--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0+
+
+Floating-point API
+==================
+
+Kernel code is normally prohibited from using floating-point (FP) registers or
+instructions, including the C float and double data types. This rule reduces
+system call overhead, because the kernel does not need to save and restore the
+userspace floating-point register state.
+
+However, occasionally drivers or library functions may need to include FP code.
+This is supported by isolating the functions containing FP code to a separate
+translation unit (a separate source file), and saving/restoring the FP register
+state around calls to those functions. This creates "critical sections" of
+floating-point usage.
+
+The reason for this isolation is to prevent the compiler from generating code
+touching the FP registers outside these critical sections. Compilers sometimes
+use FP registers to optimize inlined ``memcpy`` or variable assignment, as
+floating-point registers may be wider than general-purpose registers.
+
+Usability of floating-point code within the kernel is architecture-specific.
+Additionally, because a single kernel may be configured to support platforms
+both with and without a floating-point unit, FPU availability must be checked
+both at build time and at run time.
+
+Several architectures implement the generic kernel floating-point API from
+``linux/fpu.h``, as described below. Some other architectures implement their
+own unique APIs, which are documented separately.
+
+Build-time API
+--------------
+
+Floating-point code may be built if the option ``ARCH_HAS_KERNEL_FPU_SUPPORT``
+is enabled. For C code, such code must be placed in a separate file, and that
+file must have its compilation flags adjusted using the following pattern::
+
+ CFLAGS_foo.o += $(CC_FLAGS_FPU)
+ CFLAGS_REMOVE_foo.o += $(CC_FLAGS_NO_FPU)
+
+Architectures are expected to define one or both of these variables in their
+top-level Makefile as needed. For example::
+
+ CC_FLAGS_FPU := -mhard-float
+
+or::
+
+ CC_FLAGS_NO_FPU := -msoft-float
+
+Normal kernel code is assumed to use the equivalent of ``CC_FLAGS_NO_FPU``.
+
+Runtime API
+-----------
+
+The runtime API is provided in ``linux/fpu.h``. This header cannot be included
+from files implementing FP code (those with their compilation flags adjusted as
+above). Instead, it must be included when defining the FP critical sections.
+
+.. c:function:: bool kernel_fpu_available( void )
+
+ This function reports if floating-point code can be used on this CPU or
+ platform. The value returned by this function is not expected to change
+ at runtime, so it only needs to be called once, not before every
+ critical section.
+
+.. c:function:: void kernel_fpu_begin( void )
+ void kernel_fpu_end( void )
+
+ These functions create a floating-point critical section. It is only
+ valid to call ``kernel_fpu_begin()`` after a previous call to
+ ``kernel_fpu_available()`` returned ``true``. These functions are only
+ guaranteed to be callable from (preemptible or non-preemptible) process
+ context.
+
+ Preemption may be disabled inside critical sections, so their size
+ should be minimized. They are *not* required to be reentrant. If the
+ caller expects to nest critical sections, it must implement its own
+ reference counting.
projects / linux-block.git / commitdiff