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https://github.com/alsa-project/alsa-utils
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In 2010, ALSA PCM interface got an flag of hardware parameters to suppress periodical interrupts, according to a request from PulseAudio developer. In typical PCM operation for usual hardware, PCM drivers configure the hardware to generate the periodical interrupts to notify that the same amount of data frames as a period of PCM buffer is actually transferred via serial sound interface. The flag can suppress this if the driver support it. There's some merits of this configuration: - No interrupt context run for PCM substream. The PCM substream is handled in any process context only. No need to care of race conditions between interrupt/process contexts. This is good for developers of drivers and applications. - CPU time is not used for handlers on the interrupt context. The CPU time can be dedicated for the other tasks. This is good in a point of Time Sharing System. - Hardware is not configured to generate interrupts. This is good in a point of reduction of overall power consumption. Disabling period interrupt is used for 'Timer-based scheduling' to consume data frames on PCM buffer independently of interrupt context. As noted, no interrupt context runs for PCM substream, thus any blocking operation is not released. Furthermore, system calls for multiplexed I/O is not also released without timeout. In this scheduling model, applications need to care of available space on PCM buffer by lapse of time, typically by yielding CPU and wait for rescheduling. For the yielding, timeout is calculated for preferable amount of PCM frames to process. This is an additional merit for applications, like sound servers. when an I/O thread of the server wait for the timeout, the other threads can process data frames for server clients. Furthermore, with usage of rewinding/forwarding, applications can achieve low latency between transmission position and handling position even if they uses large size of PCM buffers. But the timeout should be calculated with enough care of hardware capabilities. To disable period interrupt, used hardware should satisfy some requirements for data transmission: 1. Even if drivers don't handle interrupts to queue next data transmission, hardware voluntarily perform the data transmission when needed (typically by requesting DMA automatically). 2. hardware has a capability to report current position of data transmission with enough accuracy against the data transmission. developers refer this as 'granularity'. If hardware can always reports updated position after the data transmission finishes, the granularity equals to the size of period of PCM buffer. 3. a fine size of data transmission in one time. This size is decided depending on configuration of hardware or DMA controller, but for efficiency it may not be one byte. Thus some amount of data frame is transferred by one data transmission. Developers refer this as 'burst-ness'. The timeout should be calculated according to the item 2 and 3, however in current ALSA PCM interface supplemental information is not delivered from drivers to applications. Although at present userspace applications should be written by a speculative way for this point, there's few problems because there're a few hardware which satisfy the above items. However, when more drivers supports this feature, the problem may largely be exposed and bothers application developers. This commit adds an option to use 'timer-based scheduling' for data transmission. This commit adds '--sched-model' option, and the scheduling mode is enabled when 'timer' is assigned to the option by equal sign. Although there's some TODOs, you can see the scheduling mode in this simple program, like: $ axfer transfer --sched-model=timer -P -d 2 -D hw:0,3 /dev/urandom -f dat -vvv $ axfer transfer --sched-model=timer -C -d 2 -D hw:1,0 /dev/null -r 48000 -vvv Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp> Signed-off-by: Takashi Iwai <tiwai@suse.de>
93 lines
2.2 KiB
C
93 lines
2.2 KiB
C
// SPDX-License-Identifier: GPL-2.0
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//
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// xfer-libasound.h - a header for receiver/transmitter of frames by alsa-lib.
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//
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// Copyright (c) 2018 Takashi Sakamoto <o-takashi@sakamocchi.jp>
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//
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// Licensed under the terms of the GNU General Public License, version 2.
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#ifndef __ALSA_UTILS_AXFER_XFER_LIBASOUND__H_
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#define __ALSA_UTILS_AXFER_XFER_LIBASOUND__H_
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#include "xfer.h"
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#include "waiter.h"
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#define logging(state, ...) \
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snd_output_printf(state->log, __VA_ARGS__)
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enum sched_model {
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SCHED_MODEL_IRQ = 0,
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SCHED_MODEL_TIMER,
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SCHED_MODEL_COUNT,
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};
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struct xfer_libasound_ops;
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struct libasound_state {
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snd_pcm_t *handle;
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snd_output_t *log;
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snd_pcm_hw_params_t *hw_params;
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snd_pcm_sw_params_t *sw_params;
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const struct xfer_libasound_ops *ops;
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void *private_data;
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bool verbose;
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char *node_literal;
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char *waiter_type_literal;
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char *sched_model_literal;
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unsigned int msec_per_period;
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unsigned int msec_per_buffer;
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unsigned int frames_per_period;
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unsigned int frames_per_buffer;
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unsigned int msec_for_avail_min;
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unsigned int msec_for_start_threshold;
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unsigned int msec_for_stop_threshold;
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bool finish_at_xrun:1;
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bool nonblock:1;
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bool mmap:1;
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bool test_nowait:1;
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bool no_auto_resample:1;
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bool no_auto_channels:1;
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bool no_auto_format:1;
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bool no_softvol:1;
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bool use_waiter:1;
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enum waiter_type waiter_type;
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struct waiter_context *waiter;
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// For scheduling type.
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enum sched_model sched_model;
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};
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// For internal use in 'libasound' module.
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struct xfer_libasound_ops {
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int (*pre_process)(struct libasound_state *state);
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int (*process_frames)(struct libasound_state *state,
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unsigned int *frame_count,
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struct mapper_context *mapper,
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struct container_context *cntrs);
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void (*post_process)(struct libasound_state *state);
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unsigned int private_size;
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};
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int xfer_libasound_wait_event(struct libasound_state *state, int timeout_msec,
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unsigned short *revents);
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extern const struct xfer_libasound_ops xfer_libasound_irq_rw_ops;
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extern const struct xfer_libasound_ops xfer_libasound_irq_mmap_r_ops;
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extern const struct xfer_libasound_ops xfer_libasound_irq_mmap_w_ops;
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extern const struct xfer_libasound_ops xfer_libasound_timer_mmap_w_ops;
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extern const struct xfer_libasound_ops xfer_libasound_timer_mmap_r_ops;
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#endif
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