linux-kernel - Re: Questions about Runtime Power Management

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Message-ID: <9E3F9C2076C45D4783F09B90D5BE77CE171D3B@BADWLRZ-SWMBX14.ads.mwn.de>
Date:	Tue, 11 Jun 2013 07:53:10 +0000
From:	"Morales, Alejandra" <alejandra.morales@....de>
To:	Oliver Neukum <oneukum@...e.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Questions about Runtime Power Management

2013/6/10 Oliver Neukum <oneukum@...e.de>
>
> On Sunday 09 June 2013 16:28:21 Morales, Alejandra wrote:
>
> > I did a test with an external USB hard drive, checking the runtime power state before and after issuing a sleep command with hdparm -Y. The drive effectively spinned down, but the runtime power state didn't change from active to suspended.
>
> Hard disks, as opposed to USB storage devices, don't do kernel based runtime PM.
> If you give a drive commands by hdparm, implementation is the job of the disk and
> nothing else in the system learns about it.
>
> > - Do scsi device drivers implement the runtime_status updates when drives effectively change their state?
> > - Is runtime power management supported by net devices?
> >
> > Any answer would be really appreciated. Thanks in advance.
>
> You are approaching this from the wrong side. "net device" is the function of
> a device. It is effectively impossible to even define what runtime PM would mean
> in that case. You need to approach the question from the side of implementation.
> Is your device USB, PCI, SCSI, FibreChannel, ... ?
>
> As far as SCSI is concerned, yes SCSI devices can do runtime PM (in principle)
> The most mature implementation is USB.
>
> You need to furthermore realise that there are forms of runtime PM independent
> of the generic kernel based runtime PM (hdparm, USB LPM, ...)


Thanks Oliver, that was helpful. I will try now to do my testing using USB devices as it will likely be easier.

Alejandra--
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