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-================
-Kernel mode NEON
-================
-
-TL;DR summary
--------------
-* Use only NEON instructions, or VFP instructions that don't rely on support
-  code
-* Isolate your NEON code in a separate compilation unit, and compile it with
-  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
-* Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
-  NEON code
-* Don't sleep in your NEON code, and be aware that it will be executed with
-  preemption disabled
-
-
-Introduction
-------------
-It is possible to use NEON instructions (and in some cases, VFP instructions) in
-code that runs in kernel mode. However, for performance reasons, the NEON/VFP
-register file is not preserved and restored at every context switch or taken
-exception like the normal register file is, so some manual intervention is
-required. Furthermore, special care is required for code that may sleep [i.e.,
-may call schedule()], as NEON or VFP instructions will be executed in a
-non-preemptible section for reasons outlined below.
-
-
-Lazy preserve and restore
--------------------------
-The NEON/VFP register file is managed using lazy preserve (on UP systems) and
-lazy restore (on both SMP and UP systems). This means that the register file is
-kept 'live', and is only preserved and restored when multiple tasks are
-contending for the NEON/VFP unit (or, in the SMP case, when a task migrates to
-another core). Lazy restore is implemented by disabling the NEON/VFP unit after
-every context switch, resulting in a trap when subsequently a NEON/VFP
-instruction is issued, allowing the kernel to step in and perform the restore if
-necessary.
-
-Any use of the NEON/VFP unit in kernel mode should not interfere with this, so
-it is required to do an 'eager' preserve of the NEON/VFP register file, and
-enable the NEON/VFP unit explicitly so no exceptions are generated on first
-subsequent use. This is handled by the function kernel_neon_begin(), which
-should be called before any kernel mode NEON or VFP instructions are issued.
-Likewise, the NEON/VFP unit should be disabled again after use to make sure user
-mode will hit the lazy restore trap upon next use. This is handled by the
-function kernel_neon_end().
-
-
-Interruptions in kernel mode
-----------------------------
-For reasons of performance and simplicity, it was decided that there shall be no
-preserve/restore mechanism for the kernel mode NEON/VFP register contents. This
-implies that interruptions of a kernel mode NEON section can only be allowed if
-they are guaranteed not to touch the NEON/VFP registers. For this reason, the
-following rules and restrictions apply in the kernel:
-* NEON/VFP code is not allowed in interrupt context;
-* NEON/VFP code is not allowed to sleep;
-* NEON/VFP code is executed with preemption disabled.
-
-If latency is a concern, it is possible to put back to back calls to
-kernel_neon_end() and kernel_neon_begin() in places in your code where none of
-the NEON registers are live. (Additional calls to kernel_neon_begin() should be
-reasonably cheap if no context switch occurred in the meantime)
-
-
-VFP and support code
---------------------
-Earlier versions of VFP (prior to version 3) rely on software support for things
-like IEEE-754 compliant underflow handling etc. When the VFP unit needs such
-software assistance, it signals the kernel by raising an undefined instruction
-exception. The kernel responds by inspecting the VFP control registers and the
-current instruction and arguments, and emulates the instruction in software.
-
-Such software assistance is currently not implemented for VFP instructions
-executed in kernel mode. If such a condition is encountered, the kernel will
-fail and generate an OOPS.
-
-
-Separating NEON code from ordinary code
----------------------------------------
-The compiler is not aware of the special significance of kernel_neon_begin() and
-kernel_neon_end(), i.e., that it is only allowed to issue NEON/VFP instructions
-between calls to these respective functions. Furthermore, GCC may generate NEON
-instructions of its own at -O3 level if -mfpu=neon is selected, and even if the
-kernel is currently compiled at -O2, future changes may result in NEON/VFP
-instructions appearing in unexpected places if no special care is taken.
-
-Therefore, the recommended and only supported way of using NEON/VFP in the
-kernel is by adhering to the following rules:
-
-* isolate the NEON code in a separate compilation unit and compile it with
-  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
-* issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
-  into the unit containing the NEON code from a compilation unit which is *not*
-  built with the GCC flag '-mfpu=neon' set.
-
-As the kernel is compiled with '-msoft-float', the above will guarantee that
-both NEON and VFP instructions will only ever appear in designated compilation
-units at any optimization level.
-
-
-NEON assembler
---------------
-NEON assembler is supported with no additional caveats as long as the rules
-above are followed.
-
-
-NEON code generated by GCC
---------------------------
-The GCC option -ftree-vectorize (implied by -O3) tries to exploit implicit
-parallelism, and generates NEON code from ordinary C source code. This is fully
-supported as long as the rules above are followed.
-
-
-NEON intrinsics
----------------
-NEON intrinsics are also supported. However, as code using NEON intrinsics
-relies on the GCC header <arm_neon.h>, (which #includes <stdint.h>), you should
-observe the following in addition to the rules above:
-
-* Compile the unit containing the NEON intrinsics with '-ffreestanding' so GCC
-  uses its builtin version of <stdint.h> (this is a C99 header which the kernel
-  does not supply);
-* Include <arm_neon.h> last, or at least after <linux/types.h>