From: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
To: Maxime Ripard <maxime@cerno.tech>
Cc: "Ulf Hansson" <ulf.hansson@linaro.org>,
"Prashant Gaikwad" <pgaikwad@nvidia.com>,
"Alexandre Belloni" <alexandre.belloni@bootlin.com>,
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Subject: Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate
Date: Mon, 07 Nov 2022 20:57:22 +0000 [thread overview]
Message-ID: <ucJ6KSBqdPTxfxUQqLUr9C9RGiQRnY1I@localhost> (raw)
In-Reply-To: <20221107085417.xrsh6xy3ouwdkp4z@houat>
Maxime Ripard <maxime@cerno.tech> writes:
> Hi,
>
> On Fri, Nov 04, 2022 at 05:35:29PM +0000, Aidan MacDonald wrote:
>>
>> Maxime Ripard <maxime@cerno.tech> 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@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.
>>
>> 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. :)
next prev parent reply other threads:[~2022-11-09 17:25 UTC|newest]
Thread overview: 115+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-04 13:17 [PATCH v2 00/65] clk: Make determine_rate mandatory for muxes Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 01/65] clk: Export clk_hw_forward_rate_request() Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 02/65] clk: lan966x: Remove unused round_rate hook Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 03/65] clk: nodrv: Add a determine_rate hook Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 04/65] clk: test: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 05/65] clk: actions: composite: Add a determine_rate hook for pass clk Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 06/65] clk: at91: main: Add a determine_rate hook Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 07/65] clk: at91: sckc: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 08/65] clk: berlin: div: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 09/65] clk: cdce706: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 10/65] clk: k210: pll: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 11/65] clk: k210: aclk: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 12/65] clk: k210: mux: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 13/65] clk: lmk04832: clkout: " Maxime Ripard
2022-11-13 22:35 ` Liam Beguin
2022-11-04 13:17 ` [PATCH v2 14/65] clk: lochnagar: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 15/65] clk: qoriq: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 16/65] clk: si5341: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 17/65] clk: stm32f4: mux: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 18/65] clk: vc5: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 19/65] clk: vc5: clkout: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 20/65] clk: wm831x: " Maxime Ripard
2022-11-07 10:58 ` Charles Keepax
2022-11-04 13:17 ` [PATCH v2 21/65] clk: davinci: da8xx-cfgchip: " Maxime Ripard
2022-11-04 16:45 ` David Lechner
2022-11-07 12:06 ` Maxime Ripard
2022-11-07 14:52 ` David Lechner
2022-11-04 13:17 ` [PATCH v2 22/65] " Maxime Ripard
2022-11-04 16:46 ` David Lechner
2022-11-04 13:17 ` [PATCH v2 23/65] clk: imx: busy: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 24/65] clk: imx: fixup-mux: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 25/65] clk: imx: scu: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 26/65] clk: mediatek: cpumux: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 27/65] clk: pxa: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 28/65] clk: renesas: r9a06g032: " Maxime Ripard
2022-11-07 7:51 ` Geert Uytterhoeven
2022-11-04 13:17 ` [PATCH v2 29/65] clk: socfpga: gate: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 30/65] clk: stm32: core: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 31/65] clk: tegra: bpmp: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 32/65] clk: tegra: super: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 33/65] clk: tegra: periph: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 34/65] clk: ux500: prcmu: " Maxime Ripard
2022-11-08 13:25 ` Linus Walleij
2022-11-09 11:05 ` Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 35/65] clk: ux500: sysctrl: " Maxime Ripard
2022-11-08 13:27 ` Linus Walleij
2022-11-10 11:28 ` Ulf Hansson
2022-11-10 11:39 ` Linus Walleij
2022-11-10 13:05 ` Ulf Hansson
2022-11-11 9:20 ` Linus Walleij
2022-11-14 9:05 ` Lee Jones
2022-11-04 13:17 ` [PATCH v2 36/65] clk: versatile: sp810: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 37/65] drm/tegra: sor: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 38/65] phy: cadence: sierra: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 39/65] phy: cadence: torrent: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 40/65] phy: ti: am654-serdes: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 41/65] phy: ti: j721e-wiz: " Maxime Ripard
2022-11-04 13:17 ` [PATCH v2 42/65] rtc: sun6i: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 43/65] ASoC: tlv320aic32x4: " Maxime Ripard
2022-11-04 15:44 ` Mark Brown
2022-11-04 15:51 ` Maxime Ripard
2022-11-04 15:59 ` Mark Brown
2022-11-07 8:43 ` Maxime Ripard
2022-11-07 12:06 ` Mark Brown
2022-11-07 15:26 ` Maxime Ripard
2022-11-07 16:02 ` Mark Brown
2023-03-22 23:31 ` Stephen Boyd
2023-03-29 19:50 ` Maxime Ripard
2023-03-29 20:04 ` Stephen Boyd
2022-11-04 13:18 ` [PATCH v2 44/65] clk: actions: composite: div: Switch to determine_rate Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 45/65] clk: actions: composite: fact: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 46/65] clk: at91: smd: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 47/65] clk: axi-clkgen: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 48/65] clk: cdce706: divider: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 49/65] clk: cdce706: clkout: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 50/65] clk: si5341: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 51/65] clk: si5351: pll: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 52/65] clk: si5351: msynth: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 53/65] clk: si5351: clkout: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 54/65] clk: da8xx: clk48: " Maxime Ripard
2022-11-04 16:49 ` David Lechner
2022-11-07 14:52 ` Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 55/65] clk: imx: scu: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 56/65] clk: ingenic: cgu: " Maxime Ripard
2022-11-04 14:31 ` Paul Cercueil
2022-11-04 14:59 ` Maxime Ripard
2022-11-04 17:35 ` Aidan MacDonald
2022-11-07 8:54 ` Maxime Ripard
2022-11-07 20:57 ` Aidan MacDonald [this message]
2022-11-09 11:00 ` Maxime Ripard
2023-03-22 23:41 ` Stephen Boyd
2023-03-23 15:35 ` Aidan MacDonald
2023-03-24 11:19 ` Maxime Ripard
2023-03-24 20:58 ` Aidan MacDonald
2023-03-27 19:24 ` Maxime Ripard
2023-04-05 12:57 ` Paul Cercueil
2023-04-05 14:50 ` Maxime Ripard
2023-04-05 15:29 ` Paul Cercueil
2022-11-05 10:33 ` Paul Cercueil
2022-11-09 10:53 ` Maxime Ripard
2022-11-09 11:36 ` Paul Cercueil
2022-11-04 13:18 ` [PATCH v2 57/65] clk: ingenic: tcu: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 58/65] clk: sprd: composite: " Maxime Ripard
2022-11-09 2:43 ` Chunyan Zhang
2022-11-04 13:18 ` [PATCH v2 59/65] clk: st: flexgen: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 60/65] clk: stm32: composite: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 61/65] clk: tegra: periph: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 62/65] clk: tegra: super: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 63/65] ASoC: tlv320aic32x4: pll: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 64/65] ASoC: tlv320aic32x4: div: " Maxime Ripard
2022-11-04 13:18 ` [PATCH v2 65/65] clk: Warn if we register a mux without determine_rate Maxime Ripard
2022-11-07 10:56 ` Charles Keepax
2023-03-21 23:55 ` [PATCH v2 00/65] clk: Make determine_rate mandatory for muxes Stephen Boyd
2023-03-22 10:01 ` Maxime Ripard
2023-03-22 15:19 ` Stephen Boyd
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