1 .. SPDX-License-Identifier: GPL-2.0+
2
3 Floating-point API
4 ==================
5
6 Kernel code is normally prohibited from using
7 instructions, including the C float and double
8 system call overhead, because the kernel does
9 userspace floating-point register state.
10
11 However, occasionally drivers or library funct
12 This is supported by isolating the functions c
13 translation unit (a separate source file), and
14 state around calls to those functions. This cr
15 floating-point usage.
16
17 The reason for this isolation is to prevent th
18 touching the FP registers outside these critic
19 use FP registers to optimize inlined ``memcpy`
20 floating-point registers may be wider than gen
21
22 Usability of floating-point code within the ke
23 Additionally, because a single kernel may be c
24 both with and without a floating-point unit, F
25 both at build time and at run time.
26
27 Several architectures implement the generic ke
28 ``linux/fpu.h``, as described below. Some othe
29 own unique APIs, which are documented separate
30
31 Build-time API
32 --------------
33
34 Floating-point code may be built if the option
35 is enabled. For C code, such code must be plac
36 file must have its compilation flags adjusted
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 bo
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 equiv
51
52 Runtime API
53 -----------
54
55 The runtime API is provided in ``linux/fpu.h``
56 from files implementing FP code (those with th
57 above). Instead, it must be included when defi
58
59 .. c:function:: bool kernel_fpu_available( voi
60
61 This function reports if floating-poin
62 platform. The value returned by this f
63 at runtime, so it only needs to be cal
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-poin
70 valid to call ``kernel_fpu_begin()`` a
71 ``kernel_fpu_available()`` returned ``
72 guaranteed to be callable from (preemp
73 context.
74
75 Preemption may be disabled inside crit
76 should be minimized. They are *not* re
77 caller expects to nest critical sectio
78 reference counting.
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