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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