1 .. SPDX-License-Identifier: GPL-2.0+ 2 3 Floating-point API 4 ================== 5 6 Kernel code is normally prohibited from using floating-point (FP) registers or 7 instructions, including the C float and double data types. This rule reduces 8 system call overhead, because the kernel does not need to save and restore the 9 userspace floating-point register state. 10 11 However, occasionally drivers or library functions may need to include FP code. 12 This is supported by isolating the functions containing FP code to a separate 13 translation unit (a separate source file), and saving/restoring the FP register 14 state around calls to those functions. This creates "critical sections" of 15 floating-point usage. 16 17 The reason for this isolation is to prevent the compiler from generating code 18 touching the FP registers outside these critical sections. Compilers sometimes 19 use FP registers to optimize inlined ``memcpy`` or variable assignment, as 20 floating-point registers may be wider than general-purpose registers. 21 22 Usability of floating-point code within the kernel is architecture-specific. 23 Additionally, because a single kernel may be configured to support platforms 24 both with and without a floating-point unit, FPU availability must be checked 25 both at build time and at run time. 26 27 Several architectures implement the generic kernel floating-point API from 28 ``linux/fpu.h``, as described below. Some other architectures implement their 29 own unique APIs, which are documented separately. 30 31 Build-time API 32 -------------- 33 34 Floating-point code may be built if the option ``ARCH_HAS_KERNEL_FPU_SUPPORT`` 35 is enabled. For C code, such code must be placed in a separate file, and that 36 file must have its compilation flags adjusted using the following pattern:: 37 38 CFLAGS_foo.o += $(CC_FLAGS_FPU) 39 CFLAGS_REMOVE_foo.o += $(CC_FLAGS_NO_FPU) 40 41 Architectures are expected to define one or both of these variables in their 42 top-level Makefile as needed. For example:: 43 44 CC_FLAGS_FPU := -mhard-float 45 46 or:: 47 48 CC_FLAGS_NO_FPU := -msoft-float 49 50 Normal kernel code is assumed to use the equivalent of ``CC_FLAGS_NO_FPU``. 51 52 Runtime API 53 ----------- 54 55 The runtime API is provided in ``linux/fpu.h``. This header cannot be included 56 from files implementing FP code (those with their compilation flags adjusted as 57 above). Instead, it must be included when defining the FP critical sections. 58 59 .. c:function:: bool kernel_fpu_available( void ) 60 61 This function reports if floating-point code can be used on this CPU or 62 platform. The value returned by this function is not expected to change 63 at runtime, so it only needs to be called once, not before every 64 critical section. 65 66 .. c:function:: void kernel_fpu_begin( void ) 67 void kernel_fpu_end( void ) 68 69 These functions create a floating-point critical section. It is only 70 valid to call ``kernel_fpu_begin()`` after a previous call to 71 ``kernel_fpu_available()`` returned ``true``. These functions are only 72 guaranteed to be callable from (preemptible or non-preemptible) process 73 context. 74 75 Preemption may be disabled inside critical sections, so their size 76 should be minimized. They are *not* required to be reentrant. If the 77 caller expects to nest critical sections, it must implement its own 78 reference counting.
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