[meta-rockchip][PATCH v2] kernel: add config fragment for lttng-modules prerequisites

[Trevor Woerner <twoerner@gmail.com>]

These machines build from an upstream defconfig via KBUILD_DEFCONFIG
rather than a yocto-kernel-cache BSP, so the standard kernel type's
configuration never applies and lttng-modules gets neither CONFIG_KPROBES
(a hard compile failure) nor CONFIG_TRACEPOINTS (builds no modules at
all). Ship the fragment in-layer so it works for any kernel recipe here,
not just ones that fetch the kernel cache.

AI-Generated: codex/claude-opus 5 (xhigh)
Signed-off-by: Trevor Woerner <twoerner@gmail.com>
---
changes in v2:
- drop the dependency on yocto-kernel-cache; v1 added
  features/kprobes/kprobes.scc to KERNEL_FEATURES, which only resolves
  for kernel recipes that fetch the cache. linux-yocto fetches it in
  linux-yocto_7.2.bb, not in linux-yocto.inc, so recipes that require
  only the .inc never get it.
- a KERNEL_FEATURES entry that is not found is a hard do_kernel_metadata
  failure, not a warning, so a shared .inc can only reference fragments
  the layer itself ships. Ship the config in-layer instead.
- widen from :rockchip:arm to :rockchip. CONFIG_KPROBES is absent on
  every machine in the layer; 32-bit merely fails loudly. arm64 also
  lacks CONFIG_TRACEPOINTS, so lttng-modules silently builds zero
  modules and ships an empty package.
- link to v1:
  https://lore.kernel.org/yocto-patches/20260326130653.478746-1-twoerner@gmail.com/
---
 recipes-kernel/linux/linux-rockchip.inc       |  1 +
 recipes-kernel/linux/linux-rockchip/lttng.cfg | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)
 create mode 100644 recipes-kernel/linux/linux-rockchip/lttng.cfg

diff --git a/recipes-kernel/linux/linux-rockchip.inc b/recipes-kernel/linux/linux-rockchip.inc
index 6c62e849834c..8e73c84a95ea 100644
--- a/recipes-kernel/linux/linux-rockchip.inc
+++ b/recipes-kernel/linux/linux-rockchip.inc
@@ -32,6 +32,7 @@ COMPATIBLE_MACHINE:rock64 = "rock64"
 COMPATIBLE_MACHINE:soquartz = "soquartz"
 
 SRC_URI:append:rockchip = " file://rockchip-kmeta;type=kmeta;name=rockchip-kmeta;destsuffix=rockchip-kmeta"
+SRC_URI:append:rockchip = " file://lttng.cfg"
 SRC_URI:append:nanopi-r4s = " file://realtek-r8169.cfg"
 
 KERNEL_FEATURES:append:rockchip:arm = " bsp/rockchip/remove-non-rockchip-arch-arm.scc"
diff --git a/recipes-kernel/linux/linux-rockchip/lttng.cfg b/recipes-kernel/linux/linux-rockchip/lttng.cfg
new file mode 100644
index 000000000000..17ba2a8116eb
--- /dev/null
+++ b/recipes-kernel/linux/linux-rockchip/lttng.cfg
@@ -0,0 +1,17 @@
+# Prerequisites for building lttng-modules.
+#
+# These machines have no BSP description in yocto-kernel-cache and build
+# from an upstream defconfig via KBUILD_DEFCONFIG, so the standard kernel
+# type's configuration is never applied and neither the arm nor the arm64
+# defconfig enables what lttng-modules needs.
+#
+# kprobes is required outright: src/wrapper/kallsyms.c fails to compile
+# without it on kernels >= 5.7.
+#
+# Tracepoints are what lttng-modules actually traces with, but that
+# symbol has no prompt and can only be selected, so enabling the function
+# tracer is what pulls it in. Without tracepoints lttng-modules builds no
+# modules at all rather than failing, leaving an empty package behind.
+CONFIG_KPROBES=y
+CONFIG_FTRACE=y
+CONFIG_FUNCTION_TRACER=y
-- 
2.50.0.173.g8b6f19ccfc3a