Date:	Fri, 12 Dec 2014 19:40:52 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	ulf.hansson@...aro.org, Kevin Hilman <khilman@...aro.org>,
	Mike Turquette <mturquette@...aro.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] of/clk: use "clkops-clocks" to specify clocks handled by clock_ops domain

Hi Grygorii,

I've found this mail deep inside my inbox :-)

On Wednesday 30 July 2014 16:25:31 Grygorii Strashko wrote:
> On 07/30/2014 03:06 AM, Laurent Pinchart wrote:
> > On Monday 28 July 2014 23:52:34 Grant Likely wrote:
> >> On Mon, Jul 28, 2014 at 11:47 AM, Grygorii Strashko wrote:
> >>> On 07/28/2014 05:05 PM, Grant Likely wrote:
> >>>> On Thu, 12 Jun 2014 19:53:43 +0300, Grygorii Strashko wrote:
> >>>>> Use "clkops-clocks" property to specify clocks handled by
> >>>>> clock_ops domain PM domain. Only clocks defined in "clkops-clocks"
> >>>>> set of clocks will be handled by Runtime PM through clock_ops
> >>>>> Pm domain.
> >>>>> 
> >>>>> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
> >>>>> ---
> >>>>> 
> >>>>>    drivers/of/of_clk.c |    7 ++-----
> >>>>>    1 file changed, 2 insertions(+), 5 deletions(-)
> >>>>> 
> >>>>> diff --git a/drivers/of/of_clk.c b/drivers/of/of_clk.c
> >>>>> index 35f5e9f..5f9b90e 100644
> >>>>> --- a/drivers/of/of_clk.c
> >>>>> +++ b/drivers/of/of_clk.c
> >>>>> @@ -86,11 +86,8 @@ int of_clk_register_runtime_pm_clocks(struct
> >>>>> device_node *np,>>>
> >>>>> 
> >>>>>       struct clk *clk;
> >>>>>       int error;
> >>>>> 
> >>>>> -    for (i = 0; (clk = of_clk_get(np, i)) && !IS_ERR(clk); i++) {
> >>>>> -            if (!clk_may_runtime_pm(clk)) {
> >>>>> -                    clk_put(clk);
> >>>>> -                    continue;
> >>>>> -            }
> >>>>> +    for (i = 0; (clk = of_clk_get_from_set(np, "clkops", i)) &&
> >>>>> +                 !IS_ERR(clk); i++) {
> >>>> 
> >>>> This really looks like an ABI break to me. What happens to all the
> >>>> existing platforms who don't have this new clkops-clocks in their
> >>>> device tree?
> >>> 
> >>> Agree. This patch as is will break such platforms.
> >>> As possible solution for above problem - the NULL can be used as clock's
> >>> prefix by default and platform code can configure new value of clock's
> >>> prefix during initialization.
> >>> In addition, to make this solution full the of_clk_get_by_name() will
> >>> need to be modified too.
> >>> 
> >>> But note pls, this is pure RFC patches which I did to find out the
> >>> answer on questions: - What is better: maintain Runtime PM clocks
> >>> configuration in DT or in code?
> >> 
> >> In code. I don't think it is workable to embed runtime PM behaviour
> >> into the DT bindings. I think there will be too much variance in what
> >> hardware requires. We can create helpers to make this simpler, but I
> >> don't think it is a good idea to set it up automatically without any
> >> control from the driver itself.
> >> 
> >>> - Where and when to call of_clk_register_runtime_pm_clocks()?
> >>> 
> >>>    Bus notifier/ platform core/ device drivers
> >> 
> >> I would say in device drivers.
> > 
> > I tend to agree with that.
> > 
> > It will help here to take a step back and remember what the problem we're
> > trying to solve is.
> > 
> > At the root is clock management. Our system comprise many clocks, and they
> > need to be handled. The Common Clock Framework nicely models the clocks,
> > and offers an API for drivers to retrieve device clocks and control them.
> > Drivers can thus implement clock management manually without much pain.
> > 
> > A clock can be managed in roughly three different ways :
> > 
> > - it can be enabled at probe time and disabled at remove time ;
> > 
> > - it can be enabled right before the device leaves its idle state and
> > disabled when the device goes back to idle ; or
> > 
> > - it can be enabled and disabled in a more fine-grained, device-specific
> > manner.
> > 
> > The selected clock management granularity depends on constraints specific
> > to the device and on how aggressive power saving needs to be. Enabling
> > the clocks at probe time and disabling them at remove time is enough for
> > most devices, but leads to a high power consumption. For that reason the
> > second clock management scheme is often desired.
> > 
> > Managing clocks manually in the driver is a valid option. However, when
> > adding runtime PM to the equation, and realizing that the clocks need to
> > be enabled in the runtime PM resume handler and disabled in the suspend
> > handler, the clock management code starts looking very similar in most
> > drivers. We're thus tempted to factorize it away from the drivers into a
> > shared location.
> > 
> > It's important to note at this point that the goal here is only to
> > simplify drivers. Moving clock management code out of the drivers doesn't
> > (unless I'm missing something) open the door to new possibilities, it just
> > serves as a simplification.
> > 
> > Now, as Grygorii mentioned, differences between how a given IP core is
> > integrated in various SoCs can make clock management SoC-dependent. In the
> > vast majority of cases (which is really what we need to target, given that
> > our target is simplifying drivers) SoC integration can be described as a
> > list of clocks that must be managed. That list can be common to all
> > devices in a given SoC, or can be device-dependent as well.
> 
> That's actually a problem - now we have static list of managed clocks
> per-SoC and not per device.
> 
> > Few locations can be used to express a per-device list of per-SoC clocks.
> > We can have clocks lists in a per-SoC and per-device location, per-device
> > clocks lists in an SoC-specific location, or per-SoC clocks lists in a
> > device- specific location.
> > 
> > The first option would require listing clocks to be managed by runtime PM
> > in DT nodes, as proposed by this patch set. I don't think this is the
> > best option, as that information is a mix of hardware description and
> > software policy, with the hardware description part being already present
> > in DT in the clocks property.
> 
> I'm not fully agree here. The clock is "functional clock" If It's managed by
> runtime PM. And all such clocks need to be enabled/disabled always when
> device is powered on/off. So, from my point of view it's HW description and
> it follows TRM.
>
> Other clocks are optional

That's actually use-case dependent, some of them might be mandatory.

> and only drivers should control them.
> And question is how to define sets of such clocks in the best way?
> 
> > The second option calls for storing the lists in SoC code under arch/. As
> > we're trying to minimize the amount of SoC code there (and even remove SoC
> > code completely when possible) I don't think that's a good option.
> > 
> > The third option would require storing the clocks lists in device drivers.
> > I believe this is our best option, as a trade-off between simplicity and
> > versatility. Drivers that use runtime PM already need to enable it
> > explicitly when probing devices. Passing a list of clock names to runtime
> > PM at that point wouldn't complicate drivers much. When the clocks list
> > isn't SoC- dependent it could be stored as static information. Otherwise
> > it could be derived from DT (or any other source of hardware description)
> > using C code, offering all the versatility we need.
> 
> Ok. if I understand right, you propose smth like this:
> 1) DT based solution:
> 
> devA {
> 	clocks = <&clkpa>, <&clkcpgmac>, <&chipclk12>;
> 	rpm-clocks = <&clkpa>, <&clkcpgmac>;
> - or -
> 	clocks = <&clkpa>, <&clkcpgmac>, <&chipclk12>;
> 	clock-names = "clk_pa", "clk_cpgmac", "cpsw_cpts_rft_clk";
> 	rpm-clocks =  "clk_pa", "clk_cpgmac";
> }

On a side note I believe the "rpm-clocks" name is too tied to the Linux 
implementation. A name similar to "functional-clocks" would be better.

> in driver:
>  pm_runtime_enable();
> 
>   |- of_clk_register_runtime_pm_clocks()
> 
> - or -
>  of_clk_register_runtime_pm_clocks()
>  pm_runtime_enable();

I prefer the second option, as an explicit opt-in is less likely to cause 
regressions, and would also offer an easy way for drivers to opt-out.

> 2) Static solution:
> char *con_ids_davinci[] =
>     { "fck", "master", "slave", NULL };
> char *con_ids_keystone[] =
>     { "clk_pa", "clk_cpgmac" };
> 
> static struct of_device_id of_match[] = {
> 	{ .compatible = "ti,keystone", con_ids_keystone},
> 	{ .compatible = "ti,davinci", con_ids_davinci},
> 	{},
> };
> 
> Personally, I like option 1 and, seems, it will not break ABI.

Is option 2 really representative of most use cases ? The list of clock inputs 
to an IP core is a property of the IP core itself. How those inputs are 
connected in the SoC is a property of the SoC integration. The clocks 
references in DT can thus vary per-SoC, but the clock names should be pretty 
much constant for a given IP core. Thus, if we have a single list of clocks to 
manager for a given IP core, it shouldn't be difficult to pass that list to 
the of_clk_register_runtime_pm_clocks() function.

> > The only drawback of this solution I can think of right now is that the
> > runtime PM core couldn't manage device clocks before probing the device.
> > Specifically device clocks couldn't be managed if no driver is loaded for
> > that device. I somehow recall that someone raised this as being a
> > problem, but I can't remember why.
> 
> I can recollect only OMAP2+ SoCs where some abstraction called HW_MOD is
> used during platform initialization to reset all devices and turn off
> unused ones before probing the devices. But clock_ops are not used by
> OMAP2+:)

-- 
Regards,

Laurent Pinchart

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