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? :)
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.
A good thing, I like this, very nice work, all of these are:
Reviewed-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
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.
thanks,
greg k-h
