linux-kernel - Re: [PATCH v6 2/3] OPP: Add support for bandwidth OPP tables

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Message-ID: <20200110065420.3bp4fjj2wbezoyrc@vireshk-i7>
Date:   Fri, 10 Jan 2020 12:24:20 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Georgi Djakov <georgi.djakov@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Sweeney, Sean" <seansw@....qualcomm.com>,
        David Dai <daidavid1@...eaurora.org>, adharmap@...eaurora.org,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Evan Green <evgreen@...omium.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] OPP: Add support for bandwidth OPP tables

On 09-01-20, 10:35, Saravana Kannan wrote:
> Agreed for the example you are giving where PM domains/voltages are
> dropped automatically when dropping the device freq to suspend freq.
> I'm just wondering about a different scenario where if some power
> domain needed to be at say 0.5v when it's suspended (no consumer using
> it)

The domain should be powered off in this case I think.

> to not lose state, or to come back up without brownouts, etc then
> suspend OPP for PM domains might be useful. But I don't know enough
> about that to speak with authority, so I'll leave it at this.
> 
> I see this suspend-opp as a way to mark to what level the bandwidth
> needs to be dropped to/brought back up from during suspend/resume by
> the driver making interconnect bandwidth requests. For example, what
> if the CPU -> DDR needed to be at some level to avoid suspend/resume
> issues (say CPU bug with respect to timing/latencies)? In this
> example, the CPU driver would be the one making bandwidth requests for
> CPU -> DDR bandwidth during normal operation and during
> suspend/resume. So it's basically exactly the same way it would treat
> CPU freq OPP.

I understand your concerns but to me it all looks hypothetical right
now. I am not saying we won't support suspend-opp for interconnect or
domains, but that we should do it only if it is required.

> Btw, I don't have a strong opinion on this. But, even if we do only a
> rate comparison, what does it even mean to compare rates for genpd or
> BW opp tables?

We will never do the comparison because those tables will never have
the suspend OPP in the respective tables.

-- 
viresh