Re: [PATCH v2 11/11] interconnect: Add devfreq support

[Saravana Kannan]

On Tue, Jul 16, 2019 at 11:13 AM Sibi Sankar <sibis@xxxxxxxxxxxxxx> wrote:
>
> Hey Saravana,
>
> On 6/18/19 2:48 AM, Saravana Kannan wrote:
> > On Mon, Jun 17, 2019 at 8:44 AM Georgi Djakov <georgi.djakov@xxxxxxxxxx> wrote:
> >>
> >> Hi Saravana,
> >>
> >> On 6/14/19 07:17, Saravana Kannan wrote:
> >>> Add a icc_create_devfreq() and icc_remove_devfreq() to create and remove
> >>> devfreq devices for interconnect paths. A driver can create/remove devfreq
> >>> devices for the interconnects needed for its device by calling these APIs.
> >>> This would allow various devfreq governors to work with interconnect paths
> >>> and the device driver itself doesn't have to actively manage the bandwidth
> >>> votes for the interconnects.
> >>
> >> Thanks for the patches, but creating devfreq devices for each interconnect path
> >> seems odd to me - at least for consumers that already use a governor.
> >
> > Each governor instance always handles one "frequency" (more like
> > performance) domain at a time. So if a consumer is already using a
> > governor to scale the hardware block, then using another governor to
> > scale the interconnect performance points is the right way to go about
> > it. In fact, that's exactly what devfreq passive governor's
> > documentation even says it's meant for. That's also what cpufreq does
> > for each cluster/CPU frequency domain too.
> >
> >> So for DDR
> >> scaling for example, are you suggesting that we add a devfreq device from the
> >> cpufreq driver in order to scale the interconnect between CPU<->DDR?
> >
> > Yes in general. Although, CPUs are a special case because CPUs don't
> > go through devfreq. So passive governor as it stands today won't work.
> > CPU<->DDR scaling might need a separate governor (unlikely) or some
> > changes to the passive governor that I'm happy to work on once we
> > settle this for general devices like GPU, etc. But the DT format for
> > CPUs will be identical to GPUs or any other device.
>
> using icc_create_devfreq from the cpufreq-hw driver on SDM845 SoC
> to scale CPU<->DDR would cause a circular dependency. (i.e) with
> the addition of cpufreq notifier to the passive governor as in
> https://patchwork.kernel.org/patch/11046147/ devm_devfreq_add_device
> would require the cpufreq transistion notifier register and cpu
> freq_cpu_get to go through. Please add your thought on addressing this.

This is an old series. So not going to dive into this much.

But to answer your question, I wrote the cpufreq_map governor a long
time ago. So not surprised if you are finding issues with it -- it
needs a rewrite anyway.

-Saravana

> >
> >> Also if the
> >> GPU is already using devfreq, should we add a devfreq per each interconnect
> >> path? What would be the benefit in this case - using different governors for
> >> bandwidth scaling maybe?
> >
> > When saying "separate/different governors" in this email, I mean both
> > different instance of the same governor logic with different tunables
> > AND actually different algorithms/governor logic entirely.
> >
> > The heuristics to use for each interconnect path might be (more like,
> > will be) different based on hardware characteristics (Eg: what voltage
> > domains the interconnect is sitting on) and what interconnect
> > information is available (Eg: Just busy time vs bandwidth count vs no
> > information etc) -- so having separate governors for each interconnect
> > path makes a lot of sense. It also allows userspace to control the
> > policy for scaling each of those paths based on product use cases.
> >
> > For example, when the GPU is just doing simple UI rendering, userspace
> > can use the max_freq sysfs file for the devfreq device to disallow high
> > bandwidth OPPs on the GPU<->DDR path, but those higher OPPs could be
> > allowed by userspace when the GPU is used for games. Having devfreq
> > device for each interconnect path also make it easy to debug
> > performance issues -- you can independently change the votes for each
> > path to figure out what is causing the bottleneck, etc.
> >
> > Adding a devfreq device for interconnect voting with a few lines gives
> > all these features "for free".
> >
> > This doesn't mean all users of interconnect framework NEED to use
> > devfreq for interconnect. They might do it simply based on
> > calculations based on the use case (Eg: display driver from display
> > resolution). But if they are trying to use any kind of
> > algorithm/heuristics, writing it as a devfreq governor should be
> > encouraged.
> >
> > Also want to point out that BW OPPs also work for drivers that don't
> > use devfreq at all. The interconnect-opp-table just lists the
> > meaningful OPP leveld for the path and the device driver can pick one
> > entry from the table based on the use case.
> >
> > Thanks,
> > Saravana
> >
> >
> >
>
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