linux-kernel - Re: [PATCH 4.19 29/74] clk: qcom: regmap-mux: fix parent clock lookup

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Date:   Wed, 15 Dec 2021 18:54:41 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH 4.19 29/74] clk: qcom: regmap-mux: fix parent clock lookup

Hi!

> > > The function mux_get_parent() uses qcom_find_src_index() to find the
> > > parent clock index, which is incorrect: qcom_find_src_index() uses src
> > > enum for the lookup, while mux_get_parent() should use cfg field (which
> > > corresponds to the register value). Add qcom_find_cfg_index() function
> > > doing this kind of lookup and use it for mux parent lookup.
> > 
> > This appears to have problems with error handling.
> > 
> > > +++ b/drivers/clk/qcom/clk-regmap-mux.c
> > > @@ -36,7 +36,7 @@ static u8 mux_get_parent(struct clk_hw *
> > >   	val &= mask;
> > >   	if (mux->parent_map)
> > > -		return qcom_find_src_index(hw, mux->parent_map, val);
> > > +		return qcom_find_cfg_index(hw, mux->parent_map, val);
> > >   	return val;
> > >   }
> > 
> > So this returns u8.
> > 
> > > +int qcom_find_cfg_index(struct clk_hw *hw, const struct parent_map *map, u8 cfg)
> > > +{
> > > +	int i, num_parents = clk_hw_get_num_parents(hw);
> > > +
> > > +	for (i = 0; i < num_parents; i++)
> > > +		if (cfg == map[i].cfg)
> > > +			return i;
> > > +
> > > +	return -ENOENT;
> > > +}
> > 
> > In case of error, -ENOENT will be cast to u8 in caller. I don't
> > believe that is correct.
> 
> Unfortunately there is no way to return proper error code from
> clk_ops->get_parent() callback. However returning -ENOENT would translate to
> 254. Then clk_core_get_parent_by_index() would determine that there is no
> such parent and return NULL. A call to clk_set_parent would reparent the
> clock.

Yeah, I guess it happens to work.

> Returning some sensible default (e.g. 0) would be much worse, since then the
> clock subsystem would assume that the clock has correct parent. A call to
> clk_set_parent would always result in ops->set_parent() call, reparenting
> the clock correctly.

Well ~0 would be sensible in this case. And a comment with explanation.

> Most probably it would be correct to make ops->get_parent() return int
> instead of u8 (either an index or an -ERROR). However this was out of scope
> for this patch.

Yep, I believe that should happen, long-term.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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