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

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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.
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