linux-kernel - Re: [linux-pm] pm loss development

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Message-Id: <201105182143.13237.rjw@sisk.pl>
Date:	Wed, 18 May 2011 21:43:13 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	markgross@...gnar.org
Cc:	Raffaele Recalcati <lamiaposta71@...il.com>,
	linux-pm@...ts.linux-foundation.org,
	davinci-linux-open-source@...ux.davincidsp.com,
	linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] pm loss development

On Wednesday, May 18, 2011, mark gross wrote:
> On Wed, May 18, 2011 at 01:07:57AM +0200, Rafael J. Wysocki wrote:
> > On Saturday, May 14, 2011, mark gross wrote:
> > > On Fri, May 13, 2011 at 06:54:57PM +0200, Rafael J. Wysocki wrote:
> > > > On Friday, May 13, 2011, Raffaele Recalcati wrote:
> > > > > Hi Rafael,
> > > > > 
> > > > > 2011/5/12 Rafael J. Wysocki <rjw@...k.pl>:
> > > > > > On Thursday, May 12, 2011, Raffaele Recalcati wrote:
> > > > > >> What happen normally in runtime pm implementation is that every devices
> > > > > >> are switched off and are enabled only when needed.
> > > > > >> In our case instead we have a completely functional embedded system and,
> > > > > >> when an asyncrhonous event appear, we have only some tens milliseconds
> > > > > >> before the actual power failure takes place.
> > > > > >> This patchset add a support in order to switch off not vital part of the system,
> > > > > >> in order to allow the board to survive longer.
> > > > > >> This allow the possibility to save important data.
> > > > > >
> > > > > > OK, so first, who decides what parts of the system are vital and what aren't?
> > > > > 
> > > > > Take a quick look at Documentation/power/loss.txt  paragrpah "2.4
> > > > > Power loss policies".
> > > > > You can decide what can be powered off.
> > > > 
> > > > I read the patches.  My question was about the general idea of who should
> > > > be responsible of making these decisions.
> > > 
> > > I would expect the system integrator would based on the application the
> > > device is getting deployed into.
> > > 
> > > A generic opportunistic policy for peripherals that are stateless and can
> > > be trivially power gated off/on from an ISR could be a default but, for
> > > peripherals that need to do some processing (like waiting on an eMMC DMA
> > > to complete) can take time to power down into a safe state.
> > 
> > What do you mean by safe state?
> >
> I need to get more details on this but I assume its a state where the
> meta data of the file system is committed to the emmc before lights go
> off such that when power is reapplied that the damage isn't too big. 

I don't think you can guarantee that the metadata won't be damaged
without notifying the filesystem of the event (and making it react
appropriately).

> I've also heard some talk of sim card corruption risks but, I don't have
> first hand info on that.

Well, I guess that might be prevented by the driver alone.

Thanks,
Rafael
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