phy: cadence: sierra: Add a determine_rate hook

The Cadence Sierra PLL 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().

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.

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.

Cc: Kishon Vijay Abraham I <[email protected]>
Cc: Vinod Koul <[email protected]>
Cc: [email protected]
Signed-off-by: Maxime Ripard <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Stephen Boyd <[email protected]>

drivers/phy/cadence/phy-cadence-sierra.c

@@ -720,6 +720,7 @@ static int cdns_sierra_pll_mux_set_parent(struct clk_hw *hw, u8 index)
 }
 
 static const struct clk_ops cdns_sierra_pll_mux_ops = {
+	.determine_rate = __clk_mux_determine_rate,
 	.set_parent = cdns_sierra_pll_mux_set_parent,
 	.get_parent = cdns_sierra_pll_mux_get_parent,
 };