1 ============
2 CPU Features
3 ============
4
5 Hollis Blanchard <hollis@austin.ibm.com>
6 5 Jun 2002
7
8 This document describes the system (including
9 PPC Linux kernel to support a variety of Power
10 compile-time selection.
11
12 Early in the boot process the ppc32 kernel det
13 chooses a set of features accordingly. Some ex
14 split instruction and data caches, and if the
15 sleep modes.
16
17 Detection of the feature set is simple. A list
18 arch/powerpc/kernel/cputable.c. The PVR regist
19 each value in the list. If a match is found, t
20 is assigned to the feature bitmask for this pr
21 function is called.
22
23 C code may test 'cur_cpu_spec[smp_processor_id
24 particular feature bit. This is done in quite
25 in ppc_setup_l2cr().
26
27 Implementing cpufeatures in assembly is a litt
28 several paths that are performance-critical an
29 index, structure dereference, and conditional
30 performance penalty but still allow for runtim
31 selection, unused code is replaced by 'nop' in
32 based on CPU 0's capabilities, so a multi-proc
33 processors will not work (but such a system wo
34 anyways).
35
36 After detecting the processor type, the kernel
37 that shouldn't be used by writing nop's over i
38 just 2 macros (found in arch/powerpc/include/a
39 transfer_to_handler::
40
41 #ifdef CONFIG_ALTIVEC
42 BEGIN_FTR_SECTION
43 mfspr r22,SPRN_VRSAVE
44 stw r22,THREAD_VRSAVE(r23)
45 END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
46 #endif /* CONFIG_ALTIVEC */
47
48 If CPU 0 supports Altivec, the code is left un
49 instructions are replaced with nop's.
50
51 The END_FTR_SECTION macro has two simpler vari
52 and END_FTR_SECTION_IFCLR. These simply test i
53 cur_cpu_spec[0]->cpu_features) or is cleared,
54 should be used in the majority of cases.
55
56 The END_FTR_SECTION macros are implemented by
57 code in the '__ftr_fixup' ELF section. When do
58 (arch/powerpc/kernel/misc.S) is invoked, it wi
59 __ftr_fixup, and if the required feature is no
60 nop's from each BEGIN_FTR_SECTION to END_FTR_S
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