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

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Message-ID: <c17450ce-fc25-e792-b91b-51106f032542@codeaurora.org>
Date:   Tue, 2 Oct 2018 11:56:56 -0700
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Georgi Djakov <georgi.djakov@...aro.org>,
        Rob Herring <robh@...nel.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
Subject: Re: [PATCH v9 2/8] dt-bindings: Introduce interconnect binding


On 10/02/2018 04:17 AM, Sudeep Holla wrote:
> On Mon, Oct 01, 2018 at 04:49:32PM -0700, Saravana Kannan wrote:
>> On 09/26/2018 07:48 AM, Sudeep Holla wrote:
>>> On Wed, Sep 26, 2018 at 05:42:15PM +0300, Georgi Djakov wrote:
>>>> Hi Rob,
>>>>
>>>> Thanks for the comments!
>>>>
>>>> On 09/25/2018 09:02 PM, 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".
>>>> Other than the 3 Qualcomm SoCs (msm8916, msm8996, sdm845) that are
>>>> currently using this binding, there is ongoing work from at least two
>>>> other vendors that would be using this same binding. I will check on
>>>> what is their progress so far.
>>>>
>>>>> 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...
>>>> I see this as a further step. It could be OPP binding which include
>>>> bandwidth values or some separate DT property. Jordan has already
>>>> proposed something, do you have any initial comments on that?
>>> I am curious as how this fits into new systems which have firmware driven
>>> CPUFreq and other DVFS. I would like to avoid using this in such systems
>>> and leave it upto the firmware to scale the bus/interconnect based on the
>>> other components that are connected to it and active.
>>>
>> You've made the same point multiple times across different patch sets. Not
>> all FW can do arbitrary functions. A lot of them are very limited in their
>> capabilities. So, as much as you and I would like to let the FW do the work,
>> it's not always possible. So, in those cases, we do need to have support for
>> the kernel scaling the interconnects correctly. Hopefully this clears up
>> your questions about FW capabilities.
> Yes, I do understand I have made the same point multiple time and it's
> intentional. We need to get the fragmented f/w support story fixed.
> Different ARM vendors are doing different things in f/w and ARM sees the
> same fragmentation story as before. We have come up with new specification
> and my annoying multiple emails are just to constantly remind the same.
>
> I do understand we have existing implementations to consider, but fixing
> the functionality in arbitrary way is not a good design and it better
> to get them fixed for future.

I believe the fragmentation you are referring to is  in the 
interface/communication protocol. I see the benefit of standardizing 
that as long as the standard actually turns out to be good. But that's 
completely separate from what the FW can/can't do. Asking to standardize 
what the FW can/can't do doesn't seem realistic as each chip vendor will 
have different priorities - power, performance, cost, chip area, etc. 
It's the conflation of these separate topics that doesn't help IMHO.

-Saravana


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