linux-kernel - Re: [PATCH v3 14/65] clk: lochnagar: Add a determine

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Date:   Thu, 4 May 2023 13:39:23 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Maxime Ripard <maxime@...no.tech>
CC:     Michael Turquette <mturquette@...libre.com>,
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Subject: Re: [PATCH v3 14/65] clk: lochnagar: Add a determine_rate hook

On Tue, Apr 04, 2023 at 12:11:04PM +0200, Maxime Ripard wrote:
> The lochnagar clocks implement 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.
> 
> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> ---

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

Thanks,
Charles