Re: ioctl assignment strategy?

From: Al Hooton
Date: Wed Dec 22 2004 - 12:17:28 EST

With permission from participants, posting the end of this thread back
to the list for the archives... . a little long, if you're not
interested in strategies to avoid creating new ioctls for calls from
userspace, hit delete now....

On Tue, 2004-12-14 at 16:46 -0800, Greg KH wrote:
> Minor one coming, why do you want to use an ioctl? ioctls are generally
> frowned upon these days, and trying to add a new one is a tough and
> arduous process, that is not for the weak, or faint of heart.

On Fri, 2004-12-17 at 15:50 -0800, Greg KH wrote:
> >
> > We use sysfs or individual /dev nodes (now that we have a huge range of
> > major and minor numbers.) We can also create a filesystem just for an
> > individual driver (takes less than 100 lines of kernel code now.)
> Excellent, this is exactly what I needed to know -- architectural-level
> changes in how the kernel architects now expect things to be done. I
> had not yet discovered the change in major/minor space during this "get
> reacquainted with the kernel" period I'm in, and was still operating
> under the old 256 minors limitation, etc., in my mind. This is perfect!
> >
> > What do your ioctls do? Usually just rethinking about what is really
> > needed from them can show us where to put them.
> In the parapin digital I/O kernel module, there are 6 operations:
> - Claim the parallel port
> - Configure individual pins to be either input or output
> - Set pin states (high or low) for one or more pins using a bitmask
> - Get pin states (high or low)
> - Respond to a pin-10 hardware interrupt
> - Release the parallel port
> My original device driver implementation uses open() to claim the port,
> close() to release it, and ioctl() for everything except setting up the
> interrupt/handler mechanisms. Sometime in the near future I will deal
> with interrupts, but not until after I get everything else stabilized.
> Thanks to your input, have decided a clean approach would be to use
> read/write for everything I was doing with ioctls (which, obviously,
> would have been possible anyway, but sometimes the whack with the
> cluebat makes the obvious more obvious...). I plan to set it up such
> that each write() requires two words (a control word specifying the
> operation to perform, and a data word with a bitmask or bit values in
> it. Each read returns a single word with current pin states, possibly
> masked by a bit mask handed down in the write(). We currently support
> building on 16-bit archs, so I don't want to combine control bits in
> words with data bits, we don't have enough space. This will be a simple
> interface that is very easy to port to just about any platform.
> My plan, now changed, for the interrupts is to not set up a
> signal-based mechanism, at least at first. Instead, I will define
> another minor on the device for letting apps know an interrupt came in
> on pin 10. When an interrupt hits, the driver will send up a single
> word to that minor. Once an app has opened the device, it can check for
> words showing up on it, either blocking or non-blocking. I realize
> there is increased latency with this approach, but a lot of the folks
> that use parapin are hardware engineers or students in universities with
> relatively little coding background. Dealing with signals is farther
> down the stack than many of them will ever get. I will probably add an
> interrupt-driven signal mechanism later for those that want to use it.
> I may have another project coming up sometime next year, however, that
> will probably require several hundred minors, and a few majors, which
> can be dynamically defined (another great improvement in device
> interfacing since I was last poking around!). You don't want to see how
> ugly that interface was planned to be before you pointed me to the
> major/minor improvements... 8^)=
> Thanks again!
> Best Regards,
> Al

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