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snd_uaudio.4: document sysctls
Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Reviewed by: dev_submerge.ch Differential Revision: https://reviews.freebsd.org/D43649
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@ -57,12 +57,88 @@ A USB audio device consists of a number of components: input terminals (e.g.\&
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USB digital input), output terminals (e.g.\& speakers), and a number of units
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in between (e.g.\& volume control).
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.Pp
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If the device supports multiple configurations, and there have been no
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user-supplied values specified through the
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.Xr sysctl 8
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interface, the driver will select the best matching configuration supported by
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the device during attach.
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"Best" means the configuration with the most channels and highest quality in
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sample rate and sample size.
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.Pp
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Refer to the
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.Ql USB Audio Class Specification
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for more information.
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.Sh SYSCTL VARIABLES
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The following settings can be entered at the
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.Xr loader 8
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prompt or in
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.Xr loader.conf 5
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and can also be changed at runtime with the
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.Xr sysctl 8
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command.
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For a change to take effect during runtime, the device has to be re-attached.
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.Bl -tag -width indent
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.It Va hw.usb.uaudio.buffer_ms
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Period of audio data processed at once, in milliseconds, from 1 to 8 (default
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is 4).
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Lower values mean less latency, but this can result in audible gaps due to
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frequent CPU wakeups.
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.It Va hw.usb.uaudio.default_bits
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Preferred sample size in bits, from 0 to 32 (default is 0).
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A value of 0 sets the sample size to the maximum supported sample size.
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.Pp
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Set this to select a smaller sample size if the device supports multiple sample
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sizes.
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.It Va hw.usb.uaudio.default_channels
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Preferred number of sample channels, from 0 to 64 (default is 0).
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USB 1.1 devices are limited to 4 channels due to bandwidth constraints, unless
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a higher value is explicitly requested.
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A value of 0 sets the sample channels to the maximum supported channel number.
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.Pp
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Set this to select a smaller channel number if the device supports multiple
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channel configurations.
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.It Va hw.usb.uaudio.default_rate
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Preferred sample rate in Hz (default is 0).
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If set to 0, the device's highest supported sample rate will be used.
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.Pp
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Note that if VCHANs are enabled, the sample rate will be overridden by
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.Pa dev.pcm.%d.[play|rec].vchanrate
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(see
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.Xr sound 4 ) ,
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which can also be used to adjust the sample rate during runtime.
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.Pp
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If
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.Pa hw.usb.uaudio.default_rate
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is non-zero,
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.Pa dev.pcm.%d.[play|rec].vchanrate
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will use it as its maximum allowed value.
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.It Va hw.usb.uaudio.handle_hid
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Let
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.Nm
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handle HID volume keys, if any (default is 1).
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.Bl -tag -width 2n
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.It 0
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Disabled.
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.It 1
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Enabled.
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.El
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.El
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.Pp
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If
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.Xr usb 4
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has been compiled with
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.Va USB_DEBUG
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on, the following setting is also available:
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.Bl -tag -width indent
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.It Va hw.usb.uaudio.debug
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Debug output level (default is 0).
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.El
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.Sh SEE ALSO
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.Xr sound 4 ,
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.Xr usb 4
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.Xr usb 4 ,
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.Xr loader.conf 5 ,
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.Xr loader 8 ,
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.Xr sysctl 8
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.Rs
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.%T "USB Audio Class Specifications"
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.%U http://www.usb.org/developers/docs/devclass_docs/
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@ -98,8 +174,7 @@ sysctls.
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.Pp
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The PCM framework in
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.Fx
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currently doesn't support the full set of USB audio mixer
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controls.
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currently does not support the full set of USB audio mixer controls.
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Some mixer controls are only available as
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.Va dev.pcm.%d.mixer
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sysctls.
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