ALSA: aloop: Add 'hrtimer' option to timer_source The snd-aloop driver currently defaults to using the system jiffies timer (struct timer_list). On systems configured with a low timer interrupt frequency (e.g., CONFIG_HZ=250), the jiffies resolution (4ms per tick) is insufficient for precise audio timing. For example, a 10ms audio period requires 2.5 jiffies ticks, causing timing jitter that leads to capture underruns. Introduce "hrtimer" as a supported timer_source option. When timer_source="hrtimer" is set, aloop uses high-resolution timers (hrtimer) to drive period updates. This provides nanosecond-level accuracy regardless of CONFIG_HZ and operates independently of other hardware audio cards. Signed-off-by: Yu-Hsuan Hsu <yuhsuan@chromium.org> Link: https://patch.msgid.link/20260705125941.1203871-1-yuhsuan@chromium.org Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c index 06bfe09..236f49a 100644 --- a/sound/drivers/aloop.c +++ b/sound/drivers/aloop.c
@@ -23,6 +23,7 @@ #include <linux/wait.h> #include <linux/module.h> #include <linux/platform_device.h> +#include <linux/hrtimer.h> #include <sound/core.h> #include <sound/control.h> #include <sound/pcm.h> @@ -55,7 +56,7 @@ MODULE_PARM_DESC(pcm_substreams, "PCM substreams # (1-8) for loopback driver."); module_param_array(pcm_notify, int, NULL, 0444); MODULE_PARM_DESC(pcm_notify, "Break capture when PCM format/rate/channels changes."); module_param_array(timer_source, charp, NULL, 0444); -MODULE_PARM_DESC(timer_source, "Sound card name or number and device/subdevice number of timer to be used. Empty string for jiffies timer [default]."); +MODULE_PARM_DESC(timer_source, "Sound card name or number and device/subdevice number of timer to be used. Empty string for jiffies timer [default], 'hrtimer' for high-resolution timer."); #define NO_PITCH 100000 @@ -161,8 +162,11 @@ struct loopback_pcm { unsigned int period_size_frac; /* period size in jiffies ticks */ unsigned int last_drift; unsigned long last_jiffies; - /* If jiffies timer is used */ + /* If jiffies / hrtimer is used */ struct timer_list timer; +#ifdef CONFIG_HIGH_RES_TIMERS + struct hrtimer hrtimer; +#endif /* size of per channel buffer in case of non-interleaved access */ unsigned int channel_buf_n; @@ -232,6 +236,31 @@ static int loopback_jiffies_timer_start(struct loopback_pcm *dpcm) return 0; } +#ifdef CONFIG_HIGH_RES_TIMERS +/* call in cable->lock */ +static int loopback_hrtimer_start(struct loopback_pcm *dpcm) +{ + unsigned long tick; + unsigned int rate_shift = get_rate_shift(dpcm); + + if (rate_shift != dpcm->pcm_rate_shift) { + dpcm->pcm_rate_shift = rate_shift; + dpcm->period_size_frac = frac_pos(dpcm, dpcm->pcm_period_size); + } + if (dpcm->period_size_frac <= dpcm->irq_pos) { + dpcm->irq_pos %= dpcm->period_size_frac; + dpcm->period_update_pending = 1; + } + tick = dpcm->period_size_frac - dpcm->irq_pos; + tick = DIV_ROUND_UP(tick, dpcm->pcm_bps); + hrtimer_start(&dpcm->hrtimer, + ns_to_ktime(div_u64((u64)tick * NSEC_PER_SEC, HZ)), + HRTIMER_MODE_REL_SOFT); + + return 0; +} +#endif + /* call in cable->lock */ static int loopback_snd_timer_start(struct loopback_pcm *dpcm) { @@ -270,6 +299,16 @@ static inline int loopback_jiffies_timer_stop(struct loopback_pcm *dpcm) return 0; } +#ifdef CONFIG_HIGH_RES_TIMERS +/* call in cable->lock */ +static inline int loopback_hrtimer_stop(struct loopback_pcm *dpcm) +{ + hrtimer_cancel(&dpcm->hrtimer); + + return 0; +} +#endif + /* call in cable->lock */ static int loopback_snd_timer_stop(struct loopback_pcm *dpcm) { @@ -300,6 +339,15 @@ static inline int loopback_jiffies_timer_stop_sync(struct loopback_pcm *dpcm) return 0; } +#ifdef CONFIG_HIGH_RES_TIMERS +static inline int loopback_hrtimer_stop_sync(struct loopback_pcm *dpcm) +{ + hrtimer_cancel(&dpcm->hrtimer); + + return 0; +} +#endif + /* call in loopback->cable_lock */ static int loopback_snd_timer_close_cable(struct loopback_pcm *dpcm) { @@ -734,6 +782,30 @@ static void loopback_jiffies_timer_function(struct timer_list *t) snd_pcm_period_elapsed(dpcm->substream); } +#ifdef CONFIG_HIGH_RES_TIMERS +static enum hrtimer_restart loopback_hrtimer_function(struct hrtimer *t) +{ + struct loopback_pcm *dpcm = container_of(t, struct loopback_pcm, hrtimer); + bool period_elapsed = false; + + scoped_guard(spinlock_irqsave, &dpcm->cable->lock) { + if (loopback_jiffies_timer_pos_update(dpcm->cable) & + (1 << dpcm->substream->stream)) { + loopback_hrtimer_start(dpcm); + if (dpcm->period_update_pending) { + dpcm->period_update_pending = 0; + period_elapsed = true; + } + } + } + + if (period_elapsed) + snd_pcm_period_elapsed(dpcm->substream); + + return HRTIMER_NORESTART; +} +#endif + /* call in cable->lock */ static int loopback_snd_timer_check_resolution(struct snd_pcm_runtime *runtime, unsigned long resolution) @@ -907,6 +979,21 @@ static void loopback_jiffies_timer_dpcm_info(struct loopback_pcm *dpcm, snd_iprintf(buffer, " timer_expires:\t%lu\n", dpcm->timer.expires); } +#ifdef CONFIG_HIGH_RES_TIMERS +static void loopback_hrtimer_dpcm_info(struct loopback_pcm *dpcm, + struct snd_info_buffer *buffer) +{ + snd_iprintf(buffer, " update_pending:\t%u\n", + dpcm->period_update_pending); + snd_iprintf(buffer, " irq_pos:\t\t%u\n", dpcm->irq_pos); + snd_iprintf(buffer, " period_frac:\t%u\n", dpcm->period_size_frac); + snd_iprintf(buffer, " last_jiffies:\t%lu (%lu)\n", + dpcm->last_jiffies, jiffies); + snd_iprintf(buffer, " timer_expires:\t%llu\n", + ktime_to_ns(hrtimer_get_expires(&dpcm->hrtimer))); +} +#endif + static void loopback_snd_timer_dpcm_info(struct loopback_pcm *dpcm, struct snd_info_buffer *buffer) { @@ -1097,6 +1184,26 @@ static const struct loopback_ops loopback_jiffies_timer_ops = { .dpcm_info = loopback_jiffies_timer_dpcm_info, }; +#ifdef CONFIG_HIGH_RES_TIMERS +static int loopback_hrtimer_open(struct loopback_pcm *dpcm) +{ + hrtimer_setup(&dpcm->hrtimer, loopback_hrtimer_function, + CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT); + + return 0; +} + +static const struct loopback_ops loopback_hrtimer_ops = { + .open = loopback_hrtimer_open, + .start = loopback_hrtimer_start, + .stop = loopback_hrtimer_stop, + .stop_sync = loopback_hrtimer_stop_sync, + .close_substream = loopback_hrtimer_stop_sync, + .pos_update = loopback_jiffies_timer_pos_update, + .dpcm_info = loopback_hrtimer_dpcm_info, +}; +#endif + static int loopback_parse_timer_id(const char *str, struct snd_timer_id *tid) { @@ -1274,7 +1381,12 @@ static int loopback_open(struct snd_pcm_substream *substream) spin_lock_init(&cable->lock); snd_refcount_init(&cable->stop_count); cable->hw = loopback_pcm_hardware; - if (loopback->timer_source) +#ifdef CONFIG_HIGH_RES_TIMERS + if (loopback->timer_source && !strcmp(loopback->timer_source, "hrtimer")) + cable->ops = &loopback_hrtimer_ops; + else +#endif + if (loopback->timer_source && loopback->timer_source[0]) cable->ops = &loopback_snd_timer_ops; else cable->ops = &loopback_jiffies_timer_ops;