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, 86 insertions, 0 deletions

diff --git a/Documentation/driver-api/sync_file.rst b/Documentation/driver-api/sync_file.rst
new file mode 100644
index 000000000..496fb2c3b
--- /dev/null
+++ b/Documentation/driver-api/sync_file.rst
@@ -0,0 +1,86 @@
+===================
+Sync File API Guide
+===================
+
+:Author: Gustavo Padovan <gustavo at padovan dot org>
+
+This document serves as a guide for device drivers writers on what the
+sync_file API is, and how drivers can support it. Sync file is the carrier of
+the fences(struct dma_fence) that are needed to synchronize between drivers or
+across process boundaries.
+
+The sync_file API is meant to be used to send and receive fence information
+to/from userspace. It enables userspace to do explicit fencing, where instead
+of attaching a fence to the buffer a producer driver (such as a GPU or V4L
+driver) sends the fence related to the buffer to userspace via a sync_file.
+
+The sync_file then can be sent to the consumer (DRM driver for example), that
+will not use the buffer for anything before the fence(s) signals, i.e., the
+driver that issued the fence is not using/processing the buffer anymore, so it
+signals that the buffer is ready to use. And vice-versa for the consumer ->
+producer part of the cycle.
+
+Sync files allows userspace awareness on buffer sharing synchronization between
+drivers.
+
+Sync file was originally added in the Android kernel but current Linux Desktop
+can benefit a lot from it.
+
+in-fences and out-fences
+------------------------
+
+Sync files can go either to or from userspace. When a sync_file is sent from
+the driver to userspace we call the fences it contains 'out-fences'. They are
+related to a buffer that the driver is processing or is going to process, so
+the driver creates an out-fence to be able to notify, through
+dma_fence_signal(), when it has finished using (or processing) that buffer.
+Out-fences are fences that the driver creates.
+
+On the other hand if the driver receives fence(s) through a sync_file from
+userspace we call these fence(s) 'in-fences'. Receiving in-fences means that
+we need to wait for the fence(s) to signal before using any buffer related to
+the in-fences.
+
+Creating Sync Files
+-------------------
+
+When a driver needs to send an out-fence userspace it creates a sync_file.
+
+Interface::
+
+	struct sync_file *sync_file_create(struct dma_fence *fence);
+
+The caller pass the out-fence and gets back the sync_file. That is just the
+first step, next it needs to install an fd on sync_file->file. So it gets an
+fd::
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+
+and installs it on sync_file->file::
+
+	fd_install(fd, sync_file->file);
+
+The sync_file fd now can be sent to userspace.
+
+If the creation process fail, or the sync_file needs to be released by any
+other reason fput(sync_file->file) should be used.
+
+Receiving Sync Files from Userspace
+-----------------------------------
+
+When userspace needs to send an in-fence to the driver it passes file descriptor
+of the Sync File to the kernel. The kernel can then retrieve the fences
+from it.
+
+Interface::
+
+	struct dma_fence *sync_file_get_fence(int fd);
+
+
+The returned reference is owned by the caller and must be disposed of
+afterwards using dma_fence_put(). In case of error, a NULL is returned instead.
+
+References:
+
+1. struct sync_file in include/linux/sync_file.h
+2. All interfaces mentioned above defined in include/linux/sync_file.h