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-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/ppc/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/ppc/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.