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

[Georgi Djakov <georgi.djakov@linaro.org>]

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.

Thanks,
Georgi