[alsa-devel] UCM {Supported,Conflicting}Device confusion

Stephen Warren swarren at wwwdotorg.org
Wed May 29 17:12:42 CEST 2013

On 05/29/2013 04:55 AM, Juho Hämäläinen wrote:
> Hello,
> Firstly as a sidenote, Supported/Conflicting is checked only one-way, if
> say devices A and B are enabled and say B has ConflictingDevice "C", and
> device C doesn't list anything, enabling of C is possible (and after
> that disabling C is not possible before B is disabled first).

It's been a long time since I touched the ConflictingDevice stuff, but
IIRC the idea was that every device would include a complete list of
Supported/Conflicting Devices, so there was only a need to check "one way".

> Now, looking at the way SupportedDevice and ConflictingDevice are
> implemented in ucm and I came to following conclusion:
> If no SupportedDevice or ConflictingDevice: All devices and combinations
> are compatible, so implicit "SupportedDevice ALL".
> If SupportedDevice is listed: Device can be enabled as long as at least
> one enabled device is in SupportedDevice list.
> If ConflictingDevice is listed: Device can be enabled if none of active
> devices are in ConflictingDevice list (noting aforementioned limit in
> checking this)
> This is at least how I figured all combinations of multiple devices can
> be defined with only zero or one either SupportedDevice or
> ConflictingDevice entry per device.
> Did I understand this right?

That sounds about right, from my rusty memory.

Oh, the commit log specifies the behaviour:

>     UCM: Implement ConflictingDevices, add device list to devices
>     Wherever SupportedDevice can appear, also allow ConflictingDevice. Only
>     one or the other (or neither) may be specified. When neither is
>     specified, allow anything. Sometimes, listing ConflictingDevices may
>     result in a shorter list than explicitly listing all SupportedDevices.
>     Add support for SupportedDevice and ConflictingDevice to SectionDevice.
>     This allows representing devices which are mutually exclusive, e.g. due
>     to a mux that switches between capturing from two different microphones,
>     without the possibility of mixing.
>     Enhance is_modifier_supported to allow ignoring SupportedDevice and
>     ConflictingDevice. This is useful when querying values from a
>     SectionModifier; there's no reason we shouldn't be able to query values
>     just because the current configuration would prevent enabling that
>     device. The new is_device_supported is implemented similarly.
>     Enhance switch_device to remove the old device from the current device
>     list before querying for the new device, and add it back immediately
>     afterwards. This allows the query for the new device to ignore any
>     conflicts caused solely by the old device.

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