linux-kernel - Re: [RFC v2 2/5] PM, Add sysfs file power_off to control device power off policy

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Date:	Sat, 5 May 2012 14:29:52 +0800
From:	huang ying <huang.ying.caritas@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Huang Ying <ying.huang@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, ming.m.lin@...el.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Zheng Yan <zheng.z.yan@...el.com>,
	Lan Tianyu <tianyu.lan@...el.com>
Subject: Re: [RFC v2 2/5] PM, Add sysfs file power_off to control device power
 off policy

On Sat, May 5, 2012 at 3:33 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> On Friday, May 04, 2012, Huang Ying wrote:
>> From: Lan Tianyu <tianyu.lan@...el.com>
>>
>> Some devices can be powered off to save more power via some platform
>> mechanism, e.g., ACPI.  But that may not work as expected for some
>> device or platform.  So, this patch adds a sysfs file named power_off
>> under <device>/power directory to provide a mechanism for user to control
>> whether to allow the device to be power off.
>>
>> power_off => "enabled" means allowing the device to be powered off if
>> possible.
>>
>> power_off => "disabled" means the device must be power on anytime.
>>
>> Also add flag power_off_user to struct dev_pm_info to record users'
>> choice. The bus layer can use this field to determine whether to
>> power off the device.
>
> It looks like the new attribute is added for all devices regardless of whether
> or not they actually can be powered off?  If so, then please don't do that,
> it's _extremely_ confusing.

Yes.  You are right.

> If you need user space to be able to control that functionality (and I can
> think of a couple of cases in which you do), there need to be 2 flags,
> can_power_off and may_power_off, where the first one is set by the kernel
> if it is known that power can be removed from the device - the attribute
> should be created when this flag is set and removed when it is unset.
>
> Then, the setting of the second flag will be controlled by the new attribute.
>
> And you'll need to patch quite a few places where devices actually have that
> capability, like where power domains are in use, so that's quire a lot of
> work.

If so, I think maybe we need 3 flags:

- can_power_off, set by kernel when it is possible to power off the device
- may_power_off_user, set by user via sysfs attribute
- may_power_off, set by kernel according to may_power_off_user, power
QoS and some other conditions

Sysfs attribute for may_power_off_user is only created if can_power_off is true.

I think we still can do that step by step.  For example, when we add
power off support to PCI devices, we set can_power_off to true for PCI
devices that is possible to be powered off;  when we add power domain
support, we set can_power_off to true for devices in power domain.  Do
you agree?

Best Regards,
Huang Ying
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