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

Will Deacon
2014-08-19T12:58:34Z

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?

[...]

> +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...

> /* 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?

[...]

> +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?

Will