On 01/09/17 12:34 AM, Takashi Iwai wrote:

On Wed, 30 Aug 2017 21:48:58 +0200, Jack Bates wrote:

...

I confirmed with Wireshark that my USB sound card responds to GET_MIN with 0x0000 and GET_MAX with 0x6300 .. The audio device class definition [1] says the units are 1/256 dB (section 5.2.2.2.3: Mixer Control) ... So by my calculation, the min and max are 0 and 99 dB (0x6300 / 256 == 99) ... Why then does alsactl print:

...

range '0 - 99' dbmin 0 dbmax 38

(/usr/sbin/aslactl -f - store)

I originally asked about this on the pulseaudio-discuss list [2] because it uses "dB data" [3].

The "0 - 99" makes sense -- it lines up with my calculation ... Where does the 38 come from? (I expect it's obvious, I just haven't found the explanation?)

Hm, what shows /proc/asound/card*/usbmixer proc file? It contains some raw data.

It also shows dBmax=38 (it matches alsactl):

nottheoilrig@debian:~$ cat /proc/asound/card2/usbmixer USB Mixer: usb_id=0x05a740fa, ctrlif=0, ctlerr=0 Card: Bose Corporation Bose Revolve SoundLink at usb-0000:00:14.0-1, full speed Unit: 2 Control: name="PCM Playback Volume", index=0 Info: id=2, control=2, cmask=0x0, channels=1, type="S16" Volume: min=0, max=99, dBmin=0, dBmax=38 Unit: 2 Control: name="PCM Playback Switch", index=0 Info: id=2, control=1, cmask=0x0, channels=1, type="INV_BOOLEAN" Volume: min=0, max=1, dBmin=0, dBmax=0 nottheoilrig@debian:~$

Basically the driver simply interprets the USB descriptor min/max unless there are overriding mappings by quirk list.

How can I inspect the device's USB descriptor min/max (to confirm that 38 *is* what's advertised)? I was using Wireshark to capture the GET_MAX request and response ([1] section 5.2.2.2.2: Get Mixer Unit Control Request) -- in which case the response data is 0x6300 ...

If the source behind the usbmixer proc file/dB data is the GET_MAX response, then how does 0x6300 turn into 38 dB? I haven't spotted any obvious quirks for usb_id=0x05a740fa (although I haven't read all of mixer.c) ... get_min_max_with_quirks() would seem to interpret it as 1/256 dB [2] (as per the spec) but 0x6300 / 256 != 38 ...

[1] http://www.usb.org/developers/docs/devclass_docs/audio10.pdf [2] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/tree/sound/u...

Here's the lsusb output:

Bus 001 Device 008: ID 05a7:40fa Bose Corp. Device Descriptor: bLength 18 bDescriptorType 1 bcdUSB 2.00 bDeviceClass 239 Miscellaneous Device bDeviceSubClass 2 ? bDeviceProtocol 1 Interface Association bMaxPacketSize0 64 idVendor 0x05a7 Bose Corp. idProduct 0x40fa bcdDevice 1.00 iManufacturer 1 iProduct 2 iSerial 3 bNumConfigurations 1 Configuration Descriptor: bLength 9 bDescriptorType 2 wTotalLength 156 bNumInterfaces 3 bConfigurationValue 1 iConfiguration 0 bmAttributes 0xc0 Self Powered MaxPower 500mA Interface Association: bLength 8 bDescriptorType 11 bFirstInterface 0 bInterfaceCount 2 bFunctionClass 1 Audio bFunctionSubClass 1 Control Device bFunctionProtocol 0 iFunction 0 Interface Descriptor: bLength 9 bDescriptorType 4 bInterfaceNumber 0 bAlternateSetting 0 bNumEndpoints 0 bInterfaceClass 1 Audio bInterfaceSubClass 1 Control Device bInterfaceProtocol 0 iInterface 0 AudioControl Interface Descriptor: bLength 9 bDescriptorType 36 bDescriptorSubtype 1 (HEADER) bcdADC 1.00 wTotalLength 43 bInCollection 1 baInterfaceNr( 0) 1 AudioControl Interface Descriptor: bLength 12 bDescriptorType 36 bDescriptorSubtype 2 (INPUT_TERMINAL) bTerminalID 1 wTerminalType 0x0101 USB Streaming bAssocTerminal 0 bNrChannels 2 wChannelConfig 0x0003 Left Front (L) Right Front (R) iChannelNames 0 iTerminal 0 AudioControl Interface Descriptor: bLength 13 bDescriptorType 36 bDescriptorSubtype 6 (FEATURE_UNIT) bUnitID 2 bSourceID 1 bControlSize 2 bmaControls( 0) 0x03 bmaControls( 0) 0x00 Mute Control Volume Control bmaControls( 1) 0x00 bmaControls( 1) 0x00 bmaControls( 2) 0x00 bmaControls( 2) 0x00 iFeature 0 AudioControl Interface Descriptor: bLength 9 bDescriptorType 36 bDescriptorSubtype 3 (OUTPUT_TERMINAL) bTerminalID 3 wTerminalType 0x0301 Speaker bAssocTerminal 0 bSourceID 2 iTerminal 0 Interface Descriptor: bLength 9 bDescriptorType 4 bInterfaceNumber 1 bAlternateSetting 0 bNumEndpoints 0 bInterfaceClass 1 Audio bInterfaceSubClass 2 Streaming bInterfaceProtocol 0 iInterface 0 Interface Descriptor: bLength 9 bDescriptorType 4 bInterfaceNumber 1 bAlternateSetting 1 bNumEndpoints 1 bInterfaceClass 1 Audio bInterfaceSubClass 2 Streaming bInterfaceProtocol 0 iInterface 0 AudioStreaming Interface Descriptor: bLength 7 bDescriptorType 36 bDescriptorSubtype 1 (AS_GENERAL) bTerminalLink 1 bDelay 0 frames wFormatTag 1 PCM AudioStreaming Interface Descriptor: bLength 14 bDescriptorType 36 bDescriptorSubtype 2 (FORMAT_TYPE) bFormatType 1 (FORMAT_TYPE_I) bNrChannels 2 bSubframeSize 2 bBitResolution 16 bSamFreqType 2 Discrete tSamFreq[ 0] 48000 tSamFreq[ 1] 44100 AudioControl Endpoint Descriptor: bLength 7 bDescriptorType 37 bDescriptorSubtype 1 (EP_GENERAL) bmAttributes 0x81 Sampling Frequency MaxPacketsOnly bLockDelayUnits 0 Undefined wLockDelay 0 Undefined Endpoint Descriptor: bLength 9 bDescriptorType 5 bEndpointAddress 0x03 EP 3 OUT bmAttributes 1 Transfer Type Isochronous Synch Type None Usage Type Data wMaxPacketSize 0x00c0 1x 192 bytes bInterval 1 bRefresh 0 bSynchAddress 0 Interface Descriptor: bLength 9 bDescriptorType 4 bInterfaceNumber 2 bAlternateSetting 0 bNumEndpoints 2 bInterfaceClass 3 Human Interface Device bInterfaceSubClass 0 No Subclass bInterfaceProtocol 0 None iInterface 0 HID Device Descriptor: bLength 9 bDescriptorType 33 bcdHID 1.11 bCountryCode 0 Not supported bNumDescriptors 1 bDescriptorType 34 Report wDescriptorLength 126 Report Descriptors: ** UNAVAILABLE ** Endpoint Descriptor: bLength 7 bDescriptorType 5 bEndpointAddress 0x81 EP 1 IN bmAttributes 3 Transfer Type Interrupt Synch Type None Usage Type Data wMaxPacketSize 0x0040 1x 64 bytes bInterval 1 Endpoint Descriptor: bLength 7 bDescriptorType 5 bEndpointAddress 0x02 EP 2 OUT bmAttributes 3 Transfer Type Interrupt Synch Type None Usage Type Data wMaxPacketSize 0x0040 1x 64 bytes bInterval 1