Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.
1 Hollis Blanchard <firstname.lastname@example.org> 2 5 Jun 2002 3 4 This document describes the system (including self-modifying code) used in the 5 PPC Linux kernel to support a variety of PowerPC CPUs without requiring 6 compile-time selection. 7 8 Early in the boot process the ppc32 kernel detects the current CPU type and 9 chooses a set of features accordingly. Some examples include Altivec support, 10 split instruction and data caches, and if the CPU supports the DOZE and NAP 11 sleep modes. 12 13 Detection of the feature set is simple. A list of processors can be found in 14 arch/powerpc/kernel/cputable.c. The PVR register is masked and compared with 15 each value in the list. If a match is found, the cpu_features of cur_cpu_spec 16 is assigned to the feature bitmask for this processor and a __setup_cpu 17 function is called. 18 19 C code may test 'cur_cpu_spec[smp_processor_id()]->cpu_features' for a 20 particular feature bit. This is done in quite a few places, for example 21 in ppc_setup_l2cr(). 22 23 Implementing cpufeatures in assembly is a little more involved. There are 24 several paths that are performance-critical and would suffer if an array 25 index, structure dereference, and conditional branch were added. To avoid the 26 performance penalty but still allow for runtime (rather than compile-time) CPU 27 selection, unused code is replaced by 'nop' instructions. This nop'ing is 28 based on CPU 0's capabilities, so a multi-processor system with non-identical 29 processors will not work (but such a system would likely have other problems 30 anyways). 31 32 After detecting the processor type, the kernel patches out sections of code 33 that shouldn't be used by writing nop's over it. Using cpufeatures requires 34 just 2 macros (found in arch/powerpc/include/asm/cputable.h), as seen in head.S 35 transfer_to_handler: 36 37 #ifdef CONFIG_ALTIVEC 38 BEGIN_FTR_SECTION 39 mfspr r22,SPRN_VRSAVE /* if G4, save vrsave register value */ 40 stw r22,THREAD_VRSAVE(r23) 41 END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC) 42 #endif /* CONFIG_ALTIVEC */ 43 44 If CPU 0 supports Altivec, the code is left untouched. If it doesn't, both 45 instructions are replaced with nop's. 46 47 The END_FTR_SECTION macro has two simpler variations: END_FTR_SECTION_IFSET 48 and END_FTR_SECTION_IFCLR. These simply test if a flag is set (in 49 cur_cpu_spec->cpu_features) or is cleared, respectively. These two macros 50 should be used in the majority of cases. 51 52 The END_FTR_SECTION macros are implemented by storing information about this 53 code in the '__ftr_fixup' ELF section. When do_cpu_ftr_fixups 54 (arch/powerpc/kernel/misc.S) is invoked, it will iterate over the records in 55 __ftr_fixup, and if the required feature is not present it will loop writing 56 nop's from each BEGIN_FTR_SECTION to END_FTR_SECTION.