Re: [PATCH v4 29/68] clk: mediatek: cpumux: Add a determine_rate hook

[Chen-Yu Tsai <wenst@chromium.org>]

On Fri, May 5, 2023 at 7:27 PM Maxime Ripard <maxime@cerno.tech> wrote:
>
> The Mediatek cpumux 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 candidates to
> trigger that parent change are either the assigned-clock-parents device
> tree property or 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.
>
> Similarly, it doesn't look like the device tree using that clock driver
> uses any of the assigned-clock properties on 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 consumer, the cpufreq driver, assumes the original parent to be
the dedicated PLL that drives it. It gets a reference to the original
parent with clk_get_parent(). It also gets an intermediate (stable) clock
via DT. It does explicit clk_set_parent() calls to switch to/from the
stable clock, and does clk_set_rate() on the dedicated PLL in between.

So yeah, they only use set_parent hook.

> The latter case would be equivalent to setting the determine_rate
> implementation to clk_hw_determine_rate_no_reparent(). 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.
>
> 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.
>
> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-mediatek@lists.infradead.org
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>

Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>