Re: [PATCH 01/13] kdbus: add documentation

[Ahmed S. Darwish]

On Fri, Jan 23, 2015 at 09:19:46PM +0800, Greg Kroah-Hartman wrote:
> On Fri, Jan 23, 2015 at 08:28:20AM +0200, Ahmed S. Darwish wrote:
> > On Fri, Jan 16, 2015 at 11:16:05AM -0800, Greg Kroah-Hartman wrote:
> > > From: Daniel Mack <daniel@xxxxxxxxxx>
> > > 
> > > kdbus is a system for low-latency, low-overhead, easy to use
> > > interprocess communication (IPC).
> > > 
> > > The interface to all functions in this driver is implemented via ioctls
> > > on files exposed through a filesystem called 'kdbusfs'. The default
> > > mount point of kdbusfs is /sys/fs/kdbus.
> > 
> > Pardon my ignorance, but we've always been told that adding
> > new ioctl()s to the kernel is a very big no-no.  But given
> > the seniority of the folks stewarding this kdbus effort,
> > there must be a good rationale ;-)
> > 
> > So, can the rationale behind introducing new ioctl()s be
> > further explained? It would be even better if it's included
> > in the documentation patch itself.
> 
> The main reason to use an ioctl is that you want to atomically set
> and/or get something "complex" through the user/kernel boundary.  For
> simple device attributes, sysfs works great, for configuring devices,
> configfs works great, but for data streams / structures / etc. an ioctl
> is the correct thing to use.
> 
> Examples of new ioctls being added to the kernel are all over the
> place, look at all of the special-purpose ioctls the filesystems keep
> creating (they aren't adding new syscalls), look at the monstrosity that
> is the DRM layer, look at other complex things like openvswitch, or
> "simpler" device-specific interfaces like the MEI one, or even more
> complex ones like the MMC interface.  These are all valid uses of ioctls
> as they are device/filesystem specific ways to interact with the kernel.
> 
> The thing is, almost no one pays attention to these new ioctls as they
> are domain-specific interfaces, with open userspace programs talking to
> them, and they work well.  ioctl is a powerful and useful interface, and
> if we were to suddenly require no new ioctls, and require everything to
> be a syscall, we would do nothing except make apis more complex (hint,
> you now have to do extra validation on your file descriptor passed to
> you to determine if it really is what you can properly operate your
> ioctl on), and cause no real benefit at all.
> 
> Yes, people abuse ioctls at times, but really, they are needed.
> 
> And remember, I was one of the people who long ago thought we should not
> be adding more ioctls, but I have seen the folly of my ways, and chalk
> it up to youthful ignorance :)
> 

Exactly, and that's why I wondered about the sudden change
of heart ;-)

Thanks for taking the time to write this.

Regards,
Darwish
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