RE: cpuidle omap 4430, some clarifications please?

[Santosh Shilimkar <santosh.shilimkar@ti.com>]

> -----Original Message-----
> From: linux-omap-owner@vger.kernel.org [mailto:linux-omap-
> owner@vger.kernel.org] On Behalf Of Etienne Le Sueur
> Sent: Monday, January 10, 2011 10:36 AM
> To: linux-omap@vger.kernel.org
> Subject: cpuidle omap 4430, some clarifications please?
>
> Hello,
>
> I'm using linux-2.6.35.3 on a pandaboard taking power measurements
> in different idle states (I've written a userspace governor for
> cpuidle).
>
> It seems like the code which is supposed to put the cores in deep
> sleep never actually gets called, see code below with my comments...
>
> Taken from arch/arm/mach-omap2/cpuidle44xx.c, omap4_enter_idle,
> which gets called by cpuidle:
>
> ===============
>
>      /*
>       * Do only WFI for non-boot CPU(aux cores)
>       */
>      if (dev->cpu) {
>          wmb();
>          do_wfi();
>          goto return_sleep_time;
>      }
>
> So on OMAP 4430, CPU1 never goes deeper than WFI?
>
>      /*
>       * Do only a WFI as long as CPU1 is online
>       */
>      if (num_online_cpus() > 1) {
>          wmb();
>          do_wfi();
>          goto return_sleep_time;
>      }
>
> So while CPU1 is online (it never goes offline due to the above
> code) CPU0 will never go deeper than WFI?
>
>      /*
>       * Hold on till CPU1 hits OFF
>       */
>      cpu1_state = pwrdm_read_pwrst(cpu1_pd);
>      if (cpu1_state != PWRDM_POWER_OFF) {
>          wmb();
>          do_wfi();
>          goto return_sleep_time;
>      }
>
> It seems like CPU1 will never 'hit off' ?
>
>      if (cx->type > OMAP4_STATE_C1)
>          clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER,
> &cpu_id);
>
> #ifdef CONFIG_PM_DEBUG
>      pwrdm_pre_transition();
> #endif
>
> The code below never gets called, therefore neither CPU0, CPU1 nor
> the MPU go into a deeper sleep state?
>
>      pwrdm_set_logic_retst(mpu_pd, cx->mpu_logic_state);
>      omap4_set_pwrdm_state(mpu_pd, cx->mpu_state);
>
>      omap4_set_pwrdm_state(core_pd, cx->core_state);
>
>      omap4_enter_sleep(dev->cpu, cx->cpu0_state);
>
> =================
>
> Can someone clarify for me?
>
Yes.
Simple rule is, no low power states are targeted when both CPUs
are online. The C-states gets targeted only when secondary
CPU gets offline through hotplug infrastrcutre. CPU1 hits
OFF mode through hot-plug path and not via idle.

With this CPU1 hits OFF state, and IDLE thread running
on master CPU can hit deeper low power states. It's more of
hardware restriction what is there on OMAP4.

To put it in slightly simpler terms,

In SMP mode, no active PM is exercised in IDLE and
gets triggered when secondary cores are out of the system.

Hope this clarifies.

Regards,
Santosh