On 2 June 2015 at 13:07, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Tue, Jun 02, 2015 at 09:51:01AM +0100, Russell King - ARM Linux wrote: >> What would resolve that would be to have the device registration succeed, >> but also to scan all devices in the system when a PM domain is attached >> and attach the PM domain to matching devices. The problem you then have >> is the same race between attaching the PM domain to all devices in the >> domain, and devices being probed. Remember, this may not be happening at >> boot time, but during module load, which on some systems is multithreaded. >> So, the problem of not having all devices in the PM domain hasn't changed >> and nothing is solved by this approach - the only thing is the code has >> become more complex. >> >> I don't think that's an improvement. > > Another idea, which I came up with while replying to Ivan on a different > but related topic is: > > * When we create a device which we know has a PM domain which should be > attached, try to look up that PM domain. > * If we find the PM domain, attach the domain, otherwise create the > domain but mark it "incomplete". > * When the device is attempted to be probed, force the probe to defer if > the PM domain is inccomplete. > * When a PM domain is initialised, search for any incomplete PM domain, > and if found, connect with the domain and arrange to handle it. This is a good description from what I had in mind and for what I have started working on. :-) > > The problem you still run into is whether all devices have been registered > into the domain, but I really think that expecting all devices to be > registered is in itself a problem. > > Consider the case where a PM domain spreads between a parent device (eg, > a bridge) and a set of child devices. We may need to probe the bridge > in order to discover and register the child devices. If we're expecting > the PM domain to be fully populated with all devices before that happens, > we're in a catch-22 situation: we can't register the child devices until > the bridge has been probed, and we can't probe the bridge because the > PM domain is incomplete. I think I understand what you have in mind and it's an important point! Although, I have some ideas for how we could allow the PM domain to be initialized independently of whether all devices has been attached or not. That would address your concerns, right!? > > I don't think there's a good answer to that one, not without going back > to today's model, where a PM domain is able to be brought up and down > without all of its associated devices attached. That, in itself, should > not be a problem since drivers should _not_ assume that the device has > any context preserved from the last time it saw it (eg, consider the > random state of a device when a crashed kernel kexecs into a new kernel.) > Doing anything else is just fragile. I agree that today's model has some positives, but there are also drawbacks. See below. 1. In the case where the PM domain driver hasn't yet been probed and thus not called pm_genpd_init() + of_genpd_add_provider_*(), we can't support probe deferral of a device which needs its PM domain powered. Simply because dev_pm_domain_attach() has no knowledge about the (not yet registered) PM domain. 2. We can't attach a devices to its PM domain via DT, unless it has a matching bus/driver which can be probed. 3. Dealing with device enumeration from HW is fragile, since there are no way to keep the PM domain powered during that sequence. That's because the PM domain hasn't yet been attached. For example amba_device_add() has this limitation. Rafael might want to add some additional reasons... Finally, regarding $subject patch, I intend to queue it for my next branch, unless someone has objections. Kind regards Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
