Channel count in Focusrite Scarlett Gen 4 (1235:8218) SplitPCM configuration

[GitHub issues - edited <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-ucm-conf issue #515 was edited from pv:

The configuration https://github.com/alsa-project/alsa-ucm-conf/blob/master/ucm2/USB-Audio/Focusrite/Scarlett-2i-HiFi.conf specifies `HWChannels 2` for the Mic1 and Mic2 devices. This gets translated to `CaptureChannels 2` for `alsaucm open '<<<SplitPCM=1>>>hw:2'`.

However, we've got a report https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/4597#note_2815906 that looks like snd_pcm_hw_params_set_channels(pcm_handle, hwparams, 2) for the CapturePCM "_ucm0001.hw:Gen,0" fails with error. The PCM can be opened with 4 channels, though. Without `SplitPCM=1`, it also probably works but I'm not sure what channel count the `dshare` plugin is using.

The issue also affects the `Direct` profile, which also specifies `CaptureChannels 2`.

Is the `HWChannels 2` for Mic1/2 input direction right?

Shall `CaptureChannels 2` in UCM device be interpreted as that the device can be opened with exactly 2 channels, or that it can be opened with some number of channels >= 2?

<details>

```
$ alsaucm open '<<<SplitPCM=1>>>hw:2' dump text
Verb.HiFi {
	Comment Default
	Device.Line1 {
		Comment "Headphones / Line 1-2"
		Values {
			CaptureCTL "_ucm0001.hw:Gen"
			PlaybackCTL "_ucm0001.hw:Gen"
			PlaybackPCM "_ucm0001.hw:Gen"
			PlaybackPriority 200
		}
	}
	Device.Mic1 {
		Comment "Input 1 Mic/Inst/Line"
		Values {
			CaptureCTL "_ucm0001.hw:Gen"
			CaptureChannel0 0
			CaptureChannelPos0 MONO
			CaptureChannels 2
			CapturePCM "_ucm0001.hw:Gen,0"
			CapturePriority 200
			PlaybackCTL "_ucm0001.hw:Gen"
		}
	}
	Device.Mic2 {
		Comment "Input 2 Mic/Inst/Line"
		Values {
			CaptureCTL "_ucm0001.hw:Gen"
			CaptureChannel0 1
			CaptureChannelPos0 MONO
			CaptureChannels 2
			CapturePCM "_ucm0001.hw:Gen,0"
			CapturePriority 100
			PlaybackCTL "_ucm0001.hw:Gen"
		}
	}
}
Verb.Direct {
	Comment "Direct Scarlett Solo 4th Gen"
	Device.Direct {
		Comment "Direct Scarlett Solo 4th Gen"
		Values {
			CaptureCTL "_ucm0001.hw:Gen"
			CaptureChannels 2
			CapturePCM "_ucm0001.hw:Gen"
			CapturePriority 1000
			PlaybackCTL "_ucm0001.hw:Gen"
			PlaybackChannels 2
			PlaybackPCM "_ucm0001.hw:Gen"
			PlaybackPriority 1000
		}
	}
}
```

</details>

Issue URL     : https://github.com/alsa-project/alsa-ucm-conf/issues/515
Repository URL: https://github.com/alsa-project/alsa-ucm-conf