

• 提供准确的时钟。必须能够提供即使samples被丢弃或在流中发现不连续时也能提供采样准确的时钟。

• 可能的话，提供DMA支持

Design

The design is based on a set of base classes and the concept of a ringbuffer of samples.

+-----------+   - provide preroll, rendering, timing
+ basesink  +   - caps nego
+-----+-----+
      |
+-----V----------+  　　 - manages ringbuffer
+ audiobasesink  +   - manages scheduling (push/pull)
+-----+----------+   　　- manages clock/query/seek
      |             　　　　 - manages scheduling of samples in the ringbuffer
      |              　　　　- manages caps parsing
      |
+-----V------+   - default ringbuffer implementation with a GThread
+ audiosink  +   - subclasses provide open/read/close methods
+------------+

The ringbuffer is a contiguous piece of memory divided into segtotal pieces of segments. Each segment has segsize bytes.

      play position 
        v          
+---+---+---+-------------------------------------+----------+
+ 0 | 1 | 2 | ....                                | segtotal |
+---+---+---+-------------------------------------+----------+
<--->
  segsize bytes = N samples * bytes_per_sample.

如上，环形缓冲器有一个play position，以segment表示。play position是设备正在从ringbuffer读取samples的位置。
ringbuffer可以进入PLAYING或STOPPED状态:
在STOPPED状态下，没有samples被put到设备，play pointer不前进。
在PLAYING状态下，samples被写入设备，并且在每个seg被写入设备之后，ringbuffer应该调用可配置的回调,
play pointer在每个段被写入之后前进。

对ringbuffer的写入操作将会把新的samples放入ringbuffer中。如果ringbuffer中没有足够的空间，
写操作将被阻塞。即使buffer为空，ringbuffer的播放也不会停止。当buffer为空时，设备会播放静音。
ringbuffer通过无锁原子操作实现，特别是在读取端，以便尽可能地降低低延迟。
每当一个new samples要put进ringbuffer，先获取read pointer，请求的write position和actural position作差值，

      /* get the currently processed segment */
      segdone = g_atomic_int_get (&buf->segdone) - buf->segbase;

      /* see how far away it is from the write segment */
      diff = writeseg - segdone;
　　　 
       .....
      
      /* segment too far ahead, writer too slow, we need to drop, hopefully UNLIKELY */
      if (G_UNLIKELY (diff < 0)) {
        /* we need to drop one segment at a time, pretend we wrote a segment. */
        skip = TRUE;
        break;
      }

      /* write segment is within writable range, we can break the loop and
       * start writing the data. */
      if (diff <= segtotal) {
        skip = FALSE;
        break;
      }

      /* else we need to wait for the segment to become writable. */
      if (!wait_segment (buf))
        goto not_started;
    }

如果差值diff<0，则samples来太慢了，丢掉来太慢的数据。如果diff > segtotal，写入部分必须等待播放指针前进。

也就是需要设备先消耗掉ringbuffer中的数据，才能继续往ringbuffer中写入新的数据。