On Wed, 6 Apr 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@xxxxxxxxxxxx> > > > > 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? ;) Neither is the case. For these subsystems, the PM dependencies _are_ known. The statement in the Documentation file refers to dependencies other than the usual parent-child relationships. PCI and SCSI have no such dependencies. In USB some do exist, but the subsystem code knows about them and takes them into account. Grep for "device_pm_wait_for_dev" in the appropriate subtrees and you'll see what I mean. Now, I have no idea what the situation is with regard to I2C... Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
