Android 4.4 Kitkat audio implementation and brief analysis

Enable recording and playing on Android 4.4
Background since the beginning of ICS, the audio system has undergone great changes. First, alsalib has been abandoned, and then AIO has been adopted. All levels of frameworks have their own characteristics, it is a step away from the Linux Audio Application, forming its own unique audio management and audio configuration functions. In general, the improvement is still very large, at least in terms of user experience has greatly surpassed the previous version. We will analyze some of the changes and implementation mechanisms in the audio Implementation of 4.4.

Requirements
First, the hardware function is normal. This is not a table. Linux supports the alsa driver to generate the alsa subsystem. It is best to test the playing and recording of audio on other file systems such as buildroot in advance.
HAL is a simple implementation of HAL. For more information, see device/asus/grouper/audio. If there is no complicated audio configuration, you can modify it based on the code, it mainly involves some playback and recording parameters. This HAL has implemented common interfaces and called standard tinyalsa interfaces, which are highly portable. Here we use wm8904, which has few functions. You can use it directly.

Android Configuration
It mainly contains four files, audio_policy.conf media_profiles.xml media_codecs.xml mixer_paths.xml, which can be directly used without major changes.
In this case, the system should not use the default stub audio HAL, but the added audio HAL. However, most of them are not supported.
Debugging
The audio system calls several underlying libraries of libtinyalsa libaudioutils libaudioroute. Some general alsa devices have been transplanted to enable the configuration function, but the specific platform may not be able to run normally. The main reason is that these libraries are easy to implement and are not fully considered, their hardware details may be ignored by them. Similarly, we do not support time stamp for our wm8904, but tinyalsa must be turned off by default.

    disable tstamp for wm8904.        Change-Id: Ia22aa6ed39ede6214657487344d0488be93e5544diff --git a/pcm.c b/pcm.cindex 4501777..94cf6ee 100644--- a/pcm.c+++ b/pcm.c@@ -691,7 +691,7 @@ struct pcm *pcm_open(unsigned int card, unsigned int device,       memset(&sparams, 0, sizeof(sparams));-    sparams.tstamp_mode = SNDRV_PCM_TSTAMP_ENABLE;+    sparams.tstamp_mode = SNDRV_PCM_TSTAMP_NONE;     sparams.period_step = 1;      if (!config->start_threshold) {

If the specific parameters are incorrect or inappropriate, the Android driver engineer needs to check the hardware design and chip manual one by one to configure the optimal working state of an audio system. So that the user experience can be the best.


Android audio system analysis is based on 4.4.2
Audio HAL
Tinyalsa and audiorouteAndroid audio systems are Linux-based ALSA drivers. tinyalsa encapsulates audio interfaces based on the alsa drivers and provides them to audio HAL and audio HAL to Android audioflinger and other frameworks. HAL needs to enable and disable the audio hardware (which android considers as hardware ).

static inline int audio_hw_device_open(const struct hw_module_t* module,                                       struct audio_hw_device** device){    return module->methods->open(module, AUDIO_HARDWARE_INTERFACE,                                 (struct hw_device_t**)device);}static inline int audio_hw_device_close(struct audio_hw_device* device){    return device->common.close(&device->common);}

Data Stream struct audio_stream_out struct audio_stream_in that needs to be implemented in and out
The in mainly includes the read method for reading audio data, and the out mainly includes the write method, which writes data to the device for recording and playing respectively. For details, see hardware/libhardware/include/hardware/audio. h.
The method is the called tinyalsa interface. The pcm operation is as follows:

/* Open and close a stream */struct pcm *pcm_open(unsigned int card, unsigned int device,                     unsigned int flags, struct pcm_config *config);int pcm_close(struct pcm *pcm);int pcm_is_ready(struct pcm *pcm);/* Obtain the parameters for a PCM */struct pcm_params *pcm_params_get(unsigned int card, unsigned int device,                                  unsigned int flags);void pcm_params_free(struct pcm_params *pcm_params);unsigned int pcm_params_get_min(struct pcm_params *pcm_params,                                enum pcm_param param);unsigned int pcm_params_get_max(struct pcm_params *pcm_params,                                enum pcm_param param);/* Set and get config */int pcm_get_config(struct pcm *pcm, struct pcm_config *config);int pcm_set_config(struct pcm *pcm, struct pcm_config *config);/* Returns a human readable reason for the last error */const char *pcm_get_error(struct pcm *pcm);/* Returns the sample size in bits for a PCM format. * As with ALSA formats, this is the storage size for the format, whereas the * format represents the number of significant bits. For example, * PCM_FORMAT_S24_LE uses 32 bits of storage. */unsigned int pcm_format_to_bits(enum pcm_format format);/* Returns the buffer size (int frames) that should be used for pcm_write. */unsigned int pcm_get_buffer_size(struct pcm *pcm);unsigned int pcm_frames_to_bytes(struct pcm *pcm, unsigned int frames);unsigned int pcm_bytes_to_frames(struct pcm *pcm, unsigned int bytes);/* Returns the pcm latency in ms */unsigned int pcm_get_latency(struct pcm *pcm);/* Returns available frames in pcm buffer and corresponding time stamp. * The clock is CLOCK_MONOTONIC if flag PCM_MONOTONIC was specified in pcm_open, * otherwise the clock is CLOCK_REALTIME. * For an input stream, frames available are frames ready for the * application to read. * For an output stream, frames available are the number of empty frames available * for the application to write. */int pcm_get_htimestamp(struct pcm *pcm, unsigned int *avail,                       struct timespec *tstamp);/* Write data to the fifo. * Will start playback on the first write or on a write that * occurs after a fifo underrun. */int pcm_write(struct pcm *pcm, const void *data, unsigned int count);int pcm_read(struct pcm *pcm, void *data, unsigned int count);/* * mmap() support. */int pcm_mmap_write(struct pcm *pcm, const void *data, unsigned int count);int pcm_mmap_read(struct pcm *pcm, void *data, unsigned int count);int pcm_mmap_begin(struct pcm *pcm, void **areas, unsigned int *offset,                   unsigned int *frames);int pcm_mmap_commit(struct pcm *pcm, unsigned int offset, unsigned int frames);/* Start and stop a PCM channel that doesn't transfer data */int pcm_start(struct pcm *pcm);int pcm_stop(struct pcm *pcm);/* Interrupt driven API */int pcm_wait(struct pcm *pcm, int timeout);/* Change avail_min after the stream has been opened with no need to stop the stream. * Only accepted if opened with PCM_MMAP and PCM_NOIRQ flags */int pcm_set_avail_min(struct pcm *pcm, int avail_min);

It is worth mentioning that HAL does not include the route operation, and audio route is handed over to libaudioroute. so, it also calls the tinyalsa interface and contains an xml parser to parse the route configuration data in mixer_paths.xml. In this way, the system can operate the pcm and mixer of alsa. In theory, the system should be able to play the audio. The tool provided by tinyalsa can be used for testing. Of course, the HAL interface cannot be tested. Tinycap tinymix tinypcminfo tinyplay

Tinyplay can play pure audio data in wav format. Tinymix allows you to view and configure the audio path:

root@sama5d3:/ # tinymix                                                       Mixer name: 'wm8904 @ SAMA5D3EK'Number of controls: 41ctl     type    num     name                                     value0       INT     1       EQ1 Volume                               121       INT     1       EQ2 Volume                               122       INT     1       EQ3 Volume                               123       INT     1       EQ4 Volume                               124       INT     1       EQ5 Volume                               125       INT     2       Digital Capture Volume                   96 966       ENUM    1       Left Caputure Mode                       Single-Ended7       ENUM    1       Right Capture Mode                       Single-Ended8       INT     2       Capture Volume                           5 59       BOOL    2       Capture Switch                           Off Off10      BOOL    1       High Pass Filter Switch                  On11      ENUM    1       High Pass Filter Mode                    Hi-fi12      BOOL    1       ADC 128x OSR Switch                      On13      INT     1       Digital Playback Boost Volume            014      INT     2       Digital Playback Volume                  96 9615      INT     2       Headphone Volume                         45 4516      BOOL    2       Headphone Switch                         On On17      BOOL    2       Headphone ZC Switch                      On On18      INT     2       Line Output Volume                       57 5719      BOOL    2       Line Output Switch                       On On20      BOOL    2       Line Output ZC Switch                    On On21      BOOL    1       EQ Switch                                Off22      BOOL    1       DRC Switch                               Off23      ENUM    1       DRC Path                                 ADC24      BOOL    1       DAC OSRx2 Switch                         Off25      BOOL    1       DAC Deemphasis Switch                    Off26      INT     2       Digital Sidetone Volume                  0 027      ENUM    1       LINER Mux                                DAC28      ENUM    1       LINEL Mux                                DAC29      ENUM    1       HPR Mux                                  DAC30      ENUM    1       HPL Mux                                  DAC31      ENUM    1       Right Sidetone                           None32      ENUM    1       Left Sidetone                            None33      ENUM    1       DACR Mux                                 Right34      ENUM    1       DACL Mux                                 Left35      ENUM    1       AIFOUTR Mux                              Right36      ENUM    1       AIFOUTL Mux                              Left37      ENUM    1       Right Capture Inverting Mux              IN1R38      ENUM    1       Right Capture Mux                        IN2R39      ENUM    1       Left Capture Inverting Mux               IN1L40      ENUM    1       Left Capture Mux                         IN2L

Audioflinger
Audioflinger is an audio server that loads audio hal and processes audio requests from the audio application. This analysis has a lot, refer to the following: http://blog.csdn.net/DroidPhone/article/details/5941344