Re: [PATCH v7 3/6] iommu/arm-smmu: Invoke pm_runtime during probe, add/remove device

[Robin Murphy <robin.murphy@arm.com>]

On 13/02/18 08:24, Tomasz Figa wrote:
> Hi Vivek,
> 
> Thanks for the patch. Please see my comments inline.
> 
> On Wed, Feb 7, 2018 at 7:31 PM, Vivek Gautam
> <vivek.gautam@codeaurora.org> wrote:
>> From: Sricharan R <sricharan@codeaurora.org>
>>
>> The smmu device probe/remove and add/remove master device callbacks
>> gets called when the smmu is not linked to its master, that is without
>> the context of the master device. So calling runtime apis in those places
>> separately.
>>
>> Signed-off-by: Sricharan R <sricharan@codeaurora.org>
>> [vivek: Cleanup pm runtime calls]
>> Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
>> ---
>>   drivers/iommu/arm-smmu.c | 42 ++++++++++++++++++++++++++++++++++++++----
>>   1 file changed, 38 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
>> index 9e2f917e16c2..c024f69c1682 100644
>> --- a/drivers/iommu/arm-smmu.c
>> +++ b/drivers/iommu/arm-smmu.c
>> @@ -913,11 +913,15 @@ static void arm_smmu_destroy_domain_context(struct iommu_domain *domain)
>>          struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>>          struct arm_smmu_device *smmu = smmu_domain->smmu;
>>          struct arm_smmu_cfg *cfg = &smmu_domain->cfg;
>> -       int irq;
>> +       int ret, irq;
>>
>>          if (!smmu || domain->type == IOMMU_DOMAIN_IDENTITY)
>>                  return;
>>
>> +       ret = pm_runtime_get_sync(smmu->dev);
>> +       if (ret)
>> +               return;
> 
> pm_runtime_get_sync() will return 0 if the device was powered off, 1
> if it was already/still powered on or runtime PM is not compiled in,
> or a negative value on error, so shouldn't the test be (ret < 0)?
> 
> Moreover, I'm actually wondering if it makes any sense to power up the
> hardware just to program it and power it down again. In a system where
> the IOMMU is located within a power domain, it would cause the IOMMU
> block to lose its state anyway.

This is generally for the case where the SMMU internal state remains 
active, but the programming interface needs to be powered up in order to 
access it.

> Actually, reflecting back on "[PATCH v7 2/6] iommu/arm-smmu: Add
> pm_runtime/sleep ops", perhaps it would make more sense to just
> control the clocks independently of runtime PM? Then, runtime PM could
> be used for real power management, e.g. really powering the block up
> and down, for further power saving.

Unfortunately that ends up pretty much unmanageable, because there are 
numerous different SMMU microarchitectures with fundamentally different 
clock/power domain schemes (multiplied by individual SoC integration 
possibilities). Since this is fundamentally a generic architectural 
driver, adding explicit clock support would probably make the whole 
thing about 50% clock code, with complicated decision trees around every 
hardware access calculating which clocks are necessary for a given 
operation on a given system. That maintainability aspect is why we've 
already nacked such a fine-grained approach in the past.

Robin.