On Thursday, October 17, 2013 01:56:23 PM Rafael J. Wysocki wrote: > On Thursday, October 17, 2013 02:57:12 PM Aaron Lu wrote: > > On 10/16/2013 09:25 PM, Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > > > > > The mechanism causing devices depending on a given power resource > > > (that is, devices that can be in D0 only if that power resource is > > > on) to be resumed automatically when the power resource is turned > > > on (and their "inferred" power state becomes D0 as a result) is > > > inherently racy and in fact unnecessary. > > > > > > It is racy, because if the power resources is turned on and then > > > immediately off, the device resume triggered by the first transition > > > to "on" may still happen, causing the power resource to be turned > > > on again. That again will trigger the "resume of dependent devices" > > > mechanism, but if the devices in question are not in use, they will > > > be suspended in the meantime causing the power resource to be turned > > > off. However, the "resume of dependent devices" will next resume > > > them again and so on. In some cases (USB port PM in particular) that > > > leads to an endless busy loop of flipping the resource on and off > > > continuously. > > > > > > It is needless, because whoever turns a power resource on will most > > > likely turn it off at some point and the devices that go into "D0" > > > as a result of turning it on will then go back into D3cold. > > > Moreover, turning all power resources a device needs to go into > > > D0 is not sufficient for a full transition into D0 in general. > > > Namely, _PS0 may need to be executed in addition to that in some > > > cases. This means that the whole rationale of the "resume of > > > dependent devices" mechanism was incorrect to begin with and it's > > > best to remove it entirely. > > > > With this patch, your previous patch could also be applied now: > > https://lkml.org/lkml/2013/6/14/173 > > [PATCH 4/4] ACPI / PM: Drop two functions that are not used any more > > Good catch and that code is dead anyway AFAICS, because power_dependent is only > used in device_pm.c (in those two functions) and in scan.c (where it is > initialized). That obviously is with the $subject patch applied. Thanks! -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
