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Subject: Re: [PATCH v10 0/8] Introduce on-chip interconnect API
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Olof Johansson <olof@lixom.net>,
        evgreen@chromium.org, Linux PM <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Rob Herring <robh+dt@kernel.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Kevin Hilman <khilman@baylibre.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Saravana Kannan <skannan@codeaurora.org>,
        bjorn.andersson@linaro.org,
        Amit Kucheria <amit.kucheria@linaro.org>,
        seansw@qti.qualcomm.com, daidavid1@codeaurora.org,
        Mark Rutland <mark.rutland@arm.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        abailon@baylibre.com, maxime.ripard@bootlin.com,
        Thierry Reding <thierry.reding@gmail.com>,
        ksitaraman@nvidia.com, sanjayc@nvidia.com,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        linux-tegra@vger.kernel.org, Doug Anderson <dianders@chromium.org>,
        Andy Gross <andy.gross@linaro.org>
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From:   Georgi Djakov <georgi.djakov@linaro.org>
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Message-ID: <199be960-032a-d72d-4293-820e23a15b7a@linaro.org>
Date:   Mon, 17 Dec 2018 13:17:30 +0200
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Hi Greg,

On 12/11/18 08:58, Greg Kroah-Hartman wrote:
> 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@linaro.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@linuxfoundation.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@linaro.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...
> 

When you get a chance to review, please keep in mind that the latest
version is v12 (from 08.Dec). The same is also available in linux-next
with no reported issues.

Thanks,
Georgi