Re: [PATCH 3/3] i2c: i2c-qcom-geni: Add support for 'assigned-performance-states'

[Rajendra Nayak <rnayak@codeaurora.org>]

On 5/7/2021 2:36 PM, Ulf Hansson wrote:
> On Tue, 4 May 2021 at 09:18, Rajendra Nayak <rnayak@codeaurora.org> wrote:
>>
>>
>> []...
>>>>>>
>>>>>> Ulf, Viresh, I think we discussed this at the time of introducing the
>>>>>> performance states.
>>>>>>
>>>>>> The client's state does not affect if its performance_state should
>>>>>> be included in the calculation of the aggregated performance_state, so
>>>>>> each driver that needs to keep some minimum performance state needs to
>>>>>> have these two snippets.
>>>>>>
>>>>>> Would it not make sense to on enable/disable re-evaluate the
>>>>>> performance_state and potentially reconfigure the hardware
>>>>>> automatically?
>>>>>
>>>>> I agree, this will be repeated across multiple drivers which would
>>>>> need some minimal vote while they are active, handling this during
>>>>> genpd enable/disable in genpd core makes sense.
>>>>
>>>> Initially that's what we tried out, but we realized that it was
>>>> difficult to deal with this internally in genpd, but more importantly
>>>> it also removed some flexibility from consumers and providers. See
>>>> commit 68de2fe57a8f ("PM / Domains: Make genpd performance states
>>>> orthogonal to the idlestates").
>>>>
>>>> As a matter of fact this was quite recently discussed [1], which also
>>>> pointed out some issues when using the "required-opps" in combination,
>>>> but perhaps that got resolved? Viresh?
>>>
>>> So I looked again at that thread in detail today. The basic idea was
>>> to enable/disable the genpd from within the OPP core and there were
>>> doubts on how to do that efficiently as there are cases where domains
>>> may be enabled for an OPP, but not for others.. etc. etc.
>>>
>>> I am not sure if I consider that thread as part of the discussion we
>>> are having here, they may be related, but that thread doesn't block
>>> anything to be done in the genpd core.
>>
>> That's true, the 2 threads are different in the sense that one talks
>> about having OPP core managing power on/off along with setting perf state,
>> while the other talks about genpd core managing a default perf state
>> along with power on/off, but they are similar in the sense that both
>> are related to the discussion whether we should treat powering on and off
>> a domain related to setting its performance state or if it should be
>> considered completely orthogonal.
>>
>> I think the clock framework treats setting clock rates and turning
>> on/off a clock orthogonal because there is an inherent assumption that
>> once the clock is turned off, what rate it was set to should not matter,
>> and it can be running at the same rate when we turn the clock back on.
>>
>> I guess we can have the same assumption here that a perf state of a
>> power domain should not matter if the power domain is turned off
>> and hence the perf state need not be dropped explicitly during power off,
>> atleast that should be true for the qcom power domains supporting perf
>> state upstream.
>>
>> Should that be the approach taken here? I guess that would mean the patch
>> I had proposed earlier [1] to manage this in the genpd core would have to set the default
>> perf state at attach and remove it only during a detach of the device to
>> the pm_domain, and not manage it during the runtime_suspend/resume of the device.
> 
> Right, I think this would be a step in the right direction, but it's
> not sufficient to solve the complete problem. As you also point out
> below.
> 
>>
>>>> A consumer driver
>>>> can no longer make its vote for its device to stick around, when the
>>>> device becomes runtime suspended - and how do we know that we never
>>>> need to support such a case?
>>
>> The above approach should take care of this but the down side of it would be,
>> unlike in the case of clocks where the devices assigning a default clock rate
>> might be doing so on a device specific clock (rarely shared with other devices)
>> in case of power domain, and especially in the qcom implementation of these
>> power domains which support perf state, these can be large domains with lots of devices,
>> and any device being active (not necessarily wanting any default perf state) will keep
>> the domain at the default perf state, requested by a device which isn't really active.
> 
> Yep, this certainly sounds suboptimal. To me, this isn't good enough.
> 
>>
>>> What about doing this just for the assigned-performance-state case as
>>> the clients don't want to play with it at all.
>>
>> well, thats possible too, but you obviously can't reuse the same bindings
>> in such cases
> 
> Not sure I understand the issue with the DT binding? Let me elaborate
> on how I think we could move forward.
> 
> It looks like we have two problems to solve:
> 
> *) We need a new DT binding.
> If that becomes a generic property along the lines of the
> "assigned-performance-state" as suggested - or if we decide to add a
> SoC specific binding via using an additional cell in "power-domains"
> (suggested by Rob), doesn't really matter much to me. The arguments
> for the new DT property are very much similar to why we added
> "assigned-clock-rates" for clocks.
> 
> **) We want to avoid boiler-plate code in drivers to manage
> "assigned-performance-state" for their devices.
> No matter what DT property we decide on (generic or SoC specific), we
> should be able to manage this from the PM domain (genpd) layer. No
> changes in the drivers should be needed.
> If a generic binding is used, we could consider to let genpd
> internally manage the whole thing (DT parsing and updating performance
> state votes for assigned-performance-state only).

Sure, so for starters does that mean I should re-spin my series which
adds the generic 'assigned-performance-states' bindings and see if Rob
is OK with that? I am guessing you are OK with the way that binding gets
used within genpd core in that series, or would you want it to be handled
differently?

> If we go for an SoC specific binding, the genpd provider needs to be
> updated. It can manage DT parsing from the ->attach|detach_dev()
> callbacks and update performance votes from the ->start|stop()
> callbacks.
> We could also consider a hybrid of these two solutions.
>>
>> [1] https://lore.kernel.org/patchwork/patch/1284042/
> 
> Kind regards
> Uffe
> 

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