On Sun, 7 Aug 2016, Lukas Wunner wrote: > Normally the device hierarchy is traversed bottom-up during suspend > and top-down during resume. However ->prepare and ->complete do it > the other way round. In the case of ->prepare, this is even documented > in Documentation/power/devices.txt but the reason thereof is not. > Could you explain this please? The purpose of ->prepare is to tell drivers that a system sleep is beginning and accordingly they should stop registering new children. This is necessary for the PM core to be able to traverse the entire device tree safely; we want to avoid races where a new child is added below a device concurrently with that device being suspended. (Or if you want to be more precise, races in which a new child is added below a device while the PM core is acquiring the device's lock just prior to invoking its ->suspend callback.) Telling drivers to stop registering new children below a device has to be done top-down, because if it were done bottom-up then it would be subject to the same race described above. Doing it top-down avoids problems; if a device registers new children while the PM core is acquiring its lock prior to invoking ->prepare, it doesn't matter. The new children will be handled later, right along with the existing ones. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
