linux-kernel - Re: [PATCH v9 2/8] dt-bindings: Introduce interconnect binding

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Date:   Mon, 1 Oct 2018 14:51:18 -0700
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Rob Herring <robh@...nel.org>,
        Georgi Djakov <georgi.djakov@...aro.org>,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        rjw@...ysocki.net, mturquette@...libre.com, khilman@...libre.com,
        vincent.guittot@...aro.org, bjorn.andersson@...aro.org,
        amit.kucheria@...aro.org, seansw@....qualcomm.com,
        daidavid1@...eaurora.org, evgreen@...omium.org,
        mark.rutland@....com, lorenzo.pieralisi@....com,
        abailon@...libre.com, maxime.ripard@...tlin.com, arnd@...db.de,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, robdclark@...il.com
Subject: Re: [PATCH v9 2/8] dt-bindings: Introduce interconnect binding



On 10/01/2018 02:26 PM, Jordan Crouse wrote:
> On Mon, Oct 01, 2018 at 01:56:32PM -0700, Saravana Kannan wrote:
>>
>> On 09/26/2018 07:34 AM, Jordan Crouse wrote:
>>> On Tue, Sep 25, 2018 at 01:02:15PM -0500, Rob Herring wrote:
>>>> On Fri, Aug 31, 2018 at 05:01:45PM +0300, Georgi Djakov wrote:
>>>>> This binding is intended to represent the relations between the interconnect
>>>>> controllers (providers) and consumer device nodes. It will allow creating links
>>>>> between consumers and interconnect paths (exposed by interconnect providers).
>>>> As I mentioned in person, I want to see other SoC families using this
>>>> before accepting. They don't have to be ready for upstream, but WIP
>>>> patches or even just a "yes, this works for us and we're going to use
>>>> this binding on X".
>>>>
>>>> Also, I think the QCom GPU use of this should be fully sorted out. Or
>>>> more generically how this fits into OPP binding which seems to be never
>>>> ending extended...
>>> This is a discussion I wouldn't mind having now.  To jog memories, this is what
>>> I posted a few weeks ago:
>>>
>>> https://patchwork.freedesktop.org/patch/246117/
>>>
>>> This seems like the easiest way to me to tie the frequency and the bandwidth
>>> quota together for GPU devfreq scaling but I'm not married to the format and
>>> I'll happily go a few rounds on the bikeshed if we can get something we can
>>> be happy with.
>>>
>>> Jordan
>> Been meaning to send this out for a while, but caught up with other stuff.
>>
>> That GPU BW patch is very specific to device to device mapping and
>> doesn't work well for different use cases (Eg: those that  can
>> calculate based on use case, etc).
>>
>> Interconnect paths have different BW (bandwidth) operating points
>> that they can support. For example: 1 GB/s, 1.7 GB/s, 5GB/s, etc.
>> Having a mapping from GPU or CPU to those are fine/necessary, but we
>> still need a separate BW OPP table for interconnect paths to list
>> what they can actually support.
>>
>> Two different ways we could represent BW OPP tables for interconnect paths:
>> 1.  Represent interconnect paths (CPU to DDR, GPU to DDR, etc) as
>> devices and have OPPs for those devices.
>>
>> 2. We can have a "interconnect-opp-tables" DT binding similar to
>> "interconnects" and "interconnect-names". So if a device GPU or
>> Video decoder or I2C device needs to vote on an interconnect path,
>> they can also list the OPP tables that those paths can support.
>>
>> I know Rob doesn't like (1). But I'm hoping at least (2) is
>> acceptable. I'm open to other suggestions too.
>>
>> Both (1) and (2) need BW OPP tables similar to frequency OPP tables.
>> That should be easy to add and Viresh is open to that. I'm open to
>> other options too, but the fundamental missing part is how to tie a
>> list of BW OPPs to interconnect paths in DT.
>>
>> Once we have one of the above two options, we can use the
>> required-opps field (already present in kernel) for the mapping
>> between GPU to a particular BW need (suggested by Viresh during an
>> in person conversation).
> Assuming we are willing to maintain the bandwidth OPP tables and the
> names / phandles needed to describe a 1:1 GPU -> bandwidth mapping
> I'm okay with required-opps but for the sake of argument how would
> required-opps work for a device that needs to vote multiple paths
> for a given OPP?

You can list multiple required-opps per device OPP. It's an array of 
phandles to OPP entries in other tables.

-Saravana

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