linux-kernel - Re: [PATCH v2 20/65] clk: wm831x: clkout: Add a determine

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Message-ID: <20221107105823.GF10437@ediswmail.ad.cirrus.com>
Date:   Mon, 7 Nov 2022 10:58:23 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Maxime Ripard <maxime@...no.tech>
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Subject: Re: [PATCH v2 20/65] clk: wm831x: clkout: Add a determine_rate hook

On Fri, Nov 04, 2022 at 02:17:37PM +0100, Maxime Ripard wrote:
> The WM381x "clkout" clock implements a mux with a set_parent hook,
> but doesn't provide a determine_rate implementation.
> 
> This is a bit odd, since set_parent() is there to, as its name implies,
> change the parent of a clock. However, the most likely candidate to
> trigger that parent change is a call to clk_set_rate(), with
> determine_rate() figuring out which parent is the best suited for a
> given rate.
> 
> The other trigger would be a call to clk_set_parent(), but it's far less
> used, and it doesn't look like there's any obvious user for that clock.
> 
> So, the set_parent hook is effectively unused, possibly because of an
> oversight. However, it could also be an explicit decision by the
> original author to avoid any reparenting but through an explicit call to
> clk_set_parent().
> 
> The latter case would be equivalent to setting the flag
> CLK_SET_RATE_NO_REPARENT, together with setting our determine_rate hook
> to __clk_mux_determine_rate(). Indeed, if no determine_rate
> implementation is provided, clk_round_rate() (through
> clk_core_round_rate_nolock()) will call itself on the parent if
> CLK_SET_RATE_PARENT is set, and will not change the clock rate
> otherwise. __clk_mux_determine_rate() has the exact same behavior when
> CLK_SET_RATE_NO_REPARENT is set.
> 
> And if it was an oversight, then we are at least explicit about our
> behavior now and it can be further refined down the line.
> 

Yeah I don't think there would be anything wrong with this clock
changing parents on a rate change, but as you say this can be
refined down the line if someone needs the behaviour. It's an
older part so probably better to stick roughly to the current
behaviour for now.

Acked-by: Charles Keepax <ckeepax@...nsource.cirrus.com>

Thanks,
Charles