Re: [RFC][PATCH] Enable async suspend/resume of i2c devices

["Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>]

On Wednesday, April 06, 2011, Jean Delvare wrote:
> On Wed, 6 Apr 2011 06:23:35 +0100, Mark Brown wrote:
> > On Mon, Apr 04, 2011 at 08:47:01PM -0700, Sonny Rao wrote:
> > > This improves our resume time when we have devices on an i2c bus
> > > that are slow to resume.  In particular we have a light sensor that
> > > adds about 50ms of resume time on one device. We have to enable it
> > > both on the i2c master and i2c client side and then we get fully async
> > > suspend/resume.  I suspect we'll see nice gains on systems with more
> > > i2c devices and will test that out soon.
> > > 
> > > Signed-off-by: Sonny Rao <sonnyrao-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> > 
> > It'd probably help if the patch explained why this is safe - my
> > immediate question is why if it's safe to just unconditionally enable
> > async suspend for all I2C clients and adaptors it's not safe to do so
> > for all devices of all types?
> 
> I have exactly the same concern. From
> Documentation/ABI/testing/sysfs-devices-power:
> 
> 	It generally is unsafe to permit the asynchronous suspend/resume
> 	of a device unless it is certain that all of the PM dependencies
> 	of the device are known to the PM core.  However, for some
> 	devices this attribute is set to "enabled" by bus type code or
> 	device drivers and in that cases it should be safe to leave the
> 	default value.
> 
> As I don't see any code being added to guarantee that "all of the PM
> dependencies of the device are known to the PM core", I am skeptical
> about the general correctness of the proposed change.
> 
> On the other hand, a quick grep on the kernel tree shows that the scsi,
> usb and pci subsystem enable async suspend unconditionally for all
> devices. This seems quite contradictory with the quoted statement
> above. Rafael, can you please clarify? Is the attribute description too
> alarmist, or are the subsystems too optimistic? ;)

Well, the short explanation is that enabling async suspend/resume for SCSI,
USB and PCI devices gives us the most visible benefit performance-wise, while
at the same time the PM dependencies for all of them generally follow the
parent-child relationships represented by the device hierarchy (i.e. each of
them has a well-defined parent device that it should not be suspended after
and resumed before).

In the few cases where there are known dependencies reaching beyond the device
hierarchy (e.g. the dependencies between the EHCI and UHCI/OHCI USB
controllers), they have been taken into account directly by the relevant
drivers.

Thanks,
Rafael