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

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Date:	Thu, 10 May 2012 08:55:49 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	huang ying <huang.ying.caritas@...il.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 Wed, 2012-05-09 at 12:38 +0200, Rafael J. Wysocki wrote:
> On Wednesday, May 09, 2012, Huang Ying wrote:
> > On Tue, 2012-05-08 at 23:34 +0200, Rafael J. Wysocki wrote:
> > > On Tuesday, May 08, 2012, Huang Ying wrote:
> > > > On Mon, 2012-05-07 at 22:53 +0200, Rafael J. Wysocki wrote:
> > > > > On Saturday, May 05, 2012, huang ying wrote:
> > > > > > 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>
> > > [...]
> > > > > I think we may add helpers for exporting/unexporting power_off_allowed
> > > > > like for the PM QoS latency attribute.  Then, whoever wants to support
> > > > > power_off_allowed and use it will export it through that helper.
> > > > 
> > > > That sounds good!
> > > > 
> > > > > Still, I'm afraid we're trying to special case something that really ins't
> > > > > a special case.  Namely, we may want to restrict devices from using some
> > > > > other low-power states as well, not only power off (eg. we may want to
> > > > > prevent devices' clocks from being stopped).
> > > > 
> > > > One step towards generalization is to provide a way for user to specify
> > > > lowest power state allowed.  For example, for PCI devices, they can
> > > > specify D1, D2, D3hot or D3cold.  But it is hard to generalize a set of
> > > > low power states for all kind of devices.  Maybe we should keep this
> > > > user space interface bus specific?
> > > 
> > > I came to the same conclusion. :-)
> > > 
> > > Besides, we already have the no_d1d2 and d1_support, d2_support flags
> > > in struct pci_dev.  We can simply add no_d3_cold in analogy and add a similar
> > > thing for ATA to cover the ZPODD case.
> > > 
> > > I would keep those things bus-type-specific and platform-specific.
> > 
> > But power-off sate seems like the only low power state that can be
> > shared between buses.
> 
> Well, the problem is that "power off" is not so well defined really.
> It geneally means "power removed", but how to achieve that, or more
> precisely what sequence of events leads to that situation, is rather
> bus/platform-specific.  It may be a direct action (like _PS3) or
> putting the parent into a low-power state (which need not mean "power off"
> for the parent), or turning off a power domain.

How to remove power is not general enough, but I think whether to power
off is more general.  So a flag like power_must_be_on can be general and
useful.

> Also, from a driver's perspective the result of putting a device into
> some non-power-off low-power state may be just as unpleasant as the
> result of removing power from it.

Yes.  From device driver or bus point of view, "power off" is just like
other low power state.  But from the "all devices" point of view, power
off can be the only low power state that is shared by all devices.  So
we can say "power off" is special from this point of view.

> > And keep power_must_be_on in dev_pm_info seems
> > good for power domain implementation.  Because one power domain may
> > contain devices comes from different buses.
> 
> But then it may go into the domain data.

That is how it is implemented for domain now.  But consider we may put a
PCIe device into power domain on some platform.  On some other platform,
D3Cold may implemented not in power domain, so we need a "no_d3cold" in
struct pci_dev.  When a PCIe device driver wants to disable "power off",
it will need to set pci_dev->no_d3cold to false and call
pm_genpd_dev_always_on().  Is it better for us to just set
dev->power_must_be_on to true instead of two operations?  After all,
duplicated state is bad smell of code.

Best Regards,
Huang Ying


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