Re: [PATCH 1/6] iommu/arm-smmu: add support for specifying clocks

Olav Haugan
2014-08-19T19:03:09Z

Hi Will,

On 8/19/2014 5:58 AM, Will Deacon wrote:
> On Wed, Aug 13, 2014 at 01:51:34AM +0100, Mitchel Humpherys wrote:
>> On some platforms with tight power constraints it is polite to only
>> leave your clocks on for as long as you absolutely need them. Currently
>> we assume that all clocks necessary for SMMU register access are always
>> on.
>>
>> Add some optional device tree properties to specify any clocks that are
>> necessary for SMMU register access and turn them on and off as needed.
>>
>> If no clocks are specified in the device tree things continue to work
>> the way they always have: we assume all necessary clocks are always
>> turned on.
>
> How does this interact with an SMMU in bypass mode?

Do you mean if you have a platform that requires clock and power
management but we leave the SMMU in bypass (i.e. no one calls into the
SMMU driver) how are the clock/power managed?

Clients of the SMMU driver are required to vote for clocks and power
when they know they need to use the SMMU. However, the clock and power
needed to be on for the SMMU to service bus masters aren't necessarily
the same as the ones needed to read/write registers...See below.

>
> [...]
>
>> +static int arm_smmu_enable_clocks(struct arm_smmu_device *smmu)
>> +{
>> + int i, ret = 0;
>> +
>> + for (i = 0; i < smmu->num_clocks; ++i) {
>> + ret = clk_prepare_enable(smmu->clocks[i]);
>> + if (ret) {
>> + dev_err(smmu->dev, "Couldn't enable clock #%d\n", i);
>> + while (i--)
>> + clk_disable_unprepare(smmu->clocks[i]);
>> + break;
>> + }
>> + }
>> +
>> + return ret;
>> +}
>> +
>> +static void arm_smmu_disable_clocks(struct arm_smmu_device *smmu)
>> +{
>> + int i;
>> +
>> + for (i = 0; i < smmu->num_clocks; ++i)
>> + clk_disable_unprepare(smmu->clocks[i]);
>> +}
>
> What stops theses from racing with each other when there are multiple
> clocks? I also assume that the clk API ignores calls to clk_enable_prepare
> for a clk that's already enabled? I couldn't find that code...

All the clock APIs are reference counted yes. Not sure what you mean by
racing with each other? When you call to enable a clock the call does
not return until the clock is already ON (or OFF).

>> /* Wait for any pending TLB invalidations to complete */
>> static void arm_smmu_tlb_sync(struct arm_smmu_device *smmu)
>> {
>> @@ -644,11 +672,15 @@ static irqreturn_t arm_smmu_context_fault(int irq, void *dev)
>> struct arm_smmu_device *smmu = smmu_domain->smmu;
>> void __iomem *cb_base;
>>
>> + arm_smmu_enable_clocks(smmu);
>
> How can I get a context interrupt from an SMMU without its clocks enabled?

Good question. At least in our SoC we usually have at least 1 "core"
clock and 1 "programming" clock. Both clocks are needed to read/write
registers but only 1 of the clocks is needed for the SMMU to service bus
master requests.

>
> [...]
>
>> +int arm_smmu_device_cfg_probe(struct arm_smmu_device *smmu)
>> {
>> unsigned long size;
>> void __iomem *gr0_base = ARM_SMMU_GR0(smmu);
>> @@ -2027,10 +2124,16 @@ static int arm_smmu_device_dt_probe(struct platform_device *pdev)
>> }
>> dev_notice(dev, "registered %d master devices\n", i);
>>
>> - err = arm_smmu_device_cfg_probe(smmu);
>> + err = arm_smmu_init_clocks(smmu);
>> if (err)
>> goto out_put_masters;
>>
>> + arm_smmu_enable_clocks(smmu);
>> +
>> + err = arm_smmu_device_cfg_probe(smmu);
>> + if (err)
>> + goto out_disable_clocks;
>> +
>> parse_driver_options(smmu);
>>
>> if (smmu->version > 1 &&
>> @@ -2039,7 +2142,7 @@ static int arm_smmu_device_dt_probe(struct platform_device *pdev)
>> "found only %d context interrupt(s) but %d required\n",
>> smmu->num_context_irqs, smmu->num_context_banks);
>> err = -ENODEV;
>> - goto out_put_masters;
>> + goto out_disable_clocks;
>> }
>>
>> for (i = 0; i < smmu->num_global_irqs; ++i) {
>> @@ -2061,12 +2164,16 @@ static int arm_smmu_device_dt_probe(struct platform_device *pdev)
>> spin_unlock(&arm_smmu_devices_lock);
>>
>> arm_smmu_device_reset(smmu);
>> + arm_smmu_disable_clocks(smmu);
>
> I wonder if this is really the right thing to do. Rather than the
> fine-grained clock enable/disable you have, why don't we just enable in
> domain_init and disable in domain_destroy, with refcounting for the clocks?
>

So the whole point of all of this is that we try to save power. As Mitch
wrote in the commit text we want to only leave the clock and power on
for as short period of time as possible.

Thanks,

Olav

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