Re: [PATCH v8 3/5] iommu/arm-smmu: Invoke pm_runtime during probe, add/remove device

[Robin Murphy]

On 07/03/18 13:52, Tomasz Figa wrote:
> On Wed, Mar 7, 2018 at 9:38 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 02/03/18 10:10, Vivek Gautam wrote:
> > > From: Sricharan R <sricharan@xxxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxx>
> > > [vivek: Cleanup pm runtime calls]
> > > Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
> > > ---
> > >    drivers/iommu/arm-smmu.c | 96
> > > ++++++++++++++++++++++++++++++++++++++++++++----
> > >    1 file changed, 88 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> > > index c8b16f53f597..3d6a1875431f 100644
> > > --- a/drivers/iommu/arm-smmu.c
> > > +++ b/drivers/iommu/arm-smmu.c
> > > @@ -209,6 +209,8 @@ struct arm_smmu_device {
> > >          struct clk_bulk_data            *clks;
> > >          int                             num_clks;
> > >    +     bool                            rpm_supported;
> > > +
> >
> >
> > Can we not automatically infer this from whether clocks and/or power domains
> > are specified or not, then just use pm_runtime_enabled() as the fast-path
> > check as Tomasz originally proposed?
>
> I wouldn't tie this to presence of clocks, since as a next step we
> would want to actually control the clocks separately. (As far as I
> understand, on QCom SoCs we might want to have runtime PM active for
> the translation to work, but clocks gated whenever access to SMMU
> registers is not needed.) Moreover, you might still have some super
> high scale thousand-core systems that require clocks to be
> prepare-enabled, but runtime PM would be undesirable for the reasons
> we discussed before.
>
> > I worry that relying on statically-defined matchdata is just going to blow
> > up the driver and DT binding into a maintenance nightmare; I really don't
> > want to start needing separate definitions for e.g. "arm,juno-etr-mmu-401"
> > and "arm,juno-hdlcd-mmu-401" just because one otherwise-identical instance
> > within the SoC is in a separate controllable power domain while the others
> > aren't.
>
> I don't see a reason why both couldn't just have RPM supported
> regardless of whether there is a real power domain. It would
> effectively be just a no-op for those that don't have one.

Because you're then effectively defining "compatible" values for the 
sake of attaching software policy to them, rather than actually 
describing different hardware implementations.

The fact that RPM can't do anything meaningful unless relevant 
clock/power aspects *are* described, however, means that we shouldn't 
need additional information redundant with that. Much like the fact that 
we don't *already* have an "arm,juno-hdlcd-mmu-401" compatible to 
account for those being integrated such that IDR0.CTTW has the wrong 
value, since the presence or not of the "dma-coherent" property already 
describes the truth in that regard.

> IMHO the
> only reason to avoid having the RPM enabled is the scalability issue
> we discussed before.

Yes, but that's kind of my point; in reality high throughput/minimal 
latency and aggressive power management are more or less mutually 
exclusive. Mobile SoCs with fine-grained clock trees and power domains 
won't have multiple 40GBe/NVMf/whatever links running flat out in 
parallel; conversely networking/infrastructure/server SoCs aren't 
designed around saving every last microamp of leakage current - even in 
the (fairly unlikely) case of the interconnect clocks being 
software-gateable at all I would be very surprised if that were ever 
exposed directly to Linux (FWIW I believe ACPI essentially *requires* 
clocks to be abstracted behind firmware).

Realistically then, explicit clocks are only expected on systems which 
care about power management. We can always revisit that assumption if 
anything crazy where it isn't the case ever becomes non-theoretical, but 
for now it's one I'm entirely comfortable with. If on the other hand it 
turns out that we can rely on just a power domain being present wherever 
we want RPM, making clocks moot, then all the better.

Robin.