1. Audio Drivers
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2. What to Expect?
Introduction to Linux' Audio Subsystem
Audio Vertical: Sound Core
Audio Horizontal
Putting them all together: Porting
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3. Audio Subsystem
ALSA conforming Apps User Space OSS conforming Apps
(aplay, arecord, mplayer, ...) (rawplay, rawrec, ...)
/dev/snd/pcmCXDYz,
/proc/asound /dev/dsp, /dev/adsp,
/dev/snd/controlCX,
/sys/class/sound /dev/mixer, /dev/audio
/dev/snd/timer
OSS Emulation Layer
(snd_pcm_oss, snd_mixer_oss)
ALSA (Sound) Core
Control Data
Audio Codec Driver
I2C Audio Controller Driver
Kernel Space
I2S
Hardware Space
MIC
Audio Controller Audio Codec
Speaker
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4. ALSA Sound Card Interface
Header: <linux/sound/core.h>
Data Structure: struct snd_card
APIs
int snd_card_create(int idx, const char *id,
struct module *module, int extra_size, struct
snd_card **card_ret);
int snd_card_register(struct snd_card *card);
int snd_card_free(struct snd_card *card);
Inverse of both the above
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5. ALSA PCM Interface
Header: <linux/sound/pcm.h>
Data Structure
struct snd_pcm
struct snd_pcm_ops
APIs
int snd_pcm_new(struct snd_card *card, const char *id, int device,
int playback_count, int capture_count, struct snd_pcm **rpcm);
void snd_pcm_set_ops(struct snd_pcm * pcm, int direction, struct
snd_pcm_ops *ops);
int snd_pcm_lib_malloc_pages(struct snd_pcm_substream
*substream, size_t size); // Typically used in hw_params
int snd_pcm_lib_free_pages(struct snd_pcm_substream
*substream); // Typically used in hw_free
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6. struct pcm_ops
int (*open)(struct snd_pcm_substream *substream);
int (*close)(struct snd_pcm_substream *substream);
int (*ioctl) /* Miscellaneous controls */
(struct snd_pcm_substream * substream, unsigned int cmd, void *arg);
int (*hw_params) /* Set h/w params & allocate the buffer */
(struct snd_pcm_substream *substream, struct snd_pcm_hw_params
*params);
int (*hw_free) /* Free the buffer */
(struct snd_pcm_substream *substream);
int (*prepare) /* Set audio parameters for transfer */
(struct snd_pcm_substream *substream);
int (*trigger) /* Trigger the transfer */
(struct snd_pcm_substream *substream, int cmd);
© 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 6
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7. ALSA Sound Card Interface
Header: <linux/sound/control.h>
Data Structure: struct snd_kcontrol_new
APIs
int snd_ctl_add(struct snd_card * card, struct
snd_kcontrol * kcontrol);
int snd_ctl_remove(struct snd_card * card,
struct snd_kcontrol * kcontrol);
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8. Porting a Audio Driver
Standard Audio Codec
Mostly involves changing pin assignments as
per the Board Design
New Audio Codec
Complete Driver as per the preceeding
discussions, need to be implemented
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9. Browse some Audio Drivers
ALSA driver examples: sound/
arm/aaci.c
soc/soc-*.c {core, pcm, utils}
Codec driver examples: sound/
arm/aaci.c
soc/codecs/twl4030.c
Browse & Discuss any
© 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 9
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10. What all have we learnt?
Introduction to Linux' Audio Subsystem
Audio Vertical
ALSA Sound Core & its Programming I/f
Audio Horizontal
Audio Codec Driver
Audio Controller Driver
Putting them all together: Porting
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11. Any Queries?
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