# Generic Kernels

postmarketOS includes a couple of generic kernel packages in the `main` device
category: `linux-postmarketos-mainline`, `linux-postmarketos-stable` and
`linux-postmarketos-lts`.

These are kernels intended to work on a wide variety of devices and are the
postmarketOS equivalents to Alpine kernels such as `linux-stable` or
`linux-lts`. Having these kernels in postmarketOS means that we have full
control over the kernel configuration and build process, which allows us to
integrate them with our [kernel configuration checks](./kconfigcheck).

In the long term, all devices using Alpine kernels should be migrated to the
postmarketOS generic kernels.

## Configuration

Since it would be very tedious to manually update the configuration for
multiple kernel packages individually, the configuration for the kernels is
automatically generated from two sources in pmaports:

* `kconfigcheck.toml`, to make sure that they pass our checks
* `kconfig-generic.toml`, a file to enable device-specific drivers and apply
  opinionated configurations

The former is a file used by pmbootstrap to verify the configuration of all
kernels in pmaports, the latter is a file specifically for the configuration of
the generic kernel packages.

`pmbootstrap kconfig generate` will look at both of these files and generate a
fragment for each of them, which are then merged with the default `defconfig`
for an architecture in the Linux kernel.

## Adding new options

If a required option for your device is missing in the generic kernels, take a
moment to consider whether this is an option that *all* kernels in postmarketOS
should have enabled. If that's the case, then you should open a MR to add it
to `kconfigcheck.toml`, see [the documentation](./kconfigcheck) on the topic
for further details.

If that's not the case, then it is probably a good fit for
`kconfig-generic.toml`. Suppose your device has an Omnivision 8856 camera and
you'd like to enable the driver for it: In Linux, this requires setting
`CONFIG_VIDEO_OV8856=m` in the kernel configuration.

Check whether `kconfig-generic.toml` has an existing category that seems to fit
this configuration option (for example, `category:video`). If not, add a new
one, otherwise add it to the existing category. Categories in this TOML file
only exist for grouping things together, no filtering is performed, so don't
think too much about it.

If this configuration option was only added recently, find out since when it
exists in Linux. Also, make sure to check whether it can be enabled on all
architectures or only on select few.

You can then add to `kconfig-generic.toml`:

```toml
["category:video".">=6.12_rc1"."all"]
VIDEO_OV8856 = "m"
```

This would result in `CONFIG_VIDEO_OV8856=m` being set for all generic kernels
newer than 6.12-rc1 and enable it on all architectures.

You can submit your change in a GitLab MR for the generic kernel maintainers to
review. Please do **not** regenerate the kernel configurations; your change
will be included as part of the next kernel upgrade. Rebuilds of the kernel
packages take a long time and they get updated regularly, so you'll only need
to wait a few days until your change makes it into the released binaries.
