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

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Message-ID: <CAPDyKFqVwSo=CoppkXTyTOY-wmNsefLq5vSw+CzUCkZ7XDLvsQ@mail.gmail.com>
Date:	Mon, 16 Mar 2015 11:12:25 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Kevin Hilman <khilman@...nel.org>, Eric Anholt <eric@...olt.net>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"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.

On 13 March 2015 at 19:01, 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.)

I do agree, that we _should_ allow PM domains to be created later/any
time. Unfortunate, that's not going to be a simple one-liner patch.
:-)

To have genpd_dev_pm_attach() return -EPROBE_DEFER, due to that the PM
domain hasn’t been _initialized_ yet, we need to know whether a PM
domain exists at all for the device. In principle we need to split the
work done by genpd_dev_pm_attach() into the two parts described below.

1.
At struct device creation time, done from the "OF core", we also need
to parse for a PM domain node. If such is found, we somehow needs to
assigned it to the device.

Normally we would have assigned the struct dev_pm_domain in the struct
device to deal with this, but that has some implications. Currently
the struct dev_pm_domain is created from SoC specific code and it's
also done at different init levels.

2. At ->probe(), genpd shall return -EPROBE_DEFER, if the device's
assigned PM domain hasn’t been initialized yet.

Implementing 2) should be trivial, but 1) could be a bit harder.
Anyway, if anyone want to have a stab on it, I will gladly review such
patches.

Kind regards
Uffe

>
>>>> drivers/base/power/domain.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> 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.
>
> Kevin
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