> > On 20 August 2012 12:50, 함명주 <myungjoo....@samsung.com> wrote:
> >
> > > Prepare devfreq core framework to support devices which
> > > can idle. When device idleness is detected perhaps through
> > > runtime-pm, need some mechanism to suspend devfreq load
> > > monitoring and resume when device is back online. Present
> > > code continues monitoring unless device is removed from
> > > devfreq core.
> > >
> > > This patch introduces following updates,
> > >
> > > - move device load monitoring logic to ondemand governor as
> > > it is specific to ondemand.
> >
> >
> > We have this ondemand governor in the mainline devfreq. However,
> > we have (not upstreamed yet) governors with specific purpose (for
> > GPU or for a specific chip) with load monitoring logic. Although
> > we don't have them upstreamed yet, why don't you keep the monitoring
> > logic sharable by other governors. (From today, I'm not objecting to
> > have individual workqueue, but I still don't want to let each active
> > governor reimplement the same things.)
> >
> >
>
> If more governors are planned based on load monitoring, possibly
> ondemand is not good enough even after adding additional
> configurable parameters. Could you please describe one such
> case which demand new governor?
>
> Please find my next comments.
GPU devfreq driver requires behaviors similar to conservative though
not exactly same.
In our internal development branch, it increases frequencies differently
according to the current usage (# GPU cores used and % each core activated).
If it's almost 100% and the queue is filling up, it goes to max.
If it's (for example) 98%, it goes to the next faster step.
Besides, GPU developers want to differ the poling intervals according
to the current frequency.
Another case is with bus + memory-interface case. We are going to use
a custom governor for them as each bus and memory-interface has its
own DVFS capability while we want them to coordinate. The two drivers
may be hooked by the custom governor. We don't have a specific detail
with this as it is developed by another division. However, what I know
is that they are going to use their own custon governor with
ondemand-like behavior in principle.
Because devfreq is meant to be used by various types of devices,
we cannot assume that the default governor is "mostly" fine.
Device driver writers can embed governors in their own devfreq
driver code ("custom governor").
> >
> >
> >
> > > - devfreq core interacts with governors via events to perform
> > > specific actions. These events include start/stop devfreq,
> > > and frequency limit changes outside devfreq. This sets ground
> > > for adding suspend/resume events.
> >
> >
> > event_handler with START/STOP/UPDATE seem fine.
> >
> > However, init() and exit() should be different from START/STOP.
> > We do not need to do init and exit every time when we do runtime
> > suspend/resume.
> >
> >
>
> I agree and its already taken care in this patch.
>
> DEVFREQ_GOV_START : Event for initializing and starting of devfreq governor
> for a device. Happens only when device is added using devfreq_add_device().
>
> DEVFREQ_GOV_STOP: Event for stopping and uninitialization of devfreq governor
> for a device. Happens only when device is removed using
> devfreq_remove_device().
>
> Events introduced in next patch (2/3) -
> DEVFREQ_GOV_SUSPEND: Event for suspending devfreq of a device. I.e
> suspending of load monitoring of device in case of ondemand governor.
> Happens only when devfreq_suspend_device() is called.
>
> DEVFREQ_GOV_RESUME: Event for resuming devfreq of a device. I.e
> continue load monitoring of already suspended device in case of ondemand
> governor. Happens only when devfreq_resume_device() is called.
Ah.. suspend/resume events are seperated. Please never mind this.
> >
> > > - use per device work instead of global work to monitor device
> > > load. This enables suspend/resume of device devfreq and
> > > reduces monitoring code complexity.
> >
> >
> > It's fine to have a delayed work struct per device.
> >
> > However, please try to keep as many things/features in devfreq.c as
> > possible; i.e., reduce features and code size of governors. Because,
> > we will be supporting various types of devices with devfreq, there
> > will be various governors and I don't want them to have shared things
> > reimplemented. Dealing with the delayed work at devfreq.c and let
> > governors choose to use it (at its struct) or not should be fine.
> >
> >
>
> Delayed work is of only relevance to ondemand governor - justifies why
> struct delayed work moved from devfreq.c to governor_simpleondemand.c.
> When delayed work is optional for governors to use, then let governors
> handle them if needed instead of devfreq core.
>
> With this change, we will not be forcing next upcoming devfreq governors
> to use delayed work for load monitoring(if required) but instead flexibility
> to have their own mechanisms. Its up to the governors to decide
> how it wants to do dvfs of a device. But imperative to respond the events.
As mentioned above, please do not assume that the default governors
(drivers/devfreq/governor_*.c) will be only governors available and
ondemand will be the only "active" governor for the future.
I think it'd be better for the driver writes to reuse code than to
copy and paste the code.
Some governors may use some other polling/sampling mechanisms; however,
still, providing delayed work at the core does not mean that governors
must use the default delayed work struct. They may simply not use it
as userspace, performance, and powersave do.
>
> >
> > In this patchset, the size of ondemand governor has been enlarged
> > too much for that purpose.
I said this because I do not want governors to act as if they are core
and I do not want to make writing governors too difficult for device
driver writers. I want them to focus on the "policy" only, not the
mechanism.
> >
> >
> > > - Force devfreq users to set min/max supported frequencies in
> > > device profile to help governors to predict target frequecy
> > > with in limits.
> >
> >
> > Is this really necessary?
> >
> >
> Yes. Mainly for two reasons,
>
> 1. Devfreq core provides sysfs interface for usespace to set min/max
> operating frequency for a device. These values must be within device
> supported min/max frequency limits.
No. they do not need to be within device supported limits.
When I say "CPU should be running at least 50MHz" when the CPU supports
1000MHz ~ 3000MHz, there is no problem. Running at 1000MHz does not
contradict with the condition.
When I say "CPU should be running at least 4000MHz" for the same CPU,
It is fine to set the CPU at max available.
> 2. Governors should know device supported min/max opps and use
> them where ever needed. Never assume UINT_MAX as max frequency.
The core and governors do not need to know such information.
They only need to provide "recommended" frequency. The corresponding
driver is going to interpret it properly. Whenever we can,
it's better to remove device specific information from governors
and cores as long as it does not significantly increase the
complexity of drivers.
UINT_MAX means "I recommend to run as fast as possible." Then, the
driver only needs to provide the fastest frequency. The core and
governor does not need to say the specific frequency.
Cheers!
MyungJoo
>
>
>
> >
> > The devfreq apis are not modified and are kept intact.
>
>
> The ABIs are not.
>
> You can no longer do "# echo 0 > ABI_path" in order to deactivate.
>
>
> Only relevant for ondemand governor. Load monitoring can be
> deactivated when polling_ms is set to zero. Next version of patch
> will fix this. Thanks.
>
>
>
> Cheers!
> MyungJoo
>
>
> >
> > Signed-off-by: Rajagopal Venkat <rajagopal.ven...@linaro.org>
> > ---
>
>
> ps. please make the patch a bit more readable. (please don't shuffle
> the location of pre-existed functions)
>
>
>
>
> --
> Regards,
> Rajagopal
>
>
>
>
>
>
> --
>
> MyungJoo Ham (함명주), PHD
>
> System S/W Lab, S/W Platform Team, Software Center
> Samsung Electronics
> Cell: +82-10-6714-2858
>
>
>
>
>
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