我有一个 MIDI 键盘“Impulse”和一个 raspi3。我想将键盘连接到raspi，并且在不接触任何东西的情况下发出声音。
所以我使用fluidsynthwithjack作为音频驱动程序。 Fluidsynth 由 systemd 服务启动。没有问题。
所以我制作了一个脚本，其中我“jack_connect”端口，并以这种方式“aconnect”我键盘的midi端口到合成器fluidsynth：
impulseport=$(aconnect -i|grep -i "IMPULSE" | cut -d ' ' -f 2)0
synthport=$(aconnect -o |grep -i "FLUID" | cut -d ' ' -f 2)0
# some verifications of existence and exit if one port is missing
aconnect ${impulseport} ${synthport}
问题是它并没有真正按照我想要的方式工作。我的 udev 规则是在加载“snd-usb-audio”接口驱动程序之前触发此脚本。因此，该变量$impulseport为空（实际上=0，最后连接）。
这是我的 udev 规则：
ACTION=="add", ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="1235", ATTRS{idProduct}=="001a", RUN+="/bin/su -c /home/pi/piano_connect - pi"
ACTION=="remove", ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="1235", ATTRS{idProduct}=="001a", RUN+="/usr/bin/aconnect -x"
在系统日志中：
1 systemd[410]: Started Sound Service.
2 fluidsynth[423]: fluidsynth: Jack sample rate mismatch, adjusting. (synth.sample-rate=44100, jackd=48000)
3 kernel: [162.772916] usb 1-1.2: new full-speed USB device number 4 using dwc_otg
4 kernel: [162.905473] usb 1-1.2: New USB device found, idVendor=1235, idProduct=001a, bcdDevice= 0.00
5 kernel: [162.905491] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
6 kernel: [162.905500] usb 1-1.2: Product: Impulse
7 kernel: [162.905509] usb 1-1.2: Manufacturer: Focusrite A.E. Ltd
8 mtp-probe: checking bus 1, device 4: "/sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2"
9 mtp-probe: bus: 1, device: 4 was not an MTP device
10 systemd[1]: Started Session c2 of user pi.
11 pi: [Piano] Jackd : connecting system to fluidsynth ports
12 pi: [Piano] Error: Port Impulse unknown, exiting
13 systemd-udevd[473]: Process '/bin/su -c /home/pi/piano_connect - pi' failed with exit code 1.
14 systemd[1]: session-c2.scope: Succeeded.15
15 kernel: [165.876060] usbcore: registered new interface driver snd-usb-audio
16 mtp-probe: checking bus 1, device 4: "/sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2"
17 mtp-probe: bus: 1, device: 4 was not an MTP device
问题出在第 12 行：$impulseport为空！因此它使脚本退出（第 13 行）。你可以看到第 15 行驱动程序是在之后加载的。所以在我的脚本中我做了一个肮脏的事情，比如一个 while 循环，我用 a 检查目录是否/sys/bus/usb/drivers/snd-usb-audio存在，sleep但它挂起系统并且模块永远不会加载。你知道吗？伙计们有什么出路吗？
注意：如果我拔下 USB MIDI 键盘，然后再次插入它，它可以正常工作，但我希望它能够在启动后的第一个插头上正常工作...或者甚至在键盘已插入时在启动后可以正常工作： D
答案1
好吧，经过一番研究，我找到了出路。一、使用udevadm monitor前插拔并插好。这是输出：
KERNEL[126.555200] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2 (usb)
KERNEL[126.555888] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0 (usb)
KERNEL[126.556508] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.1 (usb)
KERNEL[126.557829] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.2 (usb)
KERNEL[126.558188] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.3 (usb)
KERNEL[126.558548] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2 (usb)
KERNEL[129.399848] add /module/snd_seq_dummy (module)
UDEV [129.406959] add /module/snd_seq_dummy (module)
UDEV [129.493966] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2 (usb)
UDEV [129.501782] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.2 (usb)
UDEV [129.505609] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.3 (usb)
KERNEL[129.508606] add /module/snd_rawmidi (module)
UDEV [129.510396] add /module/snd_rawmidi (module)
KERNEL[129.513459] add /module/snd_usbmidi_lib (module)
UDEV [129.515605] add /module/snd_usbmidi_lib (module)
KERNEL[129.516631] add /module/snd_hwdep (module)
UDEV [129.518617] add /module/snd_hwdep (module)
KERNEL[129.535977] add /module/snd_usb_audio (module)
UDEV [129.537881] add /module/snd_usb_audio (module)
KERNEL[129.538452] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.1 (usb)
KERNEL[129.538547] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1 (sound)
KERNEL[129.539135] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/midiC1D0 (sound)
KERNEL[129.539260] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0 (snd_seq)
KERNEL[129.539985] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/controlC1 (sound)
KERNEL[129.541172] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0 (usb)
UDEV [129.541345] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.1 (usb)
KERNEL[129.541428] add /bus/usb/drivers/snd-usb-audio (drivers)
UDEV [129.542776] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0 (usb)
UDEV [129.542917] add /bus/usb/drivers/snd-usb-audio (drivers)
KERNEL[129.549489] add /module/snd_seq_midi_event (module)
UDEV [129.551648] add /module/snd_seq_midi_event (module)
KERNEL[129.552118] add /module/snd_seq_midi (module)
KERNEL[129.552213] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0 (snd_seq)
KERNEL[129.552288] add /bus/snd_seq/drivers/snd_seq_midi (drivers)
UDEV [129.554430] add /module/snd_seq_midi (module)
UDEV [129.554572] add /bus/snd_seq/drivers/snd_seq_midi (drivers)
UDEV [129.561871] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2 (usb)
UDEV [129.564936] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.1 (usb)
UDEV [129.566125] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1 (sound)
UDEV [129.568261] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0 (snd_seq)
UDEV [129.570243] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/midiC1D0 (sound)
KERNEL[129.573057] change /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1 (sound)
UDEV [129.607147] add /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/controlC1 (sound)
UDEV [129.609881] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0 (usb)
UDEV [129.611644] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0 (snd_seq)
UDEV [129.618931] change /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1 (sound)
注意到最后一行后UDEV [129.611644] bind /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0 (snd_seq)，我插入了 USB MIDI 键盘，然后执行了以下操作：
devadm info -a /sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2\:1.0/sound/card1/seq-midi-1-0/> /tmp/udev`
在输出中，第一行有：
looking at device '/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1/seq-midi-1-0':
KERNEL=="seq-midi-1-0"
SUBSYSTEM=="snd_seq"
DRIVER=="snd_seq_midi"
looking at parent device '/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0/sound/card1':
KERNELS=="card1"
SUBSYSTEMS=="sound"
DRIVERS==""
ATTRS{number}=="1"
ATTRS{id}=="Impulse"
looking at parent device '/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2/1-1.2:1.0':
KERNELS=="1-1.2:1.0"
SUBSYSTEMS=="usb"
DRIVERS=="snd-usb-audio"
ATTRS{bInterfaceProtocol}=="00"
ATTRS{bInterfaceSubClass}=="01"
ATTRS{interface}=="Novation Impulse"
ATTRS{bInterfaceNumber}=="00"
ATTRS{supports_autosuspend}=="1"
ATTRS{bAlternateSetting}==" 0"
ATTRS{authorized}=="1"
ATTRS{bNumEndpoints}=="00"
ATTRS{bInterfaceClass}=="01"
有趣的信息是SUBSYSTEM和DRIVERS。所以我将 udev 规则更改为：
ACTION=="bind", SUBSYSTEM=="snd_seq", DRIVERS=="snd-usb-audio", RUN+="/bin/su -c /home/pi/piano_connect - pi"
ACTION=="remove", ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="1235", ATTRS{idProduct}=="001a", RUN+="/usr/bin/aconnect -x"
在这里，我将 ACTION 更改为“bind”，因为第一个命令 ( ) 并与和udevadm monitor匹配，而不是产品的 id。它就像一个魅力！希望有帮助。 :)SUBSYSTEMDRIVERS