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

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Message-ID: <20221026135215.nqndtcb2unxqwwux@houat>
Date:   Wed, 26 Oct 2022 15:52:15 +0200
From:   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

Hi,

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's not clear at this point how many drivers are affected though, so
> > let's make it a warning instead of an error for now.
> > 
> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> > ---
> >  drivers/clk/clk.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > index 57b83665e5c3..11c41d987ff4 100644
> > --- a/drivers/clk/clk.c
> > +++ b/drivers/clk/clk.c
> > @@ -3700,6 +3700,11 @@ static int __clk_core_init(struct clk_core *core)
> >                 goto out;
> >         }
> >  
> > +       /* TODO: Promote to an error */
> 
> The documentation should be updated in this patch (see the table of
> hardware characteristics in Documentation/driver-api/clk.rst).
> 
> > +       if (core->ops->set_parent && !core->ops->determine_rate)
> > +               pr_warn("%s: %s must implement .set_parent & .determine_rate\n",
> 
> You can grep for it:
> 
>  $ git grep -W 'struct clk_ops .*='

TIL about -W. It's awesome, thanks

> but I'm fairly certain a coccinelle script can detect most of these
> because clk_ops are usually statically defined (although not always).
> 
> Either way I already see that 'owl_comp_div_ops' will trigger this
> warning. And 'at91sam9x5_smd_ops' looks even more likely. Given that I'm
> not super keen on applying this patch.

It's the reason why I didn't return an error at first, I wanted to
report that it's invalid and let to drivers the chance to be fixed
still.

Should I take your above comment as you'd rather have the affected
drivers fixed in this patch and we then return an error, or is it that
you don't want that patch at all?

Maxime

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