linux-kernel - Re: [PATCH v2 0/4] ata port runtime power management support

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Date:	Thu, 10 Nov 2011 10:30:53 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Lin Ming <ming.m.lin@...el.com>
cc:	linux-kernel@...r.kernel.org, <linux-ide@...r.kernel.org>,
	<linux-scsi@...r.kernel.org>, <linux-pm@...r.kernel.org>,
	Jeff Garzik <jgarzik@...ox.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <JBottomley@...allels.com>,
	Tejun Heo <tj@...nel.org>, Huang Ying <ying.huang@...el.com>,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v2 0/4] ata port runtime power management support

On Thu, 10 Nov 2011, Lin Ming wrote:

> Hi, all
> 
> These 4 patches add ata port runtime pm support.
> 
> v1:
> https://lkml.org/lkml/2011/11/2/23
> 
> v2 is totally different than v1.
> 
> v1 performed ata port runtime pm through scsi layer.
> Added hook to scsi host runtime suspend/resume code.
> 
> I realized that this is not the natural way to do ata port runtime pm.
> It does not deal with the races with ata port system suspend/resume.
> 
> With v2, ata port is made to be parent device of scsi host.
> 
> Currently, the device tree of ata port and scsi host looks as below,
> 
>         /sys/devices/pci0000:00/0000:00:1f.2    (ahci controller)
>         |-- ata1                                (ata port)
>         |-- host0                               (scsi host)
>            |-- target0:0:0                      (scsi target)
>                |-- 0:0:0:0                      (disk)
> 
> v2 changes it to:
> 
>         /sys/devices/pci0000:00/0000:00:1f.2    (ahci controller)
>         |-- ata1                                (ata port)
>             |-- host0                           (scsi host)
>                 |-- target0:0:0                 (scsi target)
>                     |-- 0:0:0:0                 (disk)
> 
> So ata port runtime PM will happen as:
> 
> disk suspend --> scsi target suspend --> scsi host suspend --> ata port
> suspend.
> 
> This is much cleaner and natural.

Have you observed any real benefit from this feature?

Currently, disks will not be runtime-suspended unless (1) the user 
requests it by setting the appropriate power/control attribute, and (2) 
the device file is closed (in particular, the disk has no mounted 
filesystems or swap partitions).  I don't imagine this combination of 
events is very common for disks using libata.

Alan Stern

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