Maxime Ripard <maxime@xxxxxxxxxx> writes: > Hi, > > On Fri, Nov 04, 2022 at 05:35:29PM +0000, Aidan MacDonald wrote: >> >> Maxime Ripard <maxime@xxxxxxxxxx> writes: >> >> > Hi Paul, >> > >> > On Fri, Nov 04, 2022 at 02:31:20PM +0000, Paul Cercueil wrote: >> >> Le ven. 4 nov. 2022 à 14:18:13 +0100, Maxime Ripard <maxime@xxxxxxxxxx> 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(). >> >> > >> >> > 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 it's partly on purpose, partly because I didn't know about >> >> .determine_rate. >> >> >> >> There's nothing odd about having a lonely .set_parent callback; in my case >> >> the clocks are parented from the device tree. >> >> >> >> Having the clocks driver trigger a parent change when requesting a rate >> >> change sounds very dangerous, IMHO. My MMC controller can be parented to the >> >> external 48 MHz oscillator, and if the card requests 50 MHz, it could switch >> >> to one of the PLLs. That works as long as the PLLs don't change rate, but if >> >> one is configured as driving the CPU clock, it becomes messy. >> >> The thing is, the clocks driver has no way to know whether or not it is >> >> "safe" to use a designated parent. >> >> >> >> For that reason, in practice, I never actually want to have a clock >> >> re-parented - it's almost always a bad idea vs. sticking to the parent clock >> >> configured in the DTS. >> > >> > Yeah, and this is totally fine. But we need to be explicit about it. The >> > determine_rate implementation I did in all the patches is an exact >> > equivalent to the round_rate one if there was one. We will never ask to >> > change the parent. >> > >> > Given what you just said, I would suggest to set the >> > CLK_SET_RATE_NO_REPARENT flag as well. >> >> Ideally there should be a way for drivers and the device tree to >> say, "clock X must be driven by clock Y", but the clock framework >> would be allowed to re-parent clocks freely as long as it doesn't >> violate any DT or driver constraints. > > I'm not really sure what you mean there, sorry. Isn't it what > assigned-clock-parents/clk_set_parent() at probe, plus a determine_rate > implementation that would affect best_parent_hw would already provide? Assigning the parent clock in the DT works once, at boot, but going off what you wrote in the commit message, if the clock driver has a .determine_rate() implementation that *can* reparent clocks then it probably *will* reparent them, and the DT assignment will be lost. What I'm suggesting is a runtime constraint that the clock subsystem would enforce, and actively prevent drivers from changing the parent. Either explicitly with clk_set_parent() or due to .determine_rate(). That way you could write a .determine_rate() implementation that *can* select a better parent, but if the DT applies a constraint to fix the clock to a particular parent, the clock subsystem will force that parent to be used so you can be sure the clock is never reparented by accident. >> That way allowing reparenting doesn't need to be an all-or-nothing >> thing, and it doesn't need to be decided at the clock driver level >> with special flags. > > Like I said, the default implementation is already working to what you > suggested if I understood properly. However, this has never been tested > for any of the drivers in that series so I don't want to introduce (and > debug ;)) regressions in all those drivers that were not setting any > constraint but never actually tested their reparenting code. > > So that series is strictly equivalent to what you had before, it's just > explicit now. > > If you find that some other decision make sense for your driver in > particular cases, feel free to change it. I barely know most of these > platforms, so I won't be able to make that decision (and test it) > unfortunately. > > Maxime That's OK, I didn't review the patch, I'm just making a general suggestion. :)
