linux-kernel - Re: [PATCH] PM / Domains: If an OF node is found but no device probed yet, defer.

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Date:	Mon, 16 Mar 2015 09:44:22 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Kevin Hilman <khilman@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Eric Anholt <eric@...olt.net>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] PM / Domains: If an OF node is found but no device probed
 yet, defer.

Hi Kevin,

On Fri, Mar 13, 2015 at 7:01 PM, Kevin Hilman <khilman@...nel.org> wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
>> On Wed, Mar 11, 2015 at 11:08 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>>> More CCes.
>>>
>>> On Wednesday, March 11, 2015 08:27:28 AM Eric Anholt wrote:
>>>> If we've declared a power domain in the OF, and the OF node is found
>>>> but the requested domain hasn't been registered on it yet, then we
>>>> probably have just tried to probe before the power domain driver has.
>>>> Defer our device's probe until it shows up.
>>>>
>>>> Signed-off-by: Eric Anholt <eric@...olt.net>
>>>
>>> Kevin, Ulf, any chance to have a look at this, please?
>>>
>>>> ---
>>>>
>>>> I ran into this when turning my ad-hoc code for BCM2835 (Raspberry Pi)
>>>> USB poweron support in the DWC2 controller to an OF-based power domain
>>>> declaration.
>>
>> I guess you are initializing the PM domains from module_init()?
>>
>> I use core_initcall() in arch/arm/mach-shmobile/pm-rmobile.c to make sure it's
>> initialized earlier, as e.g. the interrupt controller uses postcore_initcall().
>
> Yeah, I think most existing users are initizling PM domains early, but IMO
> we should be working towards supporting PM domains that are created
> later as well (as this patch does.)

Sure. When interrupt controllers are involved (yes, they can be in a PM (Clock)
Domain too), there are definitely some complex issues to resolve in the core OF
probing code first. Cfr. the bug mentioned in
http://marc.info/?l=devicetree&m=142325419327559&w=2

>>>> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
>>>> index ba4abbe..2b93c98 100644
>>>> --- a/drivers/base/power/domain.c
>>>> +++ b/drivers/base/power/domain.c
>>>> @@ -2064,7 +2064,7 @@ EXPORT_SYMBOL_GPL(of_genpd_del_provider);
>>>>  struct generic_pm_domain *of_genpd_get_from_provider(
>>>>                                       struct of_phandle_args *genpdspec)
>>>>  {
>>>> -     struct generic_pm_domain *genpd = ERR_PTR(-ENOENT);
>>>> +     struct generic_pm_domain *genpd = ERR_PTR(-EPROBE_DEFER);
>>
>> Currently platform_drv_probe() just continues if dev_pm_domain_attach() returns
>> a different error than -EPROBE_DEFER, which is what you are seeing.
>>
>> Your change does have the side effect that a new DT with PM domains won't
>> work on an older kernel that doesn't have the PM domain driver yet.
>
> Is that a real problem though?  Using newer DTs on older kernels can
> cause many types of problems.

It means we cannot describe everything in (stable) DT before all support code
has been implemented. Ignoring that, it enforces strict merge order.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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