Re: [PATCH v12 0/7] Introduce on-chip interconnect API

[Michael Turquette <mturquette@baylibre.com>]

Hi Olof, Georgi,

Happy new year! :-)

Quoting Georgi Djakov (2018-12-08 21:15:35)
> Hi Olof,
> 
> On 9.12.18 2:33, Olof Johansson wrote:
> > Hi Georgi,
> > 
> > On Sat, Dec 8, 2018 at 9:02 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.
> >>
> >> The API is using a consumer/provider-based model, where the providers are
> >> the interconnect buses and the consumers could be various drivers.
> >> The consumers request interconnect resources (path) to an endpoint and set
> >> the desired constraints on this data flow path. The provider(s) receive
> >> requests from consumers and aggregate these requests for all master-slave
> >> pairs on that path. Then the providers configure each participating in the
> >> topology node according to the requested data flow path, physical links and
> >> constraints. The topology could be complicated and multi-tiered and is SoC
> >> specific.
> > 
> > This patch series description fails to describe why you need a brand
> > new subsystem for this instead of either using one of the current
> > ones, or adapting it to fit the needs you have.
> > 
> > Primarily, I'm wondering what's missing from drivers/devfreq to fit your needs?
> 
> The devfreq subsystem seems to be more oriented towards a device (like
> GPU or CPU) that controls the power/performance characteristics by
> itself and not the performance of other devices. The main problem of
> using it is that it's using a reactive approach - for example monitor
> some performance counters and then reconfigure bandwidth after some
> bottleneck has already occurred. This is suboptimal and might not work
> well. The new solution does the opposite by allowing drivers to
> express their needs in advance and be proactive. Devfreq also does not
> seem suitable for configuring complex, multi-tiered bus topologies and
> aggregating constraints provided by drivers.

[reflowed Georgi's responses]

Agreed that devfreq is not good for this. Like any good driver
framework, the interconnect framework provides a client/consumer api to
device drivers to express their needs (in this case, throughput over a
bus or interconnect).

On modern SoCs these topologies can be quite complicated, which requires
a provider api.

I think that a dedicated framework makes sense for this.

> 
> > The series also doesn't seem to provide any kind of indication how
> > this will be used by end points. You have one driver for one SoC that
> > just contains large tables that are parsed at probe time, but no
> > driver hooks anywhere that will actually change any settings depending
> > on use cases. Also, the bindings as posted don't seem to include any
> > of this kind of information. So it's hard to get a picture of how this
> > is going to be used in reality, which makes it hard to judge whether
> > it is a good solution or not.
> 
> Here are links to some of the examples that are on the mailing list
> already. I really should have  included them in the cover letter. 
> https://lkml.org/lkml/2018/12/7/584
> https://lkml.org/lkml/2018/10/11/499
> https://lkml.org/lkml/2018/9/20/986
> https://lkml.org/lkml/2018/11/22/772
> 
> Platforms drivers for different SoCs are available:
> https://lkml.org/lkml/2018/11/17/368
> https://lkml.org/lkml/2018/8/10/380
> There is a discussion on linux-pm about supporting also Tegra
> platforms in addition to NXP and Qualcomm.

Just FYI, Alex will renew his efforts to port iMX over to this framework
after the new year.

I honestly don't know if this series is ready to be merged or not. I
stopped reviewing it a long time ago. But there is interest in the need
that it addresses for sure.

> 
> > Overall, exposing all of this to software is obviously a nightmare
> > from a complexity point of view, and one in which it will surely be
> > very very hard to make the system behave properly for generic
> > workloads beyond benchmark tuning.

Detailed SoC glue controlled by Linux is always a nightmare. This typically
falls into the power management bucket: functional clocks and interface clocks,
clock domains, voltage control, scalable power islands (for both idle & active
use cases), master initiators and slave targets across interconnects,
configuring wake-up capable interrupts and handling them, handling dynamic
dependencies such as register spaces that are not clocked/powered and must be
enabled before read/write access, reading eFuses and defining operating points
at runtime, and the inevitable "system controllers" that are a grab bag of
whatever the SoC designers couldn't fit elsewhere...

This stuff is all a nightmare to handle in Linux, and upstream Linux still
lacks the expressiveness to address much of it. Until the SoC designers replace
it all with firmware or a dedicated PM microcontroller or whatever, we'll need
to model it and implement it as driver frameworks. This is an attempt to do so
upstream, which I support.

> 
> It allows the consumer drivers to dynamically express their
> performance needs in the system in a more fine grained way (if they
> want/need to) and this helps the system to keep the lowest power
> profile. This has already been done for a long time in various
> different kernels shipping with Android devices, for example, and
> basically every vendor uses a different custom approach. So I believe
> that this is doing the generalization that was needed.

Correct, everyone does this out of tree. For example:

https://source.codeaurora.org/external/imx/linux-imx/tree/arch/arm/mach-imx/busfreq-imx.c?h=imx_4.14.62_1.0.0_beta

See you in 2019,
Mike

> 
> > Having more information about the above would definitely help tell if
> > this whole effort is a step in the right direction, or if it is
> > needless complexity that is better solved in other ways.
> 
> Sure, hope that this answers your questions.
> 
> Thanks,
> Georgi
> 
> > 
> > -Olof
> > 
>