linux-kernel - Re: [PATCH V4 16/16] ARM64: tegra: select PM_GENERIC

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Message-ID: <56714DA3.80707@nvidia.com>
Date:	Wed, 16 Dec 2015 11:40:19 +0000
From:	Jon Hunter <jonathanh@...dia.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Philipp Zabel <p.zabel@...gutronix.de>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	"Alexandre Courbot" <gnurou@...il.com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...nel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Vince Hsu <vinceh@...dia.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH V4 16/16] ARM64: tegra: select PM_GENERIC_DOMAINS


On 16/12/15 09:47, Ulf Hansson wrote:
> On 16 December 2015 at 10:40, Jon Hunter <jonathanh@...dia.com> wrote:
>> Hi Ulf,
>>
>> On 15/12/15 19:54, Ulf Hansson wrote:
>>> On 4 December 2015 at 15:57, Jon Hunter <jonathanh@...dia.com> wrote:
>>>> Enable PM_GENERIC_DOMAINS for tegra 64-bit devices. To ensure that devices
>>>> dependent upon a particular power-domain are only probed when that power
>>>> domain has been powered up, requires that PM is made mandatory for tegra
>>>> 64-bit devices and so select this option for tegra as well.
>>>>
>>>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
>>>> ---
>>>>  arch/arm64/Kconfig.platforms | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/Kconfig.platforms b/arch/arm64/Kconfig.platforms
>>>> index 9806324fa215..e0b5bd0aff0f 100644
>>>> --- a/arch/arm64/Kconfig.platforms
>>>> +++ b/arch/arm64/Kconfig.platforms
>>>> @@ -93,6 +93,8 @@ config ARCH_TEGRA
>>>>         select GENERIC_CLOCKEVENTS
>>>>         select HAVE_CLK
>>>>         select PINCTRL
>>>> +       select PM
>>>> +       select PM_GENERIC_DOMAINS
>>>
>>> If you still want to allow ARCH_TEGRA to run without PM, you should
>>> probably change to:
>>>
>>> select PM_GENERIC_DOMAINS if PM
>>
>> Per the changelog this is deliberate. If we allow !PM, then there is a
>> potential that you could probe a device when the power domain is not
>> powered on. I understand that some SoCs turn on all the power-domains
>> when !PM but this will not work for tegra because we don't register the
>> power domain until later in the boot and so we are relying upon probe
>> deferral to defer the probe of devices that use power-domains.
> 
> So what you are saying is that adding the PM domain support, will fix
> some devices to become successfully probed as they were broken before?

Not exactly. There is a legacy tegra_powergate_sequence_power_up() that
has been used to date to get around this. However, by migrating to GENPD
we really need to make PM mandatory, otherwise you could attempt to
probe a device in a power domain that is not powered. In other words,
you are probing blindly. I know some SoCs do this, but that does not
seem very robust.

Cheers
Jon
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