Hi Paul,
On Wed, Apr 05, 2023 at 03:04:05PM +0200, Paul Cercueil wrote:
Le mardi 04 avril 2023 à 12:11 +0200, Maxime Ripard a écrit :
The Ingenic CGU clocks 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().
As I said in the v2 (IIRC), clk_set_parent() is used when re-parenting from the device tree.
Yep, it's indeed an oversight in the commit log.
The driver does implement round_rate() though, which means that we can change the rate of the clock, but we will never get to change the parent.
However, It's hard to tell whether it's been done on purpose or not.
Since we'll start mandating a determine_rate() implementation, let's convert the round_rate() implementation to a determine_rate(), which will also make the current behavior explicit. And if it was an oversight, the clock behaviour can be adjusted later on.
So just to be sure, this patch won't make clk_set_rate() automatically switch parents, right?
Allowing automatic re-parenting sounds like a huge can of worms...
The behaviour is strictly equivalent before that patch and after: the driver will not reparent on a rate change. It just makes it explicit.
Maxime