Re: [PATCH] usb: ohci/uhci - add soft dependencies on ehci_hcd

On Tue, 10 Sep 2013, Lucas De Marchi wrote:

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The ordering is more of a recommendation than a necessity.  Systems
should work oksy if the modules are loaded in the wrong order.

The only issue if the drivers are loaded in the wrong order is that
full- and low-speed devices will end up being detected and enumerated
twice, with a disconnection in between.  This can result in
disconcerting messages appearing in the system log.

In theory, it is possible for a problem to arise.  For example, suppose
a full-speed flash drive is plugged in when the system starts up.
When uhci-hcd or ohci-hcd gets loaded, the drive will be detected and
it may even be mounted.  Then when ehci-hcd loads, the drive will get
disconnected and reconnected.  Any open file references to the original
mount will be orphaned.

Then IMO it should not only be advised to have this dependency, but
also require it.

Requiring the ordering won't fix all the possible problems.  For
example, ehci-pci might be compiled into the kernel while ohci-pci is
built as a module (although this probably could be prevented by
tweaking the Kconfig file).

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So this means that before installing ohci-hcd, modprobe would attempt
to install ehci-hcd?  And if the attempt failed, it would go ahead and
load ohci-hcd anyway?

Exactly.

Not really. If we try to load the module, but then fail we fail the
operation altogether. Whereas if the module simply doesn't exist, then
we carry on. In reality what we do is to assume the indexes and
softdeps in configuration are correct)

What if somebody really does want to load ohci-pci without loading 
ehci-pci?  That is a perfectly valid thing to do (and I often want to 
do it for testing).

It sounds like the only way to accomplish this would be to load both 
and then unload ehci-pci.  Yes?  Or would that unload ohci-pci as well?

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What about during unloading?  Would this mean that rmmod ohci-hcd would
automatically cause ehci-hcd to be unloaded also?  Or would unloading
ehci-hcd cause ohci-hcd to be unloaded?

No, unloading does not care about dependencies.

modprobe -r will remove then as well, in the opposite order, i.e. post
softdep, module, pre softdep.  However this applies only to modprobe,
not to the related function in libkmod since it would be weird to
other programs using this library function.

Aside from testing, people hardly ever unload drivers.  So this is a 
relatively minor concern.

Try playing with this in /etc/modprobe.d/bla.conf (and replace echo
with whatever unknown command to test a failure scenario):
install bla echo install bla
remove bla echo remove bla
softdep sr_mod pre: bla

Good recommendation.

Alan Stern