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

[Lina Iyer <lina.iyer@linaro.org>]

On Tue, Dec 02 2014 at 04:10 -0700, Catalin Marinas wrote:
>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.

Polling back to see if the register write was indeed successful, seems
a bit more than what I would like on a low latency mode like Standby.
Let me check with the hardware engineers, if a write ordering is
required. It seems to me like I am not getting much out of the wmb()
anyways. I will find out what kind of guarantees I would need to ensure
the SPM write is successful.

Thanks for your time, Catalin.

Thanks,
Lina.