For compile time configuration, it’s common Linux practice to keep board- and application-specific configuration out of the main driver file, instead putting it into the board support file.

For devices on custom boards, as typical of embedded and SoC-(system-on-chip) based hardware, Linux uses platform_data to point to board-specific structures describing devices and how they are connected to the SoC. This can include available ports, chip variants, preferred modes, default initialization, additional pin roles, and so on. This shrinks the board-support packages (BSPs) and minimizes board and application specific #ifdefs in drivers.

21 Oct 2010 16:10

Declaring I2C devices

Unlike PCI or USB devices, I2C devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each I2C bus segment, and what address these devices are using. For this reason, the kernel code must instantiate I2C devices explicitly. There are different ways to achieve this, depending on the context and requirements. However the most common method is to declare the I2C devices by bus number.

This method is appropriate when the I2C bus is a system bus, as in many embedded systems, wherein each I2C bus has a number which is known in advance. It is thus possible to pre-declare the I2C devices that inhabit this bus. This is done with an array of struct i2c_board_info, which is registered by calling i2c_register_board_info().

So, to enable such a driver one need only edit the board support file by adding an appropriate entry to i2c_board_info.

For more information see: Documentation/i2c/instantiating-devices

21 Oct 2010 16:10

The I2C address of the SSM2518 depends on the setting of the ADDR pin

ADDR	I2C Address
0	0x34
1	0x36

static struct i2c_board_info __initdata bfin_i2c_board_info[] = {
?
	[--snip--]
	{
		I2C_BOARD_INFO("ssm2518", 0x34),
	},
	[--snip--]
}

static int __init stamp_init(void)
{
	[--snip--]
	i2c_register_board_info(0, bfin_i2c_board_info,
				ARRAY_SIZE(bfin_i2c_board_info));
	[--snip--]
?
	return 0;
}
arch_initcall(board_init);

Devicetree

i2s: i2c@41600000 {
	compatible = "...;
	...

	#size-cells = <0>;
	#address-cells = <1>;
            
	ssm2518: ssm2518@34 {
		compatible = "adi,ssm2518";
		reg = <0x34>;
		gpios = <&gpio 5 0>;
	};
};

ASoC DAPM Widgets

Name	Description
OUTL	Output for Left Channel
OUTR	Output for Right Channel

ALSA Controls

Name	Description
Playback De-emphasis Switch	Enable/Disable de-emphasis
Master Playback Volume	DAC Volume Control
Master Playback Switch	Unmutes/Mutes the DAC output
Amp Low Power Mode Switch	Enables/Disables the lower power mode of the Amplifier
DAC Low Power Mode Switch	Enables/Disables the lower power mode of the DAC
DRC Switch	Enable/Disable Dynamic Range Control
DRC Limiter Switch	Enable/Disable the DRC Limiter function
DRC Compressor Switch	Enable/Disable the DRC Compressor function
DRC Expander Switch	Enable/Disable the DRC Expander function
DRC Noise Gate Switch	Enable/Disable the DRC Noise Gate function
DRC Limiter Threshold Volume	DRC Limiter Threshold Setting. Relative to input
DRC Compressor Lower Threshold Volume	DRC Compressor Lower Threshold Setting. Relative to input
DRC Expander Upper Threshold Volume	DRC Expander Upper Threshold Setting. Relative to input
DRC Noise Gate Threshold Volume	DRC Noise Gate Threshold Setting. Relative to input
DRC Upper Output Threshold Volume	DRC Limiter Threshold Setting. Relative to input
DRC Lower Output Threshold Volume	DRC Minimum Output Signal Amplitude Setting. This is the minimum output level produced by the DRC and is used to indicate the expander lower threshold, or output signal level when the input rises beyond the noise gate threshold
DRC Post Volume	Post-DRC Gain Adjust Setting. This can be used to add additional gain after the DRC function to compensate for the overall reduction of system gain due to the DRC
DRC Peak Detector Attack Time	DRC Peak Detector Attack Time
DRC Peak Detector Release Time	DRC Peak Detector Release Time
DRC Attack Time	DRC Attack Time. Used to smooth the gain curve at the thresholds (knees) of each DRC function
DRC Decay Time	DRC Decay Time. Used to smooth the gain curve at the thresholds (knees) of each DRC function
DRC Hold Time	DRC Normal Operation Hold Time. Used to prevent the gain curve calculation from increasing too quickly
DRC Noise Gate Hold Time
DRC RMS Averaging Time	DRC RMS Detector Averaging Time. This is the averaging time for the rms level that is compared to the DRC thresholds

DAI configuration

The codec driver registers one DAI named “ssm2518-hifi”.

Supported DAI formats

Name	Supported by driver	Description
SND_SOC_DAIFMT_I2S	yes	I2S mode
SND_SOC_DAIFMT_RIGHT_J	yes	Right Justified mode
SND_SOC_DAIFMT_LEFT_J	yes	Left Justified mode
SND_SOC_DAIFMT_DSP_A	yes	data MSB after FRM LRC
SND_SOC_DAIFMT_DSP_B	yes	data MSB during FRM LRC
SND_SOC_DAIFMT_AC97	no	AC97 mode
SND_SOC_DAIFMT_PDM	no	Pulse density modulation

SND_SOC_DAIFMT_NB_NF	yes	Normal bit- and frameclock
SND_SOC_DAIFMT_NB_IF	yes	Normal bitclock, inverted frameclock
SND_SOC_DAIFMT_IB_NF	yes	Inverted frameclock, normal bitclock
SND_SOC_DAIFMT_IB_IF	yes	Inverted bit- and frameclock

SND_SOC_DAIFMT_CBM_CFM	no	Codec bit- and frameclock master
SND_SOC_DAIFMT_CBS_CFM	no	Codec bitclock slave, frameclock master
SND_SOC_DAIFMT_CBM_CFS	no	Codec bitclock master, frameclock slave
SND_SOC_DAIFMT_CBS_CFS	yes	Codec bit- and frameclock slave

Supported SYSCLK rates

The CODECs system clock can be configured for various input rates. When configuring the codec system clock use SSM2518_SYSCLK for the clock id.

The following list contains the supported system clock rates and their resulting sample-rates.

SYSCLK	Supported sample-rates
2048000, 4096000, 8192000, 8192000, 3200000, 6400000, 12800000	8kHz, 16kHz, 32kHz
2822000, 5644800, 11289600, 16934400, 22579200, 33868800, 4410000, 8820000, 17640000	11.025kHz, 22.05kHz, 44.1kHz
3072000, 6144000, 38864000, 4800000, 9600000, 19200000	12kHz, 24kHz, 48kHz, 96kHz
12288000, 16384000, 24576000	8kHz, 12kHz, 16kHz, 24kHz, 32kHz, 48kHz, 96kHz

SYSCLK configuration

To configure the devices sysclk the snd_soc_codec_set_sysclk() function can be used. It needs to be called at least once before playback starts. The clk_id parameter must always be SSM2518_SYSCLK and the dir parameter must always be SND_SOC_CLOCK_IN. The freq can be set to any of the frequencies above. The system clock can either be taken from the external MCLK or the BCLK. In case the system clock is taken from the BCLK the BCLK needs to be applied to the MCLK pin and the BLCK can be left unconnected. To use the MCLK as the system clock source the source parameter should to be set to SSM2518_SYSCLK_SRC_MCLK, to use the BCLK it should be set to SSM2518_SYSCLK_SRC_BCLK

#define SSM2518_SYSCLK 0 
?
enum ssm2518_sysclk_src {
    SSM2518_SYSCLK_SRC_MCLK = 0,
    SSM2518_SYSCLK_SRC_BCLK = 1,
};

Starting with Kernel v4.18 snd_soc_codec_set_sysclk interface was removed. Instead snd_soc_component_set_sysclk should be used

TDM configuration

If you want to use the SSM2518 in TDM mode you can configure it using snd_soc_dai_set_tdm_slot() from you ASoC board driver.

The following restrictions apply to the parameters of snd_soc_dai_set_tdm_slot().

tx_mask must have exactly two bits set. The most upper bit will be the slot of right channel, the most lower bit will be the slot left channel. E.g. 0x3 means the left channel will be at slot 0 and the right channel will be at slot 1 in the TDM stream.
rx_mask must be 0.
slots must be either 1, 2, 4, 8 or 16
width must be either 16, 24, 32

Example:

static int ssm2518_link_init(struct snd_soc_pcm_runtime *rtd)
{
    int ret;
?
    ret = snd_soc_dai_set_tdm_slot(rtd->codec_dai, 0x0c, 0x00, 8, 32);
    if (ret < 0)
        return ret;
?
    return 0;
}
?
static struct snd_soc_dai_link ssm2518_dai_link = {
    ...,
    .init = ssm2518_link_init,
};

Example DAI configuration

#include "../codecs/ssm2518.h"
?
static int pmod_amp3_init(struct snd_soc_pcm_runtime *rtd)
{
    return snd_soc_codec_set_sysclk(rtd->codec, SSM2518_SYSCLK,
                SSM2518_SYSCLK_SRC_MCLK, 12288000, SND_SOC_CLOCK_IN);
}
?
static const struct snd_soc_dapm_widget pmod_amp3_widgets[] = {
    SND_SOC_DAPM_SPK("Speaker Out", NULL),
};
?
static const struct snd_soc_dapm_route pmod_amp3_routes[] = {
    { "Speaker Out", NULL, "OUTL" },
    { "Speaker Out", NULL, "OUTR" },
};
?
static struct snd_soc_dai_link pmod_amp3_dai_link = {
    .name = "ssm2518",
    .stream_name = "ssm2518",
    .codec_dai_name = "ssm2518-hifi",
    .dai_fmt = SND_SOC_DAIFMT_I2S |
            SND_SOC_DAIFMT_NB_NF |
            SND_SOC_DAIFMT_CBS_CFS,
    .init = pmod_amp3_init,
};
?
static struct snd_soc_card pmod_amp3_card = {
    .name = "PMOD AMP3 SSM2518",
    .owner = THIS_MODULE,
    .dai_link = &pmod_amp3_dai_link,
    .num_links = 1,
    .dapm_widgets = pmod_amp3_widgets,
    .num_dapm_widgets = ARRAY_SIZE(pmod_amp3_widgets),
    .dapm_routes = pmod_amp3_routes,
    .num_dapm_routes = ARRAY_SIZE(pmod_amp3_routes),
    .fully_routed = true,
};