Re: [PATCH v4 0/3] DEVFREQ, DVFS framework for non-CPU devices

["Turquette, Mike" <mturquette@ti.com>]

On Thu, Aug 4, 2011 at 11:18 PM, MyungJoo Ham <myungjoo.ham@gmail.com> wrote:
> On Fri, Aug 5, 2011 at 6:59 AM, Turquette, Mike <mturquette@ti.com> wrote:
>> On Thu, Aug 4, 2011 at 1:15 AM, MyungJoo Ham <myungjoo.ham@gmail.com> wrote:
>>>
>>> Ok.. I see.
>>>
>>> Now, I can agree with you that tickle is subset of QoS request.
>>>
>>> As long as we have QoS request feature on devices with either OPP or
>>> DEVFREQ, tickling is not needed.
>>>
>>> I'll remove tickle in the next revision (along with some bugfixes for
>>> bugs found recently).
>>>
>>>
>>> Anyway, it appears that clock-rate-wise QoS request may be dealt at
>>> OPP so that the OPPs meeting the QoS requests w/ frequency or voltage
>>> specifications are enabled and returned with opp_find_* functions.
>>> Maybe we will need to separate enable/disable by
>>> opp_enable()/opp_disable() from enable/disable by QoS requests so that
>>> the two may have different semantics. Then, QoS requests cannot
>>> override opp_disable and opp_enable cannot override QoS requests. This
>>> way, any clock-setting code properly based on OPP (including any
>>> customized devfreq governors) cannot violate QoS requests.
>>
>> If devfreq used the QoS API in it's ->target() call then this would
>> not be an issue, and further illustrates the idea of devfreq simply
>> being a policy into a proper QoS API.
>>
>> Regards,
>> Mike
>>
>
> Yes, if we choose an OPP that meets the PM-QoS requests before calling
> ->target() in devfreq_do(), there wouldn't be an issue.
>
> However, if a device is using DEVFREQ, it also means the device has
> OPPs (mandatory for DEVFREQ). If the device is using PM QoS as well as
> OPP, I guess the correctly implemented device driver needs to call
> opp_enable() and opp_disable() according to PM QoS's update_target()
> calls through the PM QoS notifier cb.
>
> Then, for such drivers, DEVFREQ automatically meets PM QoS requests
> without any modification; as long as QoS meeting OPPs are enabled at
> the device driver's PM QoS callback, there is no QoS issue.
>
> Therefore, now, it appears that neither OPP or DEVFREQ should be
> allowed to directly touch PM QoS APIs, but, the device driver should
> do so with notifier by simply calling opp-enable/disable if the
> frequency is the key QoS "actuator".
>
> If we are going to let DEVFREQ handle its corresponding devices' PM
> QoS APIs, it would mean that both device driver and its DEVFREQ codes
> will be handling PM QoS API duplicated (or worse, inconsistently).

I'm not getting too bogged down with the OPP specifics because I don't
know if that interface is going to be used in the future, and I don't
think that devfreq will need to know about OPPs once the DVFS QoS API
exists.  In that case, devfreq can just requests clock frequencies
through the QoS API.  I view devfreq's usage of the OPP library as
temporary.

The real issue here is that we don't want some weird feedback loop
with device QoS requests and devfreq targets stepping on each other.

One way to handle this is to partition QoS use in drivers away from
devfreq usage.  For example, if a GPU supports performance counters
and can introspect its own usage, then it is a perfect candidate for
devfreq; on the flip-side device drivers should *not* be allowed to
put performance/qos constraints on this particular GPU, since we
assume that the performance counters/devfreq governor will handle the
whole job for us.  Since this centralizes the decision-making for the
GPU it is safe for the devfreq->target() call to use the QoS APIs for
controlling the GPU, since no one else will.  This avoids the feedback
loop.

On the other hand, if the GPU does not support performance counters
then it should not use devfreq at all and rely 100% of QoS constraints
from various sources: the GPU driver might request a high OPP every
time work comes in, coupled with a timeout; if a QoS knob is exported
to userspace then some OpenGL library might hold constraints through
it; or some other kernel driver (video-related?) needs to use the GPU
then it can hold a constraint through the QoS API.

So there is a clear partition of QoS API usage between devices that
support performance counters and ones that do not.  We want to avoid a
feedback loop here.

Regards,
Mike

> Cheers,
> MyungJoo
>
>>> How about this concept of getting QoS requests associated with clock rates?
>>>
>>>
>>>
>>> Cheers!
>>> MyungJoo.
>>> --
>>> MyungJoo Ham, Ph.D.
>>> Mobile Software Platform Lab,
>>> Digital Media and Communications (DMC) Business
>>> Samsung Electronics
>>> cell: 82-10-6714-2858
>>> _______________________________________________
>>> linux-pm mailing list
>>> linux-pm@lists.linux-foundation.org
>>> https://lists.linux-foundation.org/mailman/listinfo/linux-pm
>>>
>>
>
>
>
> --
> MyungJoo Ham, Ph.D.
> Mobile Software Platform Lab,
> Digital Media and Communications (DMC) Business
> Samsung Electronics
> cell: 82-10-6714-2858
>