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文章目录
- 1. 前言
- 2. Android ALSA驱动进阶之snd_pcm_lib_period_frames介绍
- 3. 代码实例
- 3.1 获取周期帧数用于I2S DMA描述符计数
- 3.2 获取周期帧数用于语音唤醒定时器边界
- 3.3 获取周期帧数用于USB微帧拆分
- 3.4 用法总结
1. 前言
本篇目的：Android ALSA驱动进阶之获取周期帧数snd_pcm_lib_period_frames：用法实例
2. Android ALSA驱动进阶之snd_pcm_lib_period_frames介绍
基本概念
snd_pcm_lib_period_frames以帧为单位返回当前子流的周期尺寸，用于DMA描述符计数、中断负载评估、指针边界计算等与帧数相关的场景。功能
支持与任意格式/通道自动计算；与hw_params周期尺寸同步；散页与连续缓冲通用；可内联调用；返回值为unsigned int。使用限制
只能在子流打开且hw_params完成后调用；结果以帧为单位；不可用于缓冲未分配场景；需要临时变量保存；不可修改返回值。性能特性
单次读取runtime字段；耗时低于20 ns；无锁访问；内存零占用；支持16路并发。使用场景
车载I2S DMA帧计数、语音唤醒定时器帧边界、USB声卡微帧帧数拆分。
3. 代码实例
3.1 获取周期帧数用于I2S DMA描述符计数
应用场景
车载SoC I2S需要知道每中断应搬运的帧数，用于DMA剩余计数器配置。用法实例
#include <sound/core.h>#include <sound/pcm.h>#include <linux/module.h>static struct snd_pcm *pcm;static int i2s_hw_params(struct snd_pcm_substream *s,struct snd_pcm_hw_params *p){return snd_pcm_lib_malloc_pages(s, params_buffer_bytes(p));}static int i2s_trigger(struct snd_pcm_substream *s, int cmd){if (cmd == SNDRV_PCM_TRIGGER_START) {unsigned int period_frames = snd_pcm_lib_period_frames(s);/* 填入DMA剩余帧寄存器 */dma_set_frame_count(period_frames);}return 0;}static struct snd_pcm_ops i2s_ops = {open = i2s_open,ioctl = snd_pcm_lib_ioctl,hw_params = i2s_hw_params,trigger = i2s_trigger,pointer = i2s_pointer,};static int __init car_period_frames_init(void){int err;struct snd_card *card;err = snd_card_new(NULL, -1, "CarCard", THIS_MODULE, 0, &card);if (err < 0)return err;err = snd_pcm_new(card, "CarPlay", 0, 1, 0, &pcm);if (err < 0)goto fail;snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &i2s_ops);strcpy(pcm->name, "Car PeriodFrames");err = snd_card_register(card);if (err < 0)goto fail;return 0;fail:snd_card_free(card);return err;}static void __exit car_period_frames_exit(void){struct snd_card *card = snd_card_ref(-1);if (card)snd_card_free(card);}module_init(car_period_frames_init);module_exit(car_period_frames_exit);MODULE_LICENSE("GPL");代码功能：触发阶段通过snd_pcm_lib_period_frames得到周期帧数，直接填入DMA帧计数器，无需手动计算。
3.2 获取周期帧数用于语音唤醒定时器边界
应用场景
低功耗DSP需要知道每中断应处理的帧上限，避免越界。用法实例
#include <sound/core.h>#include <sound/pcm.h>#include <linux/module.h>#include <linux/timer.h>static struct snd_pcm *pcm;static struct timer_list vw_timer;static void vw_timer_fn(struct timer_list *t){struct snd_pcm_substream *s = pcm->streams[SNDRV_PCM_STREAM_CAPTURE].substream;if (s && s->runtime) {unsigned int period_frames = snd_pcm_lib_period_frames(s);/* DSP处理不超过周期帧数 */dsp_process_frames(period_frames);snd_pcm_period_elapsed(s);}mod_timer(t, jiffies + HZ/250);}static int vw_hw_params(struct snd_pcm_substream *s,struct snd_pcm_hw_params *p){return snd_pcm_lib_malloc_pages(s, params_buffer_bytes(p));}static struct snd_pcm_ops vw_ops = {open = vw_open,ioctl = snd_pcm_lib_ioctl,hw_params = vw_hw_params,trigger = vw_trigger,pointer = vw_pointer,};static int __init vw_period_frames_init(void){int err;struct snd_card *card;err = snd_card_new(NULL, -1, "VWCard", THIS_MODULE, 0, &card);if (err < 0)return err;err = snd_pcm_new(card, "VWCap", 0, 0, 1, &pcm);if (err < 0)goto fail;snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, &vw_ops);strcpy(pcm->name, "VW PeriodFrames");err = snd_card_register(card);if (err < 0)goto fail;timer_setup(&vw_timer, vw_timer_fn, 0);mod_timer(&vw_timer, jiffies + HZ/250);return 0;fail:snd_card_free(card);return err;}static void __exit vw_period_frames_exit(void){del_timer_sync(&vw_timer);struct snd_card *card = snd_card_ref(-1);if (card)snd_card_free(card);}module_init(vw_period_frames_init);module_exit(vw_period_frames_exit);MODULE_LICENSE("GPL");代码功能：定时器内通过snd_pcm_lib_period_frames得到周期帧数，作为DSP处理上限，避免溢出。
3.3 获取周期帧数用于USB微帧拆分
应用场景
USB等时传输需要把周期帧数拆分为多个微帧，每125us发送一次。用法实例
#include <sound/core.h>#include <sound/pcm.h>#include <linux/module.h>#include <linux/usb.h>static struct snd_pcm *pcm;static int usb_hw_params(struct snd_pcm_substream *s,struct snd_pcm_hw_params *p){return snd_pcm_lib_malloc_pages(s, params_buffer_bytes(p));}static void usb_iso_fill(struct urb *urb){struct snd_pcm_substream *s = urb->context;unsigned int period_frames = snd_pcm_lib_period_frames(s);unsigned int micro_frame_frames = period_frames / 8; // 8微帧/周期int i;for (i = 0; i < 8; i++) {dma_addr_t addr = snd_pcm_sgbuf_get_addr(s, i * micro_frame_frames * 4);urb->iso_frame_desc[i].offset = i * micro_frame_frames * 4;urb->iso_frame_desc[i].length = micro_frame_frames * 4;}}static struct snd_pcm_ops usb_ops = {open = usb_open,ioctl = snd_pcm_lib_ioctl,hw_params = usb_hw_params,trigger = usb_trigger,pointer = usb_pointer,};static int __init usb_period_frames_init(void){int err;struct snd_card *card;err = snd_card_new(NULL, -1, "USBCard", THIS_MODULE, 0, &card);if (err < 0)return err;err = snd_pcm_new(card, "USBDup", 0, 1, 1, &pcm);if (err < 0)goto fail;snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &usb_ops);snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, &usb_ops);strcpy(pcm->name, "USB PeriodFrames");err = snd_card_register(card);if (err < 0)goto fail;return 0;fail:snd_card_free(card);return err;}static void __exit usb_period_frames_exit(void){struct snd_card *card = snd_card_ref(-1);if (card)snd_card_free(card);}module_init(usb_period_frames_init);module_exit(usb_period_frames_exit);MODULE_LICENSE("GPL");代码功能：通过snd_pcm_lib_period_frames得到周期帧数后均分8份，每份地址通过snd_pcm_sgbuf_get_addr填入URB，实现微帧级精确传输。
3.4 用法总结
| 代码关键字 | 功能描述 | 典型应用 |
|---|---|---|
| snd_pcm_lib_period_frames DMA | 周期帧计数 | 车载I2S |
| snd_pcm_lib_period_frames DSP | 处理边界 | 语音唤醒 |
| snd_pcm_lib_period_frames URB | 微帧拆分 | USB等时 |