Hi Maxime,
Le mer. 9 nov. 2022 à 11:53:01 +0100, Maxime Ripard maxime@cerno.tech a écrit :
Hi Paul,
On Sat, Nov 05, 2022 at 10:33:54AM +0000, Paul Cercueil wrote:
Hi Maxime,
Le ven. 4 nov. 2022 à 15:59:46 +0100, Maxime Ripard maxime@cerno.tech a écrit :
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@cerno.tech 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.
But that would introduce policy into the driver...
I'm not sure why you're bringing policies into that discussion. There's plenty of policy in the driver already, and the current code doesn't do something that the old wasn't doing (implicitly).
Yes, I was just talking about the CLK_SET_RATE_NO_REPARENT flag adding policy. The fact that there's plenty of policy in the driver already is not an argument for adding some more.
And there's plenty of policies in drivers in general. Whether you limit the rate or not, whether you allow reparenting or not, even the CLK_SET_RATE_NO_REPARENT flag mentioned above is a policy decision set by drivers.
Allowing reparenting and not limiting the rates is not a policy, it's just following what the hardware allows you to do. The absence of policy means that the driver allows you to configure the hardware in any way you might want to.
Limiting rates, forbidding reparenting, that's policy, and it doesn't belong in a driver.
You can argue that choosing not to reparent on rate change is a policy, and it is. That's why we need a way to enforce these policies outside the driver.
The fact that I don't want the MMC parented to the PLLs, doesn't mean that it's an invalid configuration per se.
Sure, and that's another policy :)
A policy that is not enforced by the driver.
Going back to the patch itself... I am fine with the change, although the patch description should probably be updated. We have .set_parent callbacks to configure clocks from DT, there's nothing more to it.
Cheers, -Paul