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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
