linux-kernel - Re: [PATCH v3 43/65] ASoC: tlv320aic32x4: Add a determine

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Date:   Tue, 4 Apr 2023 16:26:18 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Andreas Färber <afaerber@...e.de>,
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Subject: Re: [PATCH v3 43/65] ASoC: tlv320aic32x4: Add a determine_rate hook

On Tue, Apr 04, 2023 at 12:11:33PM +0200, Maxime Ripard wrote:
> The tlv320aic32x4 clkin 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.

It could be configured from device tree as well couldn't it?

> 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().

Historically clk_set_rate() wouldn't reparent IIRC.

> 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.

To be honest it's surprising that we'd have to manually specify this, I
would expect to be able to reparent.  I suspect it'd be better to go the
other way here and allow reparenting.

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