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

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:

quoted

On Fri, Jan 16, 2015 at 11:16:05AM -0800, Greg Kroah-Hartman wrote:

quoted

From: Daniel Mack <redacted>

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.

Oops, I meant, MIC, not MMC, sorry about that.