Re: [PATCH v13 03/10] qcom: spm: Add Subsystem Power Manager driver

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Dec 01, 2014 at 06:57:05PM +0000, Lina Iyer wrote:
> On Thu, Nov 27 2014 at 08:01 -0700, Lorenzo Pieralisi wrote:
> >On Thu, Nov 27, 2014 at 05:24:07AM +0000, Lina Iyer wrote:
> >
> >[...]
> >
> >> +static int spm_set_low_power_mode(enum pm_sleep_mode mode)
> >> +{
> >> +       struct spm_driver_data *drv = per_cpu(cpu_spm_drv,
> >> +                                               smp_processor_id());
> >> +       u32 start_index;
> >> +       u32 ctl_val;
> >> +
> >> +       if (!drv)
> >> +               return -ENXIO;
> >> +
> >> +       start_index = drv->reg_data->start_index[mode];
> >> +
> >> +       ctl_val = spm_register_read(drv, SPM_REG_SPM_CTL);
> >> +       ctl_val &= ~(SPM_CTL_INDEX << SPM_CTL_INDEX_SHIFT);
> >> +       ctl_val |= start_index << SPM_CTL_INDEX_SHIFT;
> >> +       ctl_val |= SPM_CTL_EN;
> >> +       spm_register_write(drv, SPM_REG_SPM_CTL, ctl_val);
> >> +
> >> +       /* Ensure we have written the start address */
> >> +       wmb();
> >
> >Can you explain please what this wmb is meant to achieve ? If it is there
> >to make sure the write made it to the SPM that's not a proper way
> >to achieve it, the barrier ensures ordering, not write completion.
>
> A write completion would be great, but assuming that the context switch
> using SCM or WFI could ensure the flush, then ordering is probably good
> enough for this.

I assume you want the write to the device to complete (side-effects are
observed) before the WFI (which would raise a StandbyWFI signal). The
safer way is to read back from the device (or check/poll for the device
state change) rather than a barrier.

-- 
Catalin