On Mon, Jan 17, 2011 at 10:15:25AM +0000, Catalin Marinas wrote:
> On 15 January 2011 16:11, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
> > SMP requires at least the ARMv6K extensions to be present, so if we're
> > running on SMP, the WFE and SEV instructions must be available.
> >
> > However, when we run on UP, the v6K extensions may not be available,
> > and so we don't want WFE/SEV to be in the instruction stream. Use the
> > SMP alternatives infrastructure to replace these instructions with NOPs
> > if we build for SMP but run on UP.
> [...]
> > --- a/arch/arm/include/asm/spinlock.h
> > +++ b/arch/arm/include/asm/spinlock.h
> > @@ -5,17 +5,36 @@
> > #error SMP not supported on pre-ARMv6 CPUs
> > #endif
> >
> > +/*
> > + * sev and wfe are ARMv6K extensions. Uniprocessor ARMv6 may not have the K
> > + * extensions, so when running on UP, we have to patch these instructions away.
> > + */
> > +#define ALT_SMP(smp, up) \
> > + "9998: " smp "\n" \
> > + " .pushsection \".alt.smp.init\", \"a\"\n" \
> > + " .long 9998b\n" \
> > + " " up "\n" \
> > + " .popsection\n"
> > +
> > +#ifdef CONFIG_THUMB2_KERNEL
> > +#define SEV ALT_SMP("sev.w", "nop.w")
> > +#define WFE(cond) ALT_SMP("wfe" cond ".w", "nop.w")
> > +#else
> > +#define SEV ALT_SMP("sev", "nop")
> > +#define WFE(cond) ALT_SMP("wfe" cond, "nop")
> > +#endif
>
> In the SEV macro definition, can you also include the dsb?
No, you can't do preprocessor conditionals in the middle of a macro
definition, and I don't want to have 4 versions of the SEV stuff.
> This barrier is only there because of sev, otherwise we don't need it
> (we have a dmb prior to releasing the lock).
1. Does it make sense to have sev and wfe instructions in non-SMP kernels?
We now have in asm/system.h:
#if __LINUX_ARM_ARCH__ >= 7 || \
(__LINUX_ARM_ARCH__ == 6 && defined(CONFIG_CPU_32v6K))
#define sev() __asm__ __volatile__ ("sev" : : : "memory")
#define wfe() __asm__ __volatile__ ("wfe" : : : "memory")
#define wfi() __asm__ __volatile__ ("wfi" : : : "memory")
#endif
2. Should we have a smp_dsb() which makes the dsb conditional elsewhere?
3. Do we always need a dsb prior to a sev? Maybe the SPEAR patches need
another review to determine how they're using sev()?
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