kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 107 insertions, 0 deletions

diff --git a/Documentation/driver-api/thermal/cpu-cooling-api.rst b/Documentation/driver-api/thermal/cpu-cooling-api.rst
new file mode 100644
index 000000000..645d914c4
--- /dev/null
+++ b/Documentation/driver-api/thermal/cpu-cooling-api.rst
@@ -0,0 +1,107 @@
+=======================
+CPU cooling APIs How To
+=======================
+
+Written by Amit Daniel Kachhap <amit.kachhap@linaro.org>
+
+Updated: 6 Jan 2015
+
+Copyright (c)  2012 Samsung Electronics Co., Ltd(http://www.samsung.com)
+
+0. Introduction
+===============
+
+The generic cpu cooling(freq clipping) provides registration/unregistration APIs
+to the caller. The binding of the cooling devices to the trip point is left for
+the user. The registration APIs returns the cooling device pointer.
+
+1. cpu cooling APIs
+===================
+
+1.1 cpufreq registration/unregistration APIs
+--------------------------------------------
+
+    ::
+
+	struct thermal_cooling_device
+	*cpufreq_cooling_register(struct cpumask *clip_cpus)
+
+    This interface function registers the cpufreq cooling device with the name
+    "thermal-cpufreq-%x". This api can support multiple instances of cpufreq
+    cooling devices.
+
+   clip_cpus:
+	cpumask of cpus where the frequency constraints will happen.
+
+    ::
+
+	struct thermal_cooling_device
+	*of_cpufreq_cooling_register(struct cpufreq_policy *policy)
+
+    This interface function registers the cpufreq cooling device with
+    the name "thermal-cpufreq-%x" linking it with a device tree node, in
+    order to bind it via the thermal DT code. This api can support multiple
+    instances of cpufreq cooling devices.
+
+    policy:
+	CPUFreq policy.
+
+
+    ::
+
+	void cpufreq_cooling_unregister(struct thermal_cooling_device *cdev)
+
+    This interface function unregisters the "thermal-cpufreq-%x" cooling device.
+
+    cdev: Cooling device pointer which has to be unregistered.
+
+2. Power models
+===============
+
+The power API registration functions provide a simple power model for
+CPUs.  The current power is calculated as dynamic power (static power isn't
+supported currently).  This power model requires that the operating-points of
+the CPUs are registered using the kernel's opp library and the
+`cpufreq_frequency_table` is assigned to the `struct device` of the
+cpu.  If you are using CONFIG_CPUFREQ_DT then the
+`cpufreq_frequency_table` should already be assigned to the cpu
+device.
+
+The dynamic power consumption of a processor depends on many factors.
+For a given processor implementation the primary factors are:
+
+- The time the processor spends running, consuming dynamic power, as
+  compared to the time in idle states where dynamic consumption is
+  negligible.  Herein we refer to this as 'utilisation'.
+- The voltage and frequency levels as a result of DVFS.  The DVFS
+  level is a dominant factor governing power consumption.
+- In running time the 'execution' behaviour (instruction types, memory
+  access patterns and so forth) causes, in most cases, a second order
+  variation.  In pathological cases this variation can be significant,
+  but typically it is of a much lesser impact than the factors above.
+
+A high level dynamic power consumption model may then be represented as::
+
+	Pdyn = f(run) * Voltage^2 * Frequency * Utilisation
+
+f(run) here represents the described execution behaviour and its
+result has a units of Watts/Hz/Volt^2 (this often expressed in
+mW/MHz/uVolt^2)
+
+The detailed behaviour for f(run) could be modelled on-line.  However,
+in practice, such an on-line model has dependencies on a number of
+implementation specific processor support and characterisation
+factors.  Therefore, in initial implementation that contribution is
+represented as a constant coefficient.  This is a simplification
+consistent with the relative contribution to overall power variation.
+
+In this simplified representation our model becomes::
+
+	Pdyn = Capacitance * Voltage^2 * Frequency * Utilisation
+
+Where `capacitance` is a constant that represents an indicative
+running time dynamic power coefficient in fundamental units of
+mW/MHz/uVolt^2.  Typical values for mobile CPUs might lie in range
+from 100 to 500.  For reference, the approximate values for the SoC in
+ARM's Juno Development Platform are 530 for the Cortex-A57 cluster and
+140 for the Cortex-A53 cluster.