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SubjectRe: [RFC PATCH 05/10] alpha: Primitive support for CPU power down.
On Tue, Jul 16, 2013 at 10:34 AM, Richard Henderson <rth@twiddle.net> wrote:
> Use WTINT to wait for the next interrupt. Squash the WTINT call
> if the PALcode doesn't support it (e.g. MILO). No attempt is yet
> made to skip clock ticks during normal scheduling in order to stay
> in power down mode longer.

The architecture reference manual says

> The counter, PCC, may increment at a lower rate or may stop entirely
> during wtint execution. This side effect is implementation dependent.

Is that anything to worry about?

>
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
> arch/alpha/include/asm/pal.h | 1 +
> arch/alpha/include/uapi/asm/pal.h | 1 +
> arch/alpha/kernel/process.c | 15 +++++++++++++++
> arch/alpha/kernel/traps.c | 12 ++++++++++++
> 4 files changed, 29 insertions(+)
>
> diff --git a/arch/alpha/include/asm/pal.h b/arch/alpha/include/asm/pal.h
> index 6fcd2b5..e78ec9b 100644
> --- a/arch/alpha/include/asm/pal.h
> +++ b/arch/alpha/include/asm/pal.h
> @@ -89,6 +89,7 @@ __CALL_PAL_W1(wrmces, unsigned long);
> __CALL_PAL_RW2(wrperfmon, unsigned long, unsigned long, unsigned long);
> __CALL_PAL_W1(wrusp, unsigned long);
> __CALL_PAL_W1(wrvptptr, unsigned long);
> +__CALL_PAL_RW1(wtint, unsigned long, unsigned long);
>
> /*
> * TB routines..
> diff --git a/arch/alpha/include/uapi/asm/pal.h b/arch/alpha/include/uapi/asm/pal.h
> index 3c0ce08..dfc8140 100644
> --- a/arch/alpha/include/uapi/asm/pal.h
> +++ b/arch/alpha/include/uapi/asm/pal.h
> @@ -46,6 +46,7 @@
> #define PAL_rdusp 58
> #define PAL_whami 60
> #define PAL_retsys 61
> +#define PAL_wtint 62
> #define PAL_rti 63
>
>
> diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
> index f2360a7..3130f13 100644
> --- a/arch/alpha/kernel/process.c
> +++ b/arch/alpha/kernel/process.c
> @@ -46,6 +46,21 @@
> void (*pm_power_off)(void) = machine_power_off;
> EXPORT_SYMBOL(pm_power_off);
>
> +/*
> + * Sleep the CPU.
> + * EV6, LCA45 and QEMU know how to power down, skipping N timer interrupts.
> + */
> +void arch_cpu_idle(void)
> +{
> + wtint(0);
> + local_irq_enable();
> +}
> +
> +void arch_cpu_idle_dead(void)
> +{
> + wtint(INT_MAX);
> +}
> +
> struct halt_info {
> int mode;
> char *restart_cmd;
> diff --git a/arch/alpha/kernel/traps.c b/arch/alpha/kernel/traps.c
> index affccb9..991f6c3 100644
> --- a/arch/alpha/kernel/traps.c
> +++ b/arch/alpha/kernel/traps.c
> @@ -243,6 +243,18 @@ do_entIF(unsigned long type, struct pt_regs *regs)
> (const char *)(data[1] | (long)data[2] << 32),
> data[0]);
> }
> + if (type == 4) {
> + /* If CALL_PAL WTINT is not supported by the PALcode,
> + "emulate" it by overwriting the insn. */

The pseudo-code for WTINT contains an IF(implemented) check, where the
ELSE case just does v0 <- 0. So is overwriting with nop just an
optimization to avoid the (expensive) PAL call? If it is, could we
clarify the comment?

> + unsigned int *pinsn
> + = (unsigned int *) regs->pc - 1;
> + if (*pinsn == PAL_wtint) {
> + *pinsn = 0x47e01400; /* mov 0,$0 */
> + imb();
> + regs->r0 = 0;
> + return;
> + }
> + }
> die_if_kernel((type == 1 ? "Kernel Bug" : "Instruction fault"),
> regs, type, NULL);
> }
> --
> 1.8.1.4
>


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