Re: 26 ways to set a device driver variable from userland

linux-os (Dick Johnson)
2005-10-19T20:09:20Z

On Wed, 19 Oct 2005, Rick Niles wrote:

> There are so many ways to set a configuration value in a device driver. I'm wondering
> which are "recommended" methods. I'm looking for some sort of guidance when writing a
> new driver. I kinda assume this is a FAQ, but I didn't see it anyway, maybe it should
> be added to the FAQ.
>
> OK there might not be 26 ways, but there's a few major ones. I'm thinking in term of
> char devices so some of these might not apply to block and network drivers.
>
> (1) ioctl, probably the oldest.
> (2) use read/write to a special configuration-only /dev file (e.g. /dev/dvb)
> (3) /proc filesystem
> (4) sysfs
> (5) module load-time command line options.
>
> I understand that flexibility is a good thing, but some guidance would be helpful.
>
> I sorta got the idea that /proc is "out" this year and sysfs is the "in" thing, but
> what about the others? Would you say that (2) should be be discouraged? Did anyone
> tell the DVB people that? Or maybe more is better, that is, a good driver should
> allow for ALL of the above! (OK, yeah that was flame bait.) Should EVERY variable
> that can be modified by say sysfs also be settable by insmod command line?
>
> Any guidance would be greatly appreciated,
> Rick Niles.
>

There is no question that ioctl() will always exist. You will never
have a problem compiling your module when Linux-99.99 rolls out
in the year 2054. That said, you want to start your ioctl control
values above anything the kernel uses and return -ENOTTY for anything
out of range. This will allow somebody to do `od /dev/YourDevice` without
breaking your device!

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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