Re: [PATCH v2 13/65] clk: lmk04832: clkout: Add a determine_rate hook

Liam Beguin
2022-11-13T22:35:56Z

Hi Maxime,

On Fri, Nov 04, 2022 at 02:17:30PM +0100, Maxime Ripard wrote:
> The LKM04832 "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().

This is correct, the set_parent hook is effectively unused at the
moment. It was implemented as a way for consumers to select the parent
themselves.

The LMK04832 is used in JESD204 applications where devices need a device
clock as well as a sysref clock. Since this is determined by the
hardware layout, a devicetree option is used to select the inital state
of the clkout mux. This is set at the end of lmk04832_register_clkout().

> 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.
>
> Since the CLK_SET_RATE_NO_REPARENT flag was already set though, it seems
> unlikely.
>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>

Reviewed-by: Liam Beguin <liambeguin@gmail.com>

Cheers,
Liam

> ---
> drivers/clk/clk-lmk04832.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/clk/clk-lmk04832.c b/drivers/clk/clk-lmk04832.c
> index f416f8bc2898..f68bb0affdad 100644
> --- a/drivers/clk/clk-lmk04832.c
> +++ b/drivers/clk/clk-lmk04832.c
> @@ -1281,6 +1281,7 @@ static const struct clk_ops lmk04832_clkout_ops = {
> .is_enabled = lmk04832_clkout_is_enabled,
> .prepare = lmk04832_clkout_prepare,
> .unprepare = lmk04832_clkout_unprepare,
> + .determine_rate = __clk_mux_determine_rate,
> .set_parent = lmk04832_clkout_set_parent,
> .get_parent = lmk04832_clkout_get_parent,
> };
>
> --
> b4 0.11.0-dev-99e3a
>