Subject: Making udev emit a signal when it is done loading modules

From: [email-masked] (Hans de Goede)
Date: Sat, 17 Jan 2015 09:44:00 +0100

Dear udev developers,

We (me and some kernel devs mostly) would like to add support to
the kernel for userspace telling the kernel that it is done with
the *initial* loading of modules, with the purpose of cleaning up
(disabling) unused harware resources like e.g. regulators and
clocks.

Currently the kernel does this cleanup just before it starts init
(which may very well be init from a ramdisk). In some cases this
is too early really, because later on a module may get loaded
which needs this resources, these resources will then get turned
on again by the loaded driver, and most of the time this is not
an issue, but sometimes it is.

I realize very well that there is no magic moment where udev is
really ever done loading modules, but the case which we want to
support only involves devices which are *already enumerated*, but
may not yet have a driver loaded, when udev starts. We would like
udev to emit a signal (ABI to be discussed) when it is done
trying to load modules for everything which was already enumerated
when it starts, iow when there are no new device events pending
anymore when udev does its initial hotplug replay.

So the question to you is would you be willing to include such
functionality in udev ?  Note this signal would need to be emitted
when udev from the real rootfs is done with the initial module
loading, as the real rootfs may very well have more modules
available then the initrd.

###

With the generic story above told let me also give the concrete
example / problem which has let to me asking this (note this has
been brought up before on various kernel lists, it is a
re-occuring theme, this is just an example really) :

The problem at hand is a sata connector which also has a sata-power
connector on an embedded (ish) board where the sata-power is
controlled through a gpio. The sata-power connector is modeled
in devicetree as a power-supply and this supply gets controlled
by the ahci_platform driver.

The disk power may very well have already been turned on by the
bootloader, so we add a regulator-boot-on property to the regulator
node in devicetree to make sure that it is left untouched when the
regulator driver loads. If the ahci_platform driver is build into
the kernel, it will then take control of the regulator and
everything works well.

If however the ahci_platform driver is a module, then as soon as
the kernel is ready to start init, unused regulators are turned off
and the disk looses its power while spinning and ends up doing an
emergency heads park. Then as soon as the ahci_platform module
loads the disk is spin up again. This is not good (quite bad
actually) for the lifetime of the disk.

Regards,

Hans