Update documentation for Scarlett 4th Gen and Vocaster

docs/OLDKERNEL.md

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+# ALSA Scarlett2 Usage With Old Kernels
+
+Linux kernel 6.7 (check your version with `uname -r`) was the first
+kernel version with this driver enabled by default. It’s recommended
+that you run 6.7 or later, or build the backported driver for your
+kernel. If you do, then these instructions aren’t relevant; continue
+with [INSTALL.md](INSTALL.md) for prerequisites, how to build,
+install, and run `alsa-scarlett-gui`.
+
+If you’ve got a Scarlett Gen 2 or 3 or a Clarett+ 8Pre and don’t mind
+the level meters not working, then the minimum kernel versions are:
+
+- **Scarlett Gen 2**: Linux 5.4 (bugs fixed in Linux 5.14)
+- **Scarlett Gen 3**: Linux 5.14
+- **Clarett+ 8Pre**: Linux 6.1
+
+## Linux Kernel with Backported Driver (recommended)
+
+Install the latest version of the backported driver from here:
+
+https://github.com/geoffreybennett/scarlett-gen2/releases
+
+then you can ignore the instructions below.
+
+## Linux Kernel before 6.7 without Backported Driver
+
+If you’re running a kernel before 6.7 without the backported driver,
+you need to enable it at module load time with the `device_setup=1`
+option to insmod/modprobe. Create a file
+`/etc/modprobe.d/scarlett.conf` containing the appropriate line for
+your device:
+
+Scarlett Gen 2:
+
+- **6i6**: `options snd_usb_audio vid=0x1235 pid=0x8203 device_setup=1`
+- **18i8**: `options snd_usb_audio vid=0x1235 pid=0x8204 device_setup=1`
+- **18i20**: `options snd_usb_audio vid=0x1235 pid=0x8201 device_setup=1`
+
+Scarlett Gen 3:
+
+- **Solo**: `options snd_usb_audio vid=0x1235 pid=0x8211 device_setup=1`
+- **2i2**: `options snd_usb_audio vid=0x1235 pid=0x8210 device_setup=1`
+- **4i4**: `options snd_usb_audio vid=0x1235 pid=0x8212 device_setup=1`
+- **8i6**: `options snd_usb_audio vid=0x1235 pid=0x8213 device_setup=1`
+- **18i8**: `options snd_usb_audio vid=0x1235 pid=0x8214 device_setup=1`
+- **18i20**: `options snd_usb_audio vid=0x1235 pid=0x8215 device_setup=1`
+
+Clarett+:
+
+- **8Pre**: `options snd_usb_audio vid=0x1235 pid=0x820c device_setup=1`
+
+Or you can use a sledgehammer:
+```
+options snd_usb_audio device_setup=1,1,1,1
+```
+to pass that option to the first 4 USB audio devices.
+
+To see if the driver is present and enabled: `dmesg | grep -i -A 5 -B
+5 focusrite` should display information like:
+
+```
+New USB device found, idVendor=1235, idProduct=8215, bcdDevice= 6.0b
+Product: Scarlett 18i20 USB
+Focusrite Scarlett Gen 2/3 Mixer Driver enabled pid=0x8215
+```
+
+If the driver is disabled you’ll see a message like:
+
+```
+Focusrite Scarlett Gen 2/3 Mixer Driver disabled; use options
+snd_usb_audio vid=0x1235 pid=0x8215 device_setup=1 to enable and
+report any issues to g@b4.vu
+```