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, 235 insertions, 0 deletions

diff --git a/Documentation/gpu/afbc.rst b/Documentation/gpu/afbc.rst
new file mode 100644
index 000000000..4d38dc49d
--- /dev/null
+++ b/Documentation/gpu/afbc.rst
@@ -0,0 +1,235 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+===================================
+ Arm Framebuffer Compression (AFBC)
+===================================
+
+AFBC is a proprietary lossless image compression protocol and format.
+It provides fine-grained random access and minimizes the amount of
+data transferred between IP blocks.
+
+AFBC can be enabled on drivers which support it via use of the AFBC
+format modifiers defined in drm_fourcc.h. See DRM_FORMAT_MOD_ARM_AFBC(*).
+
+All users of the AFBC modifiers must follow the usage guidelines laid
+out in this document, to ensure compatibility across different AFBC
+producers and consumers.
+
+Components and Ordering
+=======================
+
+AFBC streams can contain several components - where a component
+corresponds to a color channel (i.e. R, G, B, X, A, Y, Cb, Cr).
+The assignment of input/output color channels must be consistent
+between the encoder and the decoder for correct operation, otherwise
+the consumer will interpret the decoded data incorrectly.
+
+Furthermore, when the lossless colorspace transform is used
+(AFBC_FORMAT_MOD_YTR, which should be enabled for RGB buffers for
+maximum compression efficiency), the component order must be:
+
+ * Component 0: R
+ * Component 1: G
+ * Component 2: B
+
+The component ordering is communicated via the fourcc code in the
+fourcc:modifier pair. In general, component '0' is considered to
+reside in the least-significant bits of the corresponding linear
+format. For example, COMP(bits):
+
+ * DRM_FORMAT_ABGR8888
+
+   * Component 0: R(8)
+   * Component 1: G(8)
+   * Component 2: B(8)
+   * Component 3: A(8)
+
+ * DRM_FORMAT_BGR888
+
+   * Component 0: R(8)
+   * Component 1: G(8)
+   * Component 2: B(8)
+
+ * DRM_FORMAT_YUYV
+
+   * Component 0: Y(8)
+   * Component 1: Cb(8, 2x1 subsampled)
+   * Component 2: Cr(8, 2x1 subsampled)
+
+In AFBC, 'X' components are not treated any differently from any other
+component. Therefore, an AFBC buffer with fourcc DRM_FORMAT_XBGR8888
+encodes with 4 components, like so:
+
+ * DRM_FORMAT_XBGR8888
+
+   * Component 0: R(8)
+   * Component 1: G(8)
+   * Component 2: B(8)
+   * Component 3: X(8)
+
+Please note, however, that the inclusion of a "wasted" 'X' channel is
+bad for compression efficiency, and so it's recommended to avoid
+formats containing 'X' bits. If a fourth component is
+required/expected by the encoder/decoder, then it is recommended to
+instead use an equivalent format with alpha, setting all alpha bits to
+'1'. If there is no requirement for a fourth component, then a format
+which doesn't include alpha can be used, e.g. DRM_FORMAT_BGR888.
+
+Number of Planes
+================
+
+Formats which are typically multi-planar in linear layouts (e.g. YUV
+420), can be encoded into one, or multiple, AFBC planes. As with
+component order, the encoder and decoder must agree about the number
+of planes in order to correctly decode the buffer. The fourcc code is
+used to determine the number of encoded planes in an AFBC buffer,
+matching the number of planes for the linear (unmodified) format.
+Within each plane, the component ordering also follows the fourcc
+code:
+
+For example:
+
+ * DRM_FORMAT_YUYV: nplanes = 1
+
+   * Plane 0:
+
+     * Component 0: Y(8)
+     * Component 1: Cb(8, 2x1 subsampled)
+     * Component 2: Cr(8, 2x1 subsampled)
+
+ * DRM_FORMAT_NV12: nplanes = 2
+
+   * Plane 0:
+
+     * Component 0: Y(8)
+
+   * Plane 1:
+
+     * Component 0: Cb(8, 2x1 subsampled)
+     * Component 1: Cr(8, 2x1 subsampled)
+
+Cross-device interoperability
+=============================
+
+For maximum compatibility across devices, the table below defines
+canonical formats for use between AFBC-enabled devices. Formats which
+are listed here must be used exactly as specified when using the AFBC
+modifiers. Formats which are not listed should be avoided.
+
+.. flat-table:: AFBC formats
+
+   * - Fourcc code
+     - Description
+     - Planes/Components
+
+   * - DRM_FORMAT_ABGR2101010
+     - 10-bit per component RGB, with 2-bit alpha
+     - Plane 0: 4 components
+              * Component 0: R(10)
+              * Component 1: G(10)
+              * Component 2: B(10)
+              * Component 3: A(2)
+
+   * - DRM_FORMAT_ABGR8888
+     - 8-bit per component RGB, with 8-bit alpha
+     - Plane 0: 4 components
+              * Component 0: R(8)
+              * Component 1: G(8)
+              * Component 2: B(8)
+              * Component 3: A(8)
+
+   * - DRM_FORMAT_BGR888
+     - 8-bit per component RGB
+     - Plane 0: 3 components
+              * Component 0: R(8)
+              * Component 1: G(8)
+              * Component 2: B(8)
+
+   * - DRM_FORMAT_BGR565
+     - 5/6-bit per component RGB
+     - Plane 0: 3 components
+              * Component 0: R(5)
+              * Component 1: G(6)
+              * Component 2: B(5)
+
+   * - DRM_FORMAT_ABGR1555
+     - 5-bit per component RGB, with 1-bit alpha
+     - Plane 0: 4 components
+              * Component 0: R(5)
+              * Component 1: G(5)
+              * Component 2: B(5)
+              * Component 3: A(1)
+
+   * - DRM_FORMAT_VUY888
+     - 8-bit per component YCbCr 444, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(8)
+              * Component 1: Cb(8)
+              * Component 2: Cr(8)
+
+   * - DRM_FORMAT_VUY101010
+     - 10-bit per component YCbCr 444, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(10)
+              * Component 1: Cb(10)
+              * Component 2: Cr(10)
+
+   * - DRM_FORMAT_YUYV
+     - 8-bit per component YCbCr 422, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(8)
+              * Component 1: Cb(8, 2x1 subsampled)
+              * Component 2: Cr(8, 2x1 subsampled)
+
+   * - DRM_FORMAT_NV16
+     - 8-bit per component YCbCr 422, two plane
+     - Plane 0: 1 component
+              * Component 0: Y(8)
+       Plane 1: 2 components
+              * Component 0: Cb(8, 2x1 subsampled)
+              * Component 1: Cr(8, 2x1 subsampled)
+
+   * - DRM_FORMAT_Y210
+     - 10-bit per component YCbCr 422, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(10)
+              * Component 1: Cb(10, 2x1 subsampled)
+              * Component 2: Cr(10, 2x1 subsampled)
+
+   * - DRM_FORMAT_P210
+     - 10-bit per component YCbCr 422, two plane
+     - Plane 0: 1 component
+              * Component 0: Y(10)
+       Plane 1: 2 components
+              * Component 0: Cb(10, 2x1 subsampled)
+              * Component 1: Cr(10, 2x1 subsampled)
+
+   * - DRM_FORMAT_YUV420_8BIT
+     - 8-bit per component YCbCr 420, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(8)
+              * Component 1: Cb(8, 2x2 subsampled)
+              * Component 2: Cr(8, 2x2 subsampled)
+
+   * - DRM_FORMAT_YUV420_10BIT
+     - 10-bit per component YCbCr 420, single plane
+     - Plane 0: 3 components
+              * Component 0: Y(10)
+              * Component 1: Cb(10, 2x2 subsampled)
+              * Component 2: Cr(10, 2x2 subsampled)
+
+   * - DRM_FORMAT_NV12
+     - 8-bit per component YCbCr 420, two plane
+     - Plane 0: 1 component
+              * Component 0: Y(8)
+       Plane 1: 2 components
+              * Component 0: Cb(8, 2x2 subsampled)
+              * Component 1: Cr(8, 2x2 subsampled)
+
+   * - DRM_FORMAT_P010
+     - 10-bit per component YCbCr 420, two plane
+     - Plane 0: 1 component
+              * Component 0: Y(10)
+       Plane 1: 2 components
+              * Component 0: Cb(10, 2x2 subsampled)
+              * Component 1: Cr(10, 2x2 subsampled)