Date:   Tue, 11 Dec 2018 07:58:08 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Georgi Djakov <georgi.djakov@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
        evgreen@...omium.org, Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Saravana Kannan <skannan@...eaurora.org>,
        bjorn.andersson@...aro.org,
        Amit Kucheria <amit.kucheria@...aro.org>,
        seansw@....qualcomm.com, daidavid1@...eaurora.org,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        abailon@...libre.com, maxime.ripard@...tlin.com,
        Thierry Reding <thierry.reding@...il.com>,
        ksitaraman@...dia.com, sanjayc@...dia.com,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-tegra@...r.kernel.org, Doug Anderson <dianders@...omium.org>,
        Andy Gross <andy.gross@...aro.org>
Subject: Re: [PATCH v10 0/8] Introduce on-chip interconnect API

On Mon, Dec 10, 2018 at 04:50:00PM +0200, Georgi Djakov wrote:
> On 12/10/18 13:00, Rafael J. Wysocki wrote:
> > On Mon, Dec 10, 2018 at 11:18 AM Georgi Djakov <georgi.djakov@...aro.org> 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@...uxfoundation.org> 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@...aro.org> 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].
> > 
> > OK
> > 
> >> 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.
> 
> That was the initial idea, but the problem is that there is a recent
> change in the cmd_db API (needed by the sdm845 provider driver), which
> is going through arm-soc/qcom/drivers. So either Greg pulls also the
> qcom-drivers-for-4.21 tag from Andy or the whole series goes via Olof
> and Arnd. Maybe there are other options. I don't have any preference and
> don't want to put extra burden on any maintainers, so i am ok with what
> they prefer.

Let me take the time later this week to review the code, which I haven't
done in a while...

thanks,

greg k-h

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