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author | Takashi Iwai <tiwai@suse.de> | 2019-09-10 14:03:08 +0300 |
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committer | Takashi Iwai <tiwai@suse.de> | 2019-09-10 14:03:08 +0300 |
commit | 7711fb7dac1ab77fd1b4d948f4647a569e4a1ae2 (patch) | |
tree | dd35ad21e0eb13572757dab279ae41f6d953baed /Documentation/powerpc/cpu_features.rst | |
parent | 789492f0c86505e63369907bcb1afdf52dec9366 (diff) | |
parent | bb831786117519fc16dfd3eaa7b84e4f6bbb8d99 (diff) | |
download | linux-7711fb7dac1ab77fd1b4d948f4647a569e4a1ae2.tar.xz |
Merge tag 'asoc-v5.4' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound into for-next
ASoC: Updates for v5.4
Quite a big update this time around, particularly in the core
where we've had a lot of cleanups from Morimoto-san - there's
not much functional change but quite a bit of modernization
going on. We've also seen a lot of driver work, a lot of it
cleanups but also some particular drivers.
- Lots and lots of cleanups from Morimoto-san and Yue Haibing.
- Lots of cleanups and enhancements to the Freescale, sunxi dnd
Intel rivers.
- Initial Sound Open Firmware suppot for i.MX8.
- Removal of w90x900 and nuc900 drivers as the platforms are
being removed.
- New support for Cirrus Logic CS47L15 and CS47L92, Freescale
i.MX 7ULP and 8MQ, Meson G12A and NXP UDA1334
Diffstat (limited to 'Documentation/powerpc/cpu_features.rst')
-rw-r--r-- | Documentation/powerpc/cpu_features.rst | 60 |
1 files changed, 60 insertions, 0 deletions
diff --git a/Documentation/powerpc/cpu_features.rst b/Documentation/powerpc/cpu_features.rst new file mode 100644 index 000000000000..b7bcdd2f41bb --- /dev/null +++ b/Documentation/powerpc/cpu_features.rst @@ -0,0 +1,60 @@ +============ +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. |