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When building feature controls from a unit without a name, we try to
derive a name first from the feature unit's input, then fall back to the
output terminal.
If a feature unit connects directly to a "USB Streaming" input terminal
rather than a mixer or other virtual type, the control receives the
somewhat meaningless name "PCM", even if the output had a descriptive
type such as "Headset" or "Speaker".
Here is an example of such AudioControl descriptor from a USB headset
which ends up named "PCM Playback" and is therefore not recognized as
headphones by userspace:
AudioControl Interface Descriptor:
bLength 12
bDescriptorType 36
bDescriptorSubtype 2 (INPUT_TERMINAL)
bTerminalID 4
wTerminalType 0x0101 USB Streaming
bAssocTerminal 5
bNrChannels 2
wChannelConfig 0x0003
Left Front (L)
Right Front (R)
iChannelNames 0
iTerminal 0
AudioControl Interface Descriptor:
bLength 9
bDescriptorType 36
bDescriptorSubtype 3 (OUTPUT_TERMINAL)
bTerminalID 5
wTerminalType 0x0402 Headset
bAssocTerminal 4
bSourceID 6
iTerminal 0
AudioControl Interface Descriptor:
bLength 13
bDescriptorType 36
bDescriptorSubtype 6 (FEATURE_UNIT)
bUnitID 6
bSourceID 4
bControlSize 2
bmaControls(0) 0x0002
Volume Control
bmaControls(1) 0x0000
bmaControls(2) 0x0000
iFeature 0
Other headsets and DACs I tried that used their output terminal for
naming only did so due to their input being an unnamed sidetone mixer.
Instead of always starting with the input terminal, check the type of it
first. If it seems uninteresting, invert the order and use the output
terminal first for naming.
This makes userspace recognize headsets with simple controls as
headphones, and leads to more consistent naming of playback devices
based on their outputs irrespective of sidetone mixers.
Signed-off-by: Kenny Levinsen <kl@kl.wtf>
Link: https://lore.kernel.org/r/20240301231107.42679-1-kl@kl.wtf
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.