linux-kernel - Re: [PATCH] usb: ohci/uhci - add soft dependencies on ehci

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Date:	Tue, 10 Sep 2013 13:31:50 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tom Gundersen <teg@...m.no>
cc:	linux-usb@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	"linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] usb: ohci/uhci - add soft dependencies on ehci_hcd

On Tue, 10 Sep 2013, Tom Gundersen wrote:

> Hi Alan,
> 
> On Tue, Sep 10, 2013 at 7:02 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Tue, 10 Sep 2013, Tom Gundersen wrote:
> >
> >> Support for specifying soft dependencies in the modules themselves was
> >> introduced in commit 7cb14ba.
> >>
> >> In Arch we have always been shipping a module.d(5) fragment ordering ohci/uhci
> >> after ehci. If this ordering is really necessary, it would be great to move it
> >> to the kernel and getting the correct fragment generated by depmod.
> >
> > 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.
> 
> Thanks for the explanation. Sounds to me that a softdep is indeed appropriate:
> 
> //
> The softdep command allows you to specify soft, or optional, module
> dependencies. modulename can be used without these optional modules
> installed, but usually with some features missing. For example, a
> driver for a storage HBA might require another module be loaded in
> order to use management features.
> 
> pre-deps and post-deps modules are lists of names and/or aliases of
> other modules that modprobe will attempt to install (or remove) in
> order before and after the main module given in the modulename
> argument.
> //

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?

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?

> >> Signed-off-by: Tom Gundersen <teg@...m.no>
> >> Cc: Alan Stern <stern@...land.harvard.edu>
> >> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >> ---
> >>  drivers/usb/host/ohci-hcd.c | 1 +
> >>  drivers/usb/host/uhci-hcd.c | 1 +
> >>  2 files changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/usb/host/ohci-hcd.c b/drivers/usb/host/ohci-hcd.c
> >> index 8f6b695..783b50a 100644
> >> --- a/drivers/usb/host/ohci-hcd.c
> >> +++ b/drivers/usb/host/ohci-hcd.c
> >> @@ -1171,6 +1171,7 @@ EXPORT_SYMBOL_GPL(ohci_init_driver);
> >>  MODULE_AUTHOR (DRIVER_AUTHOR);
> >>  MODULE_DESCRIPTION(DRIVER_DESC);
> >>  MODULE_LICENSE ("GPL");
> >> +MODULE_SOFTDEP("pre: ehci_hcd");
> >>
> >>  #if defined(CONFIG_ARCH_SA1100) && defined(CONFIG_SA1111)
> >>  #include "ohci-sa1111.c"
> >> diff --git a/drivers/usb/host/uhci-hcd.c b/drivers/usb/host/uhci-hcd.c
> >> index 4a86b63..c2babf3 100644
> >> --- a/drivers/usb/host/uhci-hcd.c
> >> +++ b/drivers/usb/host/uhci-hcd.c
> >> @@ -937,3 +937,4 @@ module_exit(uhci_hcd_cleanup);
> >>  MODULE_AUTHOR(DRIVER_AUTHOR);
> >>  MODULE_DESCRIPTION(DRIVER_DESC);
> >>  MODULE_LICENSE("GPL");
> >> +MODULE_SOFTDEP("pre: ehci_hcd");
> >
> > Where is MODULE_SOFTDEP defined?  It isn't mentioned in any .h files in
> > my kernel tree.
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/include/linux/module.h#n104

I think I get the picture.

The MODULE_SOFTDEP lines should be moved to uhci-pci.c and ohci-pci.c,
because the ordering restrictions apply only to the PCI versions of
these drivers.  And they should specify "pre: ehci-pci", not ehci-hcd.

Alan Stern

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