linux-kernel - Re: [PATCH 4/4] clk: Warn if we register a mux without determine

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Message-ID: <20221103123328.stzhtq5e2jscjdxd@houat>
Date:   Thu, 3 Nov 2022 13:33:28 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH 4/4] clk: Warn if we register a mux without determine_rate

Going back to this mail.

On Tue, Oct 25, 2022 at 07:07:58PM -0700, Stephen Boyd wrote:
> Quoting Maxime Ripard (2022-10-18 06:52:59)
> > The determine_rate hook allows to select the proper parent and its rate
> > for a given clock configuration. On another hand, set_parent is there to
> > change the parent of a mux.
> > 
> > Some clocks provide a set_parent hook but don't implement
> > determine_rate. In such a case, set_parent is pretty much useless since
> > the clock framework will always assume the current parent is to be used,
> > and we will thus never change it.
> > 
> > This situation can be solved in two ways:
> >   - either we don't need to change the parent, and we thus shouldn't
> >     implement set_parent;
> >   - or we don't want to change the parent, in this case we should set
> >     CLK_SET_RATE_NO_REPARENT;
> >   - or we're missing a determine_rate implementation.
> > 
> > The latter is probably just an oversight from the driver's author, and
> > we should thus raise their awareness about the fact that the current
> > state of the driver is confusing.
> 
> There is another case which is a leaf clk that is a mux where you only
> expect clk_set_parent() to be used, and not clk_set_rate(). This use
> case is odd though, so I'm not sure how much we care.

It looks like there's a good number of clocks that do indeed only
provide get_parent / set_parent. It's hard to tell if it's an oversight
or a choice.

I think we can make that decision explicit by providing a determine_rate
helper that always returns the current parent and its rate. It shouldn't
change anything from a CCF behavior point of view, and it makes it clear
what the behavior is. And if someone wants something else, then they can
change it to whatever they want.

Maxime