linux-kernel - Re: [PATCH v9 07/22] clk: Add API to get index of the clock parent

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Message-ID: <20191107152115.GA2580600@ulmo>
Date:   Thu, 7 Nov 2019 16:21:15 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        jason@...edaemon.net, jonathanh@...dia.com,
        linus.walleij@...aro.org, marc.zyngier@....com,
        mark.rutland@....com, stefan@...er.ch, tglx@...utronix.de,
        pdeschrijver@...dia.com, pgaikwad@...dia.com,
        linux-clk@...r.kernel.org, linux-gpio@...r.kernel.org,
        jckuo@...dia.com, josephl@...dia.com, talho@...dia.com,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        mperttunen@...dia.com, spatra@...dia.com, robh+dt@...nel.org,
        devicetree@...r.kernel.org, rjw@...ysocki.net,
        viresh.kumar@...aro.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v9 07/22] clk: Add API to get index of the clock parent

On Thu, Nov 07, 2019 at 03:54:03AM +0300, Dmitry Osipenko wrote:
> 07.11.2019 02:10, Stephen Boyd пишет:
> > Quoting Sowjanya Komatineni (2019-08-16 12:41:52)
> >> This patch adds an API clk_hw_get_parent_index to get index of the
> >> clock parent to use during the clock restore operations on system
> >> resume.
> >  
> > Is there a reason we can't save the clk hw index at suspend time by
> > reading the hardware to understand the current parent? The parent index
> > typically doesn't matter unless we're trying to communicate something
> > from the framework to the provider driver. Put another way, I would
> > think the provider driver can figure out the index itself without having
> > to go through the framework to do so.
> 
> Isn't it a bit wasteful to duplicate information about the parent within
> a provider if framework already has that info? The whole point of this
> new API is to allow providers to avoid that unnecessary duplication.
> 
> Please note that clk_hw_get_parent_index is getting used only at the
> resume time and not at suspend.

I agree with this. All of the information that we need is already cached
in the framework. Doing this in the driver would mean essentially adding
a "saved parent" field along with code to read the value at suspend time
to the three types of clocks that currently use this core helper.

That's certainly something that we *can* do, but it doesn't sound like a
better option than simply querying the framework for the value that we
need.

Thierry

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