Re: [PATCH v6 07/10] iommu/arm-smmu-v3: Implement pm_runtime & system sleep ops

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Apr 22, 2026 at 01:40:53PM +0000, Pranjal Shrivastava wrote:

> > > +/*
> > > + * This should always return true if devlinks are setup correctly
> > > + * and the client using the SMMU is active.
> > > + */
> > > +__maybe_unused bool arm_smmu_rpm_get_if_active(struct arm_smmu_device *smmu)
> > > +{
> > > +       if (!arm_smmu_is_active(smmu))
> > > +               return false;
> > 
> > I'm wondering if this check is redundant. What would happen if we removed it?
> > 
> 
> You're right that pm_runtime_get_if_active() shall suffice. However, I
> added the arm_smmu_is_active() check as a lockless fast path for unmaps
> 
> Consider a scneario where there're high-frequency unmaps (unmap-storm)
> while the SMMU is suspended. Without this check, every thread would 
> contend for the dev->power.lock spinlock inside the RPM core 
> (pm_runtime_get_if_active) just to discover the device is not active.
> 
> This check allows CPUs to elide the command immediately without
> contending for the dev->power.lock spinlock inside the RPM core
> (get_if_active call) and avoids unnecessary cacheline bouncing when the
> SMMU is suspended.

But these sorts of pre checks are almost always racy, I think you need a
comment or lockdep statement explaining how the locking works.

Jason