# kernel-surface

A Fedora kernel RPM for the Microsoft Surface Pro 12" Gen 1 (Snapdragon
X1P-42-100): vanilla upstream Linux with the device's patch set from the
[miasvanklei Gentoo overlay](https://github.com/miasvanklei/Gentoo-overlay)
applied.

Used in [Fedora KIWI descriptions, modified by me for Surface Pro 12"](https://sayag.it/fedora-linux-surface-pro-12in/kiwi-descriptions).

## What gets applied

`patch-order.txt` lists the patch directories in the order the overlay's
`vanilla-kernel-*.ebuild` applies them, minus `gpu` (does not exist) and
`rockchip` (irrelevant).

## Configuration

The starting point is `config/base-aarch64.config`: Fedora's own aarch64
kernel config, taken verbatim from the `config` file inside
`kernel-core-7.2.0-61.fc45.aarch64.rpm`. It is vendored rather than downloaded,
so the build depends on no third-party host and a refresh shows up as a
reviewable diff. To refresh it for a newer Fedora, replace everything below the
comment header with:

```bash
dnf download kernel-core --arch aarch64
rpm2cpio kernel-core-*.aarch64.rpm | cpio -idm './usr/lib/modules/*/config'
```

The overlay's ebuild seeds itself from the same Fedora config by way of a
Gentoo-side fork, `projg2/fedora-kernel-config-for-gentoo`. That repository no
longer exists, which is why the base config lives here instead.

Three fragments are merged on top with `scripts/kconfig/merge_config.sh`:

* `config/surface.config` – the drivers the patch set enables (SAM stack and
  its new RTC, CAMSS and the CSI-2 D-PHY, fastrpc, ath12k, AudioReach).
* `config/live-image.config` – what the KIWI image types need: erofs with lzma
  and dm-snapshot/overlayfs built in for `dracut`'s `dmsquash-live`, the EFI
  stub because GRUB's aarch64 `linux` command loads the kernel as an EFI
  application, and xz module compression.
* `config/no-debug-info.config` – DWARF and BTF off. The package produces no
  `-debuginfo` subpackage (`%global debug_package %{nil}`) and strips modules
  at install time, so the DWARF was compiled only to be discarded, and it is
  what makes the build too big for the 6 CPU / 8 GB arm64 builder. The cost is
  BTF, the one part that would have shipped: no `/sys/kernel/btf/vmlinux`, so
  eBPF CO-RE tooling does not work, and `CONFIG_BPF_LSM` switches itself off
  with it, which disables systemd settings such as `RestrictFileSystems=`.

`%prep` also resets `CONFIG_BUILD_SALT`, which the base config inherits from
the Fedora build it came from, and which feeds the kernel's build ID.

`%prep` then asserts that `CONFIG_RTC_DRV_SURFACE` and
`CONFIG_PHY_QCOM_MIPI_CSI2` made it into `.config`. Those symbols only exist if
the patch set applied, so a silently mis-applied patch fails the build instead
of producing a kernel that is quietly missing drivers.

There is no module signing and the kernel is not signed for Secure Boot, which
matches the ISO (the project README notes Secure Boot is not available for it).

## Installing

```bash
dnf config-manager addrepo --from-repofile=\
  https://f003.backblazeb2.com/file/dist-sayagit-fedora-rpm/fedora/45/kernel-sp12in.repo
```

## Building

The build must run on aarch64 – `ExclusiveArch: aarch64`. On an x86_64 host,
use `binfmt`-based emulation, at a large cost in build time.

```bash
# Fetch the miasvanklei/Gentoo-overlay
git submodule update --init overlay

# Collect the patch tarball, the kernel tarball and the configs
./make-sources.sh

# Build
sudo dnf builddep kernel-surface.spec
rpmbuild -bb kernel-surface.spec
```

The build is pinned to `-j4` rather than the node's CPU count, because four
parallel compilers is what the builder's memory limit holds:

```bash
rpmbuild -bb --define '_smp_mflags -j4' kernel-surface.spec
```

`make-sources.sh -n` skips the downloads and only rebuilds the patch tarball,
which is what you want while iterating on `patch-order.txt`. `-o DIR` writes
somewhere other than rpm's `%_sourcedir`.

To move to a newer kernel, change `%global upstream_ver` in the spec, update the
`overlay` submodule, and re-run `make-sources.sh`. `%global patchset_ver` is for
the case where the patches change but the kernel version does not.

### Package layout

One binary package with the kernel image, every module and the device trees. It
does not reproduce Fedora's `kernel-core` / `kernel-modules` /
`kernel-modules-extra` split – for a single-device image there is nothing to
gain from it, and the split is driven by module lists that would need to be
maintained by hand.

It `Provides` all three of those names plus `kernel` and the `*-uname-r` forms,
so packages that `Requires: kernel-core` resolve against it. It deliberately
does **not** `Obsoletes` them: the KIWI description selects it by name, which
keeps the repository safe to enable on a normal Fedora system without hijacking
kernel updates there.

## Releasing

The Jenkinsfile builds and publishes nothing except for tags of the form
`fedora-<fedora_ver>-kernel-<upstream_ver>-patchset-<patchset_ver>`, e.g.
`fedora-45-kernel-7.2.3-patchset-1`. If it is `rawhide`, use the latest Fedora
tag (on the moment of writing this it was `46`).

The Fedora version selects both the build container and the bucket prefix, and
is checked against the container's own `%{fedora}` for a numbered release.

What the tag does *not* override is the `overlay` submodule, and the patch set
there is written against one kernel version. Tagging a kernel the pinned overlay
commit does not target gets a `%prep` full of rejected patches – see the
`patch-order.txt` and `%if %{undefined}` comments for how the two are kept in
step.