Re: [PATCH v3 43/65] ASoC: tlv320aic32x4: Add a determine_rate hook

Maxime Ripard
2023-04-05T15:17:35Z

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Hi Mark,

On Tue, Apr 04, 2023 at 04:26:18PM +0100, Mark Brown wrote:
> 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?

Yep, indeed.

> > 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.

Yeah, I think I'd prefer to allow reparenting too, but as can be seen
from the other reviewers in that thread, it seems like we have a very
split community here, so that doesn't sound very realistic without some
major pushback :)

Maxime

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