Re: [RFC PATCH 2/2] of/clk: use "clkops-clocks" to specify clocks handled by clock_ops domain

[Laurent Pinchart]

Hi Kevin,

On Monday 08 September 2014 13:13:25 Kevin Hilman wrote:
> Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> writes:
> > On Monday 28 July 2014 23:52:34 Grant Likely wrote:
> >> On Mon, Jul 28, 2014 at 11:47 AM, Grygorii Strashko wrote:
> >> > On 07/28/2014 05:05 PM, Grant Likely wrote:
> >> >> On Thu, 12 Jun 2014 19:53:43 +0300, Grygorii Strashko wrote:
> [...]
> 
> >> > - Where and when to call of_clk_register_runtime_pm_clocks()?
> >> > 
> >> >   Bus notifier/ platform core/ device drivers
> >> 
> >> I would say in device drivers.
> > 
> > I tend to agree with that.
> > 
> > It will help here to take a step back and remember what the problem we're
> > trying to solve is.
> 
> [jumping in late, after Grygorii ping'd me about looking at this]
> 
> Laurent, thanks for summarizing the problem so well.  It helped me
> catchup on the discussion.

You're welcome. Sorry for the very late reply.

> > At the root is clock management. Our system comprise many clocks, and they
> > need to be handled. The Common Clock Framework nicely models the clocks,
> > and offers an API for drivers to retrieve device clocks and control them.
> > Drivers can thus implement clock management manually without much pain.
> > 
> > A clock can be managed in roughly three different ways :
> > 
> > - it can be enabled at probe time and disabled at remove time ;
> > 
> > - it can be enabled right before the device leaves its idle state and
> > disabled when the device goes back to idle ; or
> > 
> > - it can be enabled and disabled in a more fine-grained, device-specific
> > manner.
> > 
> > The selected clock management granularity depends on constraints specific
> > to the device and on how aggressive power saving needs to be. Enabling
> > the clocks at probe time and disabling them at remove time is enough for
> > most devices, but leads to a high power consumption. For that reason the
> > second clock management scheme is often desired.
> > 
> > Managing clocks manually in the driver is a valid option. However, when
> > adding runtime PM to the equation, and realizing that the clocks need to
> > be enabled in the runtime PM resume handler and disabled in the suspend
> > handler, the clock management code starts looking very similar in most
> > drivers. We're thus tempted to factorize it away from the drivers into a
> > shared location.
> > 
> > It's important to note at this point that the goal here is only to
> > simplify drivers. Moving clock management code out of the drivers doesn't
> > (unless I'm missing something) open the door to new possibilities, it just
> > serves as a simplification.
> 
> I disagree. Actually, it opens up the door to lots of new possibilities
> that are crucial for fine-grained PM with QoS. It is not just
> simplification. There are many good reasons that some SoCs have moved all
> the management of PM-related clocks *out* of device drivers. More on that
> below...
> 
> > Now, as Grygorii mentioned, differences between how a given IP core is
> > integrated in various SoCs can make clock management SoC-dependent. In the
> > vast majority of cases (which is really what we need to target, given that
> > our target is simplifying drivers) SoC integration can be described as a
> > list of clocks that must be managed. That list can be common to all
> > devices in a given SoC, or can be device-dependent as well.
> 
> If we care about fine-grained PM, this is a way-too oversimplified
> version of what SoC integragion means.
> 
> There are lots of pieces which fall under "SoC integration", for
> example: clock domains, power domains, voltage domains, SoC-specific
> wakeup capabilities, etc. etc.  Then, for fun throw in QoS constraints,
> and things get really exciting.
> 
> IOW, if you care about fine-grained PM and QoS, you simply can't reduce
> SoC integration down to "a list of clocks to be managed."

Of course. I was talking about SoC integration for clocks, not about SoC 
integration in general.

> QoS makes this interesting as well because a device driver's decision to
> gate its own clocks may have serious repercussions on the wakeup latency
> of *other* devices in the same power domain. For example, the clock gating
> of the last active device in a powerdomain may cause the enclosing power-
> domain to be power gated, having a major impact on the wakup latency of
> *all* devices in that power domain.
> 
> So if we're going to manage the list of PM-related clocks in the device
> driver, we'll also keep track of all the other devices in the same power
> domain, whether or not they're active, whether or not they have QoS
> constraints, etc. etc. Hopefully you can see that we're quickly way outside
> the scope of the IP block that the device driver is intended to manage.
> 
> All of this is "SoC integration" knowledge, and IMO doen't belong in the
> device drivers.  It belongs at the SoC integration level, and in todays
> kernel frameworks that means pm_domain/genpd.

Ok, there's more to it than I initially thought. Let's see how we can make 
this happen then :-)

> > Few locations can be used to express a per-device list of per-SoC clocks.
> > We can have clocks lists in a per-SoC and per-device location, per-device
> > clocks lists in an SoC-specific location, or per-SoC clocks lists in a
> > device- specific location.
> > 
> > The first option would require listing clocks to be managed by runtime PM
> > in DT nodes, as proposed by this patch set. I don't think this is the
> > best option, as that information is a mix of hardware description and
> > software policy, with the hardware description part being already present
> > in DT in the clocks property.
> 
> I'm not seeing which part you think is software policy here?  Which clocks
> are driving which IP blocks is a hardware description.
> 
> Which clocks are actually gated, and if/when is software policy and should
> be decided by the SoC-specific runtime PM and genpd implementations, but
> describing which clocks are wired to which IP blocks is certainly hardware
> description IMO.

The clocks property is a hardware description, but a new property that would 
hold a list of clocks to be managed by the runtime PM core is less so.

> > The second option calls for storing the lists in SoC code under arch/. As
> > we're trying to minimize the amount of SoC code there (and even remove SoC
> > code completely when possible) I don't think that's a good option.
> > 
> > The third option would require storing the clocks lists in device drivers.
> > I believe this is our best option, as a trade-off between simplicity and
> > versatility. Drivers that use runtime PM already need to enable it
> > explicitly when probing devices. Passing a list of clock names to runtime
> > PM at that point wouldn't complicate drivers much. When the clocks list
> > isn't SoC- dependent it could be stored as static information. Otherwise
> > it could be derived from DT (or any other source of hardware description)
> > using C code, offering all the versatility we need.
> 
> As is probably clear from above, I don't like this approach at all.

Do we at least agree that option 2 (storing the lists in arch/) is a bad idea 
and should be ruled out ?

> > The only drawback of this solution I can think of right now is that the
> > runtime PM core couldn't manage device clocks before probing the device.
> > Specifically device clocks couldn't be managed if no driver is loaded for
> > that device. I somehow recall that someone raised this as being a
> > problem, but I can't remember why.
> 
> The bigger drawback of this approach is that the device-drivers become a
> repository for SoC integration details that IMO don't belong in a device
> driver for a specific IP block.

I agree that SoC integration details don't belong in device drivers, but I 
don't see this as SoC integration details. The driver knows that the IP core 
it manages has two functional clock inputs, and knows how those clock inputs 
are named. The name is a property of the IP core, not of the SoC. Only how 
(and whether) those clocks are connected in the SoC is integration 
information.

Now, if the IP core itself can be synthesized with different clock option, 
those synthesis options belong to DT, either explicitly in the clock-names 
property or implicitly in the compatible property.

> Over the last few years, we've created abstractions for this kind of SoC
> integration information (pm_domains) as well as frameworks for handling
> much of the common parts (genpd) and in doing so, have been able to
> remove PM-related clock management from device drivers altogether.
> 
> I think managing this stuff back in device drivers would be a major step
> backwards.

-- 
Regards,

Laurent Pinchart

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