最近在一個項目中需要使用到服務器上的聲卡，當然也不是需要真的物理聲卡(真有物理聲卡，服務器在機房中播放音樂，放電影？)。我們需要的是虛擬語音聲卡，能處理客戶端到服務器的語音流，不需要真正的播放。

我們需要的這個虛擬聲卡，在linux中作為一個內核模塊存在、運行，snd-dummy。

默認的snd-dummy模塊的聲道數是8,遠少于項目需要的聲道數，現在試著將其改到128.

這個模塊現在已經包含在內核模塊中，但它是從alsa-driver包中提取出來的，與其同步，與其重新編譯內核，不如使用alsa-driver包編譯這個模塊。

下載解壓alsa-driver-1.0.23.tar.bz2，轉到alsa-driver-1.0.23/alsa-kernel/drivers,修改dummy.c源文件，第66行，將賦值的8改為128，

static int pcm_substreams[SNDRV_CARDS] = {[0 ... (SNDRV_CARDS - 1)] = 128};

轉到源碼根目錄，cd alsa-driver-1.0.23,執行configure腳本，

./configure --with-cards=dummy,usb-audio,hda-intel,hrtimer,rtctimer --with-card-options=all

make

find -name *.ko

./pci/hda/snd-hda-codec-intelhdmi.ko

./pci/hda/snd-hda-codec-nvhdmi.ko

./pci/hda/snd-hda-codec-realtek.ko

./pci/hda/snd-hda-codec-idt.ko

./pci/hda/snd-hda-intel.ko

./pci/hda/snd-hda-codec-ca0110.ko

./pci/hda/snd-hda-codec-via.ko

./pci/hda/snd-hda-codec-atihdmi.ko

./pci/hda/snd-hda-codec-conexant.ko

./pci/hda/snd-hda-codec.ko

./pci/hda/snd-hda-codec-cirrus.ko

./pci/hda/snd-hda-codec-si3054.ko

./pci/hda/snd-hda-codec-analog.ko

./pci/hda/snd-hda-codec-cmedia.ko

./usb/snd-usb-audio.ko

./usb/snd-usbmidi-lib.ko

./acore/oss/snd-mixer-oss.ko

./acore/oss/snd-pcm-oss.ko

./acore/seq/oss/snd-seq-oss.ko

./acore/seq/snd-seq-midi.ko

./acore/seq/snd-seq-midi-event.ko

./acore/seq/snd-seq.ko

./acore/seq/snd-seq-device.ko

./acore/snd-hwdep.ko

./acore/snd-page-alloc.ko

./acore/snd-rawmidi.ko

./acore/snd-timer.ko

./acore/snd.ko

./acore/snd-pcm.ko

./drivers/snd-dummy.ko

./modules/snd-hda-codec-intelhdmi.ko

./modules/snd-seq-oss.ko

./modules/snd-hda-codec-nvhdmi.ko

./modules/snd-hwdep.ko

./modules/snd-seq-midi.ko

./modules/snd-hda-codec-realtek.ko

./modules/snd-seq-midi-event.ko

./modules/snd-page-alloc.ko

./modules/snd-hda-codec-idt.ko

./modules/snd-hda-intel.ko

./modules/snd-hda-codec-ca0110.ko

./modules/snd-rawmidi.ko

./modules/snd-usb-audio.ko

./modules/snd-hda-codec-via.ko

./modules/snd-hda-codec-atihdmi.ko

./modules/snd-timer.ko

./modules/snd-mixer-oss.ko

./modules/snd-dummy.ko

./modules/snd-hda-codec-conexant.ko

./modules/snd-hda-codec.ko

./modules/snd-usbmidi-lib.ko

./modules/snd-hda-codec-cirrus.ko

./modules/snd-hda-codec-si3054.ko

./modules/snd.ko

./modules/snd-pcm-oss.ko

./modules/snd-pcm.ko

./modules/snd-seq.ko

./modules/snd-seq-device.ko

./modules/snd-hda-codec-analog.ko

./modules/snd-hda-codec-cmedia.ko

snd-dummy.ko已經編譯好了，首先卸載該內核模塊，

rmmod snd-dummy

把modules/snd-dummy.ko拷貝到/lib/modules/`uname -r`/kernel/sound/drivers/目錄，

加載內核模塊，insmod snd-dummy。

不過這個過程可能需要同時更新其他的模塊，按照相應的目錄存儲結構覆蓋舊版本內核模塊。

直到能正常加載snd-dummy模塊，

最后使用aplay -L, aplay -l命令檢測最新的dummy虛擬聲卡及聲道狀態，

debian118:~/alsa-driver-1.0.23# aplay -L

default:CARD=Dummy

Dummy, Dummy PCM

Default Audio Device

null

Discard all samples (playback) or generate zero samples (capture)

debian118:~/alsa-driver-1.0.23# aplay -l

**** List of PLAYBACK Hardware Devices ****

card 0: Dummy [Dummy], device 0: Dummy PCM [Dummy PCM]

Subdevices: 128/128

Subdevice #0: subdevice #0

Subdevice #1: subdevice #1

Subdevice #2: subdevice #2

Subdevice #3: subdevice #3

Subdevice #4: subdevice #4

Subdevice #5: subdevice #5

Subdevice #6: subdevice #6

Subdevice #7: subdevice #7

Subdevice #8: subdevice #8

Subdevice #9: subdevice #9

Subdevice #10: subdevice #10

Subdevice #11: subdevice #11

Subdevice #12: subdevice #12

Subdevice #13: subdevice #13

Subdevice #14: subdevice #14

Subdevice #15: subdevice #15

Subdevice #16: subdevice #16

Subdevice #17: subdevice #17

Subdevice #18: subdevice #18

Subdevice #19: subdevice #19

Subdevice #20: subdevice #20

Subdevice #21: subdevice #21

Subdevice #22: subdevice #22

Subdevice #23: subdevice #23

Subdevice #24: subdevice #24

Subdevice #25: subdevice #25

Subdevice #26: subdevice #26

Subdevice #27: subdevice #27

Subdevice #28: subdevice #28

Subdevice #29: subdevice #29

Subdevice #30: subdevice #30

Subdevice #31: subdevice #31

Subdevice #32: subdevice #32

Subdevice #33: subdevice #33

Subdevice #34: subdevice #34

Subdevice #35: subdevice #35

Subdevice #36: subdevice #36

Subdevice #37: subdevice #37

Subdevice #38: subdevice #38

Subdevice #39: subdevice #39

Subdevice #40: subdevice #40

Subdevice #41: subdevice #41

Subdevice #42: subdevice #42

Subdevice #43: subdevice #43

Subdevice #44: subdevice #44

Subdevice #45: subdevice #45

Subdevice #46: subdevice #46

Subdevice #47: subdevice #47

Subdevice #48: subdevice #48

Subdevice #49: subdevice #49

Subdevice #50: subdevice #50

Subdevice #51: subdevice #51

Subdevice #52: subdevice #52

Subdevice #53: subdevice #53

Subdevice #54: subdevice #54

Subdevice #55: subdevice #55

Subdevice #56: subdevice #56

Subdevice #57: subdevice #57

Subdevice #58: subdevice #58

Subdevice #59: subdevice #59

Subdevice #60: subdevice #60

Subdevice #61: subdevice #61

Subdevice #62: subdevice #62

Subdevice #63: subdevice #63

Subdevice #64: subdevice #64

Subdevice #65: subdevice #65

Subdevice #66: subdevice #66

Subdevice #67: subdevice #67

Subdevice #68: subdevice #68

Subdevice #69: subdevice #69

Subdevice #70: subdevice #70

Subdevice #71: subdevice #71

Subdevice #72: subdevice #72

Subdevice #73: subdevice #73

Subdevice #74: subdevice #74

Subdevice #75: subdevice #75

Subdevice #76: subdevice #76

Subdevice #77: subdevice #77

Subdevice #78: subdevice #78

Subdevice #79: subdevice #79

Subdevice #80: subdevice #80

Subdevice #81: subdevice #81

Subdevice #82: subdevice #82

Subdevice #83: subdevice #83

Subdevice #84: subdevice #84

Subdevice #85: subdevice #85

Subdevice #86: subdevice #86

Subdevice #87: subdevice #87

Subdevice #88: subdevice #88

Subdevice #89: subdevice #89

Subdevice #90: subdevice #90

Subdevice #91: subdevice #91

Subdevice #92: subdevice #92

Subdevice #93: subdevice #93

Subdevice #94: subdevice #94

Subdevice #95: subdevice #95

Subdevice #96: subdevice #96

Subdevice #97: subdevice #97

Subdevice #98: subdevice #98

Subdevice #99: subdevice #99

Subdevice #100: subdevice #100

Subdevice #101: subdevice #101

Subdevice #102: subdevice #102

Subdevice #103: subdevice #103

Subdevice #104: subdevice #104

Subdevice #105: subdevice #105

Subdevice #106: subdevice #106

Subdevice #107: subdevice #107

Subdevice #108: subdevice #108

Subdevice #109: subdevice #109

Subdevice #110: subdevice #110

Subdevice #111: subdevice #111

Subdevice #112: subdevice #112

Subdevice #113: subdevice #113

Subdevice #114: subdevice #114

Subdevice #115: subdevice #115

Subdevice #116: subdevice #116

Subdevice #117: subdevice #117

Subdevice #118: subdevice #118

Subdevice #119: subdevice #119

Subdevice #120: subdevice #120

Subdevice #121: subdevice #121

Subdevice #122: subdevice #122

Subdevice #123: subdevice #123

Subdevice #124: subdevice #124

Subdevice #125: subdevice #125

Subdevice #126: subdevice #126

Subdevice #127: subdevice #127

這時看到系統識別了一個dummy聲卡，走道數為128,完全實現了聲道數修改。

總結

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