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snd_uaudio(4): Adapt buffer length to buffer_ms tunable.

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Adapt the length of the driver side audio buffer to the USB transfer
interval, which is adjustable through the buffer_ms tunable. This
eliminates unnecessary latency in USB audio playback.

To reduce power consumption caused by frequent CPU wakeups, increase the
default buffer_ms value to 4ms. In combination with adaptive buffer
length, this still results in less roundtrip latency compared to the
previous 2ms default.

Extend the buffer_ms value range to 1ms for low latency applications.

MFC after:	2 weeks
Reviewed by:	christos
Differential Revision:	https://reviews.freebsd.org/D41942
This commit is contained in:
Florian Walpen 2024-02-12 13:04:57 +02:00 committed by Christos Margiolis
parent 53c768e683
commit b2e97edffd
1 changed files with 60 additions and 10 deletions
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70
sys/dev/sound/usb/uaudio.c
View file

@ -96,7 +96,7 @@
static int uaudio_default_rate = 0; /* use rate list */
static int uaudio_default_bits = 32;
static int uaudio_default_channels = 0; /* use default */
static int uaudio_buffer_ms = 2;
static int uaudio_buffer_ms = 4;
static bool uaudio_handle_hid = true;
static SYSCTL_NODE(_hw_usb, OID_AUTO, uaudio, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
@ -110,6 +110,9 @@ SYSCTL_INT(_hw_usb_uaudio, OID_AUTO, default_bits, CTLFLAG_RWTUN,
SYSCTL_INT(_hw_usb_uaudio, OID_AUTO, default_channels, CTLFLAG_RWTUN,
&uaudio_default_channels, 0, "uaudio default sample channels");
#define UAUDIO_BUFFER_MS_MIN 1
#define UAUDIO_BUFFER_MS_MAX 8
static int
uaudio_buffer_ms_sysctl(SYSCTL_HANDLER_ARGS)
{
@ -121,10 +124,10 @@ uaudio_buffer_ms_sysctl(SYSCTL_HANDLER_ARGS)
if (err != 0 || req->newptr == NULL || val == uaudio_buffer_ms)
return (err);
if (val > 8)
val = 8;
else if (val < 2)
val = 2;
if (val > UAUDIO_BUFFER_MS_MAX)
val = UAUDIO_BUFFER_MS_MAX;
else if (val < UAUDIO_BUFFER_MS_MIN)
val = UAUDIO_BUFFER_MS_MIN;
uaudio_buffer_ms = val;
@ -133,7 +136,7 @@ uaudio_buffer_ms_sysctl(SYSCTL_HANDLER_ARGS)
SYSCTL_PROC(_hw_usb_uaudio, OID_AUTO, buffer_ms,
CTLTYPE_INT | CTLFLAG_RWTUN | CTLFLAG_MPSAFE, 0, sizeof(int),
uaudio_buffer_ms_sysctl, "I",
"uaudio buffering delay from 2ms to 8ms");
"uaudio buffering delay in milliseconds, from 1 to 8");
#ifdef USB_DEBUG
static int uaudio_debug;
@ -1310,13 +1313,60 @@ uaudio_detach(device_t dev)
return (0);
}
static uint32_t
uaudio_get_interval_frames(const usb_endpoint_descriptor_audio_t *ed)
{
uint32_t frames = 1;
/* Isochronous transfer interval is 2^(bInterval - 1) frames. */
if (ed->bInterval >= 1 && ed->bInterval <= 16)
frames = (1 << (ed->bInterval - 1));
/* Limit transfer interval to maximum number of frames. */
if (frames > UAUDIO_NFRAMES)
frames = UAUDIO_NFRAMES;
return (frames);
}
static uint32_t
uaudio_get_buffer_ms(struct uaudio_softc *sc, uint32_t int_frames)
{
uint32_t ms = 1;
uint32_t fps = usbd_get_isoc_fps(sc->sc_udev);
/* Make sure a whole USB transfer interval fits into the buffer. */
if (fps >= 1000 && int_frames > 0 && int_frames <= UAUDIO_NFRAMES) {
/* Convert interval frames to milliseconds. */
ms = ((int_frames * 1000) / fps);
}
/* Respect minimum buffer length set through buffer_ms tunable. */
if (ms < uaudio_buffer_ms)
ms = uaudio_buffer_ms;
/* Limit buffer length to 8 milliseconds. */
if (ms > UAUDIO_BUFFER_MS_MAX)
ms = UAUDIO_BUFFER_MS_MAX;
return (ms);
}
static uint32_t
uaudio_get_buffer_size(struct uaudio_chan *ch, uint8_t alt)
{
struct uaudio_chan_alt *chan_alt = &ch->usb_alt[alt];
/* We use 2 times 8ms of buffer */
uint32_t buf_size = chan_alt->sample_size *
howmany(chan_alt->sample_rate * (UAUDIO_NFRAMES / 8), 1000);
uint32_t int_frames, ms, buf_size;
/* USB transfer interval in frames, from endpoint descriptor. */
int_frames = uaudio_get_interval_frames(chan_alt->p_ed1);
/* Buffer length in milliseconds, and in bytes of audio data. */
ms = uaudio_get_buffer_ms(ch->priv_sc, int_frames);
buf_size = chan_alt->sample_size *
howmany(chan_alt->sample_rate * ms, 1000);
return (buf_size);
}
static uint32_t
uaudio_max_buffer_size(struct uaudio_chan *ch, uint8_t alt)
{
struct uaudio_chan_alt *chan_alt = &ch->usb_alt[alt];
uint32_t buf_size;
/* Maximum buffer length is 8 milliseconds. */
buf_size = chan_alt->sample_size *
howmany(chan_alt->sample_rate * UAUDIO_BUFFER_MS_MAX, 1000);
return (buf_size);
}
@ -2626,7 +2676,7 @@ uaudio_chan_init(struct uaudio_chan *ch, struct snd_dbuf *b,
buf_size = 0;
for (x = 0; x != ch->num_alt; x++) {
uint32_t temp = uaudio_get_buffer_size(ch, x);
uint32_t temp = uaudio_max_buffer_size(ch, x);
if (temp > buf_size)
buf_size = temp;
}