On Thu, May 23, 2019 at 10:52 PM Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, May 23, 2019 at 06:01:11PM -0700, Saravana Kannan wrote: > > Add a generic "depends-on" property that allows specifying mandatory > > functional dependencies between devices. Add device-links after the > > devices are created (but before they are probed) by looking at this > > "depends-on" property. > > > > This property is used instead of existing DT properties that specify > > phandles of other devices (Eg: clocks, pinctrl, regulators, etc). This > > is because not all resources referred to by existing DT properties are > > mandatory functional dependencies. Some devices/drivers might be able > > to operate with reduced functionality when some of the resources > > aren't available. For example, a device could operate in polling mode > > if no IRQ is available, a device could skip doing power management if > > clock or voltage control isn't available and they are left on, etc. > > > > So, adding mandatory functional dependency links between devices by > > looking at referred phandles in DT properties won't work as it would > > prevent probing devices that could be probed. By having an explicit > > depends-on property, we can handle these cases correctly. > > > > Having functional dependencies explicitly called out in DT and > > automatically added before the devices are probed, provides the > > following benefits: > > > > - Optimizes device probe order and avoids the useless work of > > attempting probes of devices that will not probe successfully > > (because their suppliers aren't present or haven't probed yet). > > > > For example, in a commonly available mobile SoC, registering just > > one consumer device's driver at an initcall level earlier than the > > supplier device's driver causes 11 failed probe attempts before the > > consumer device probes successfully. This was with a kernel with all > > the drivers statically compiled in. This problem gets a lot worse if > > all the drivers are loaded as modules without direct symbol > > dependencies. > > > > - Supplier devices like clock providers, regulators providers, etc > > need to keep the resources they provide active and at a particular > > state(s) during boot up even if their current set of consumers don't > > request the resource to be active. This is because the rest of the > > consumers might not have probed yet and turning off the resource > > before all the consumers have probed could lead to a hang or > > undesired user experience. > > > > Some frameworks (Eg: regulator) handle this today by turning off > > "unused" resources at late_initcall_sync and hoping all the devices > > have probed by then. This is not a valid assumption for systems with > > loadable modules. Other frameworks (Eg: clock) just don't handle > > this due to the lack of a clear signal for when they can turn off > > resources. This leads to downstream hacks to handle cases like this > > that can easily be solved in the upstream kernel. > > > > By linking devices before they are probed, we give suppliers a clear > > count of the number of dependent consumers. Once all of the > > consumers are active, the suppliers can turn off the unused > > resources without making assumptions about the number of consumers. > > > > By default we just add device-links to track "driver presence" (probe > > succeeded) of the supplier device. If any other functionality provided > > by device-links are needed, it is left to the consumer/supplier > > devices to change the link when they probe. > > Somewhere in this wall of text you need to say: > MAKES DEVICES BOOT FASTER! > right? :) I'm sure it will, but I can't easily test and measure this number because I don't have a device with 100s of devices (common in mobile SoCs) where I can load all the drivers as modules and are supported upstream. And the current ones I have mostly workaround this in their downstream tree by manually ordering with initcalls and link order. But I see the avoidance of useless probes that'll fail as more of a free side benefit and not the main goal of this patch series. Getting modules to actually work and crash the system while booting is the main goal. > So in short, this solves the issue of deferred probing with systems with > loads of modules for platform devices and device tree, in that now you > have a chance to probe devices in the correct order saving loads of busy > loops. Yes, definitely saves loads of busy work. > A good thing, I like this, very nice work, all of these are: > Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Thanks! > but odds are I'll take this through my tree, so I'll add my s-o-b then. > But only after the DT people agree on the new entry. Yup! Trying to do that. :) -Saravana
