forked from Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
60 lines
2.7 KiB
60 lines
2.7 KiB
============ |
|
CPU Features |
|
============ |
|
|
|
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/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.
|
|
|