audio write pcm ioctl - SNDRV_PCM_IOCTL_WRITEI_FRAMES

http://androidxref.com/9.0.0_r3/xref/external/tinyalsa/pcm.c#535

http://androidxref.com/kernel_3.18/xref/sound/core/pcm_native.c#2745

 

https://blog.csdn.net/Vincentywj/article/details/77849442

对于回放的情形，PCM 数据流向大致是：

        copy_from_user           DMA                 I2S           DAC
              ^                   ^                   ^             ^
+---------+   |    +----------+   |   +-----------+   |   +-----+   |   +------+
|userspace+-------->DMA Buffer+------->I2S TX FIFO+------->CODEC+------->SPK/HP|
+---------+        +----------+       +-----------+       +-----+       +------+

 

来源：https://blog.csdn.net/chenqiang0721/article/details/102761326