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

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Date:   Thu, 07 Nov 2019 11:19:32 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     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

Quoting Thierry Reding (2019-11-07 07:21:15)
> 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.

Don't we already have a "saved parent" field by storing the pointer to
the clk_hw?

> 
> 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.
> 

Let me say this another way. Why does this driver want to know the index
that the framework uses for some clk_hw pointer? Perhaps it happens to
align with the same value that hardware uses, but I still don't
understand why the driver wants to know what the framework has decided
is the index for some clk_hw pointer.

Or is this something like "give me the index for the parent that the
framework thinks I currently have but in reality don't have anymore
because the register contents were wiped and we need to reparent it"? A
generic API to get any index for this question is overkill and we should
consider adding some sort of API like clk_hw_get_current_parent_index(),
or a framework flag that tells the framework this parent is incorrect
and we need to call the .set_parent() op again to reconfigure it.