Re: [PATCH v10 0/8] Introduce on-chip interconnect API

From: Rafael J. Wysocki
Date: Mon Dec 10 2018 - 06:00:26 EST

On Mon, Dec 10, 2018 at 11:18 AM Georgi Djakov <georgi.djakov@xxxxxxxxxx> wrote:
> Hi Rafael,
> On 12/10/18 11:04, Rafael J. Wysocki wrote:
> > On Thu, Dec 6, 2018 at 3:55 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> On Wed, Dec 05, 2018 at 12:41:35PM -0800, Evan Green wrote:
> >>> On Tue, Nov 27, 2018 at 10:03 AM Georgi Djakov <georgi.djakov@xxxxxxxxxx> wrote:
> >>>>
> >>>> Modern SoCs have multiple processors and various dedicated cores (video, gpu,
> >>>> graphics, modem). These cores are talking to each other and can generate a
> >>>> lot of data flowing through the on-chip interconnects. These interconnect
> >>>> buses could form different topologies such as crossbar, point to point buses,
> >>>> hierarchical buses or use the network-on-chip concept.
> >>>>
> >>>> These buses have been sized usually to handle use cases with high data
> >>>> throughput but it is not necessary all the time and consume a lot of power.
> >>>> Furthermore, the priority between masters can vary depending on the running
> >>>> use case like video playback or CPU intensive tasks.
> >>>>
> >>>> Having an API to control the requirement of the system in terms of bandwidth
> >>>> and QoS, so we can adapt the interconnect configuration to match those by
> >>>> scaling the frequencies, setting link priority and tuning QoS parameters.
> >>>> This configuration can be a static, one-time operation done at boot for some
> >>>> platforms or a dynamic set of operations that happen at run-time.
> >>>>
> >>>> This patchset introduce a new API to get the requirement and configure the
> >>>> interconnect buses across the entire chipset to fit with the current demand.
> >>>> The API is NOT for changing the performance of the endpoint devices, but only
> >>>> the interconnect path in between them.
> >>>
> >>> For what it's worth, we are ready to land this in Chrome OS. I think
> >>> this series has been very well discussed and reviewed, hasn't changed
> >>> much in the last few spins, and is in good enough shape to use as a
> >>> base for future patches. Georgi's also done a great job reaching out
> >>> to other SoC vendors, and there appears to be enough consensus that
> >>> this framework will be usable by more than just Qualcomm. There are
> >>> also several drivers out on the list trying to add patches to use this
> >>> framework, with more to come, so it made sense (to us) to get this
> >>> base framework nailed down. In my experiments this is an important
> >>> piece of the overall power management story, especially on systems
> >>> that are mostly idle.
> >>>
> >>> I'll continue to track changes to this series and we will ultimately
> >>> reconcile with whatever happens upstream, but I thought it was worth
> >>> sending this note to express our "thumbs up" towards this framework.
> >>
> >> Looks like a v11 will be forthcoming, so I'll wait for that one to apply
> >> it to the tree if all looks good.
> >
> > I'm honestly not sure if it is ready yet.
> >
> > New versions are coming on and on, which may make such an impression,
> > but we had some discussion on it at the LPC and some serious questions
> > were asked during it, for instance regarding the DT binding introduced
> > here. I'm not sure how this particular issue has been addressed here,
> > for example.
> There have been no changes in bindings since v4 (other than squashing
> consumer and provider bindings into a single patch and fixing typos).
> The last DT comment was on v9 [1] where Rob wanted confirmation from
> other SoC vendors that this works for them too. And now we have that
> confirmation and there are patches posted on the list [2].


> The second thing (also discussed at LPC) was about possible cases where
> some consumer drivers can't calculate how much bandwidth they actually
> need and how to address that. The proposal was to extend the OPP
> bindings with one more property, but this is not part of this patchset.
> It is a future step that needs more discussion on the mailing list. If a
> driver really needs some bandwidth data now, it should be put into the
> driver and not in DT. After we have enough consumers, we can discuss
> again if it makes sense to extract something into DT or not.

That's fine by me.

Admittedly, I have some reservations regarding the extent to which
this approach will turn out to be useful in practice, but I guess as
long as there is enough traction, the best way to find out it to try
and see. :-)

>From now on I will assume that this series is going to be applied by Greg.