Re: [PATCH V3 0/5] iommu/arm-smmu: Add runtime pm/sleep support

[Jordan Crouse <jcrouse@codeaurora.org>]

On Tue, Apr 04, 2017 at 12:39:14PM -0700, Stephen Boyd wrote:
> On 04/03, Will Deacon wrote:
> > On Fri, Mar 31, 2017 at 10:58:16PM -0400, Rob Clark wrote:
> > > On Fri, Mar 31, 2017 at 1:54 PM, Will Deacon <will.deacon@arm.com> wrote:
> > > > On Thu, Mar 09, 2017 at 09:05:43PM +0530, Sricharan R wrote:
> > > >> This series provides the support for turning on the arm-smmu's
> > > >> clocks/power domains using runtime pm. This is done using the
> > > >> recently introduced device links patches, which lets the symmu's
> > > >> runtime to follow the master's runtime pm, so the smmu remains
> > > >> powered only when the masters use it.
> > > >
> > > > Do you have any numbers for the power savings you achieve with this?
> > > > How often do we actually manage to stop the SMMU clocks on an SoC with
> > > > a handful of masters?
> > > >
> > > > In other words, is this too coarse-grained to be useful, or is it common
> > > > that all the devices upstream of the SMMU are suspended?
> > > 
> > > well, if you think about a phone/tablet with a command mode panel,
> > > pretty much all devices will be suspended most of the time ;-)
> > 
> > Well, that's really what I was asking about. I assumed that periodic
> > modem/radio transactions would keep the SMMU clocked, so would like to get a
> > rough idea of the power savings achieved with this coarse-grained approach.
> 
> Sometimes we distribute SMMUs to each IP block in the system and
> let each one of those live in their own clock + power domain. In
> these cases, the SMMU can be powered down along with the only IP
> block that uses it. Furthermore, sometimes we put the IP block
> and the SMMU inside the same power domain to really tie the two
> together, so we definitely have cases where all devices (device?)
> upstream of the SMMU are suspended. And in the case of
> multimedia, it could be very often that something like the camera
> app isn't open and thus the SMMU dedicated for the camera can be
> powered down.
> 
> Other times we have two SMMUs in the system where one is
> dedicated to GPU and the other is "everything else". Even in
> these cases, we can suspend the GPU one when the GPU is inactive
> because it's the only consumer. The other SMMU might not be as
> fine grained, but I think we still power it down quite often
> because the consumers are mostly multimedia devices that aren't
> active when the display is off.

And just to confuse things even further: with per-instance pagetables we have an
interest in forcing the SMMU clocks *on* because we don't know when the GPU
might try to hit the registers to switch a pagetable and if somebody in the
pipeline is actively trying to do power management at the same time hilarity
will ensue.

The alternative to pm_runtime is the downstream driver that probes the SMMU
clocks from DT and frobs them itself. I think we can agree that is far less
reasonable.

Jordan

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