| Hollis Blanchard <[email protected]> |
| 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. |
