linux-kernel - Re: Put unused PCI devices in D3

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Message-ID: <20080930000923.GA29775@srcf.ucam.org>
Date:	Tue, 30 Sep 2008 01:09:23 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	"Jeffrey W. Baker" <jwbaker@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Put unused PCI devices in D3

On Mon, Sep 29, 2008 at 01:11:03PM -0700, Jeffrey W. Baker wrote:

> Currently it looks to me from browsing the code that D3 is only used in
> Linux when the whole machine is headed for S3, and the drivers are asked
> to suspend.  I think the drivers should be able to enter D3 in other
> circumstances:
> 
> * During pci_unregister_driver ... i.e. on module unload

The fact that Linux isn't using a device doesn't inherently mean that 
the system isn't using it. For example, the smbus controller will 
probably still be used by ACPI even if there's no Linux driver loaded.

> * When a network interface is downed

Yes. Drivers are moving towards this model.

> * If the device is a bridge or hub with no downstream device

Potential problems with hotplugging? But sure, this kind of thing is 
being implemented in USB and SCSI.

> * Whenever userspace requests D3 via sysfs

This functionality was explicitly removed a few years back.

> So, what fundamental problem prevents me from, for example, calling
> pci_set_power_state() from ohci1394's __exit?

Nothing I'm aware of. It sounds pretty safe in that case. But that would 
require you to load and unload the driver - better to have the driver 
loaded the entire time and make sure it does as much runtime power 
management as possible. I don't know enough about the firewire OHCI 
hardware, but can you power down most of the chip and still get hotplug 
events?
-- 
Matthew Garrett | mjg59@...f.ucam.org
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