diff options
Diffstat (limited to 'Documentation/powerpc/cpu_features.rst')
| -rw-r--r-- | Documentation/powerpc/cpu_features.rst | 60 |
1 files changed, 0 insertions, 60 deletions
diff --git a/Documentation/powerpc/cpu_features.rst b/Documentation/powerpc/cpu_features.rst deleted file mode 100644 index b7bcdd2f41..0000000000 --- a/Documentation/powerpc/cpu_features.rst +++ /dev/null @@ -1,60 +0,0 @@ -============ -CPU Features -============ - -Hollis Blanchard <hollis@austin.ibm.com> -5 Jun 2002 - -This document describes the system (including self-modifying code) used in the -PPC Linux kernel to support a variety of PowerPC CPUs without requiring -compile-time selection. - -Early in the boot process the ppc32 kernel detects the current CPU type and -chooses a set of features accordingly. Some examples include Altivec support, -split instruction and data caches, and if the CPU supports the DOZE and NAP -sleep modes. - -Detection of the feature set is simple. A list of processors can be found in -arch/powerpc/kernel/cputable.c. The PVR register is masked and compared with -each value in the list. If a match is found, the cpu_features of cur_cpu_spec -is assigned to the feature bitmask for this processor and a __setup_cpu -function is called. - -C code may test 'cur_cpu_spec[smp_processor_id()]->cpu_features' for a -particular feature bit. This is done in quite a few places, for example -in ppc_setup_l2cr(). - -Implementing cpufeatures in assembly is a little more involved. There are -several paths that are performance-critical and would suffer if an array -index, structure dereference, and conditional branch were added. To avoid the -performance penalty but still allow for runtime (rather than compile-time) CPU -selection, unused code is replaced by 'nop' instructions. This nop'ing is -based on CPU 0's capabilities, so a multi-processor system with non-identical -processors will not work (but such a system would likely have other problems -anyways). - -After detecting the processor type, the kernel patches out sections of code -that shouldn't be used by writing nop's over it. Using cpufeatures requires -just 2 macros (found in arch/powerpc/include/asm/cputable.h), as seen in head.S -transfer_to_handler:: - - #ifdef CONFIG_ALTIVEC - BEGIN_FTR_SECTION - mfspr r22,SPRN_VRSAVE /* if G4, save vrsave register value */ - stw r22,THREAD_VRSAVE(r23) - END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC) - #endif /* CONFIG_ALTIVEC */ - -If CPU 0 supports Altivec, the code is left untouched. If it doesn't, both -instructions are replaced with nop's. - -The END_FTR_SECTION macro has two simpler variations: END_FTR_SECTION_IFSET -and END_FTR_SECTION_IFCLR. These simply test if a flag is set (in -cur_cpu_spec[0]->cpu_features) or is cleared, respectively. These two macros -should be used in the majority of cases. - -The END_FTR_SECTION macros are implemented by storing information about this -code in the '__ftr_fixup' ELF section. When do_cpu_ftr_fixups -(arch/powerpc/kernel/misc.S) is invoked, it will iterate over the records in -__ftr_fixup, and if the required feature is not present it will loop writing -nop's from each BEGIN_FTR_SECTION to END_FTR_SECTION. |