src/kernel/linux/v4.14/Documentation/sound/soc/codec.rst

=======================
ASoC Codec Class Driver
=======================

The codec class driver is generic and hardware independent code that configures
the codec, FM, MODEM, BT or external DSP to provide audio capture and playback.
It should contain no code that is specific to the target platform or machine.
All platform and machine specific code should be added to the platform and
machine drivers respectively.

Each codec class driver *must* provide the following features:-

1. Codec DAI and PCM configuration
2. Codec control IO - using RegMap API
3. Mixers and audio controls
4. Codec audio operations
5. DAPM description.
6. DAPM event handler.

Optionally, codec drivers can also provide:-

7. DAC Digital mute control.

Its probably best to use this guide in conjunction with the existing codec
driver code in sound/soc/codecs/

ASoC Codec driver breakdown
===========================

Codec DAI and PCM configuration
-------------------------------
Each codec driver must have a struct snd_soc_dai_driver to define its DAI and
PCM capabilities and operations. This struct is exported so that it can be
registered with the core by your machine driver.

e.g.
::

  static struct snd_soc_dai_ops wm8731_dai_ops = {
	.prepare	= wm8731_pcm_prepare,
	.hw_params	= wm8731_hw_params,
	.shutdown	= wm8731_shutdown,
	.digital_mute	= wm8731_mute,
	.set_sysclk	= wm8731_set_dai_sysclk,
	.set_fmt	= wm8731_set_dai_fmt,
  };
  
  struct snd_soc_dai_driver wm8731_dai = {
	.name = "wm8731-hifi",
	.playback = {
		.stream_name = "Playback",
		.channels_min = 1,
		.channels_max = 2,
		.rates = WM8731_RATES,
		.formats = WM8731_FORMATS,},
	.capture = {
		.stream_name = "Capture",
		.channels_min = 1,
		.channels_max = 2,
		.rates = WM8731_RATES,
		.formats = WM8731_FORMATS,},
	.ops = &wm8731_dai_ops,
	.symmetric_rates = 1,
  };


Codec control IO
----------------
The codec can usually be controlled via an I2C or SPI style interface
(AC97 combines control with data in the DAI). The codec driver should use the
Regmap API for all codec IO. Please see include/linux/regmap.h and existing
codec drivers for example regmap usage.


Mixers and audio controls
-------------------------
All the codec mixers and audio controls can be defined using the convenience
macros defined in soc.h.
::

    #define SOC_SINGLE(xname, reg, shift, mask, invert)

Defines a single control as follows:-
::

  xname = Control name e.g. "Playback Volume"
  reg = codec register
  shift = control bit(s) offset in register
  mask = control bit size(s) e.g. mask of 7 = 3 bits
  invert = the control is inverted

Other macros include:-
::

    #define SOC_DOUBLE(xname, reg, shift_left, shift_right, mask, invert)

A stereo control
::

    #define SOC_DOUBLE_R(xname, reg_left, reg_right, shift, mask, invert)

A stereo control spanning 2 registers
::

    #define SOC_ENUM_SINGLE(xreg, xshift, xmask, xtexts)

Defines an single enumerated control as follows:-
::

   xreg = register
   xshift = control bit(s) offset in register
   xmask = control bit(s) size
   xtexts = pointer to array of strings that describe each setting

   #define SOC_ENUM_DOUBLE(xreg, xshift_l, xshift_r, xmask, xtexts)

Defines a stereo enumerated control


Codec Audio Operations
----------------------
The codec driver also supports the following ALSA PCM operations:-
::

  /* SoC audio ops */
  struct snd_soc_ops {
	int (*startup)(struct snd_pcm_substream *);
	void (*shutdown)(struct snd_pcm_substream *);
	int (*hw_params)(struct snd_pcm_substream *, struct snd_pcm_hw_params *);
	int (*hw_free)(struct snd_pcm_substream *);
	int (*prepare)(struct snd_pcm_substream *);
  };

Please refer to the ALSA driver PCM documentation for details.
http://www.alsa-project.org/~iwai/writing-an-alsa-driver/


DAPM description
----------------
The Dynamic Audio Power Management description describes the codec power
components and their relationships and registers to the ASoC core.
Please read dapm.txt for details of building the description.

Please also see the examples in other codec drivers.


DAPM event handler
------------------
This function is a callback that handles codec domain PM calls and system
domain PM calls (e.g. suspend and resume). It is used to put the codec
to sleep when not in use.

Power states:-
::

	SNDRV_CTL_POWER_D0: /* full On */
	/* vref/mid, clk and osc on, active */

	SNDRV_CTL_POWER_D1: /* partial On */
	SNDRV_CTL_POWER_D2: /* partial On */

	SNDRV_CTL_POWER_D3hot: /* Off, with power */
	/* everything off except vref/vmid, inactive */

	SNDRV_CTL_POWER_D3cold: /* Everything Off, without power */


Codec DAC digital mute control
------------------------------
Most codecs have a digital mute before the DACs that can be used to
minimise any system noise.  The mute stops any digital data from
entering the DAC.

A callback can be created that is called by the core for each codec DAI
when the mute is applied or freed.

i.e.
::

  static int wm8974_mute(struct snd_soc_dai *dai, int mute)
  {
	struct snd_soc_codec *codec = dai->codec;
	u16 mute_reg = snd_soc_read(codec, WM8974_DAC) & 0xffbf;

	if (mute)
		snd_soc_write(codec, WM8974_DAC, mute_reg | 0x40);
	else
		snd_soc_write(codec, WM8974_DAC, mute_reg);
	return 0;
  }