Re: Driver Model 2 Proposal - Linux Kernel Performance v Usability

Richard B. Johnson
2003-09-15T11:38:15Z

On Fri, 12 Sep 2003, Timothy Miller wrote:

>
>
> Richard B. Johnson wrote:
> > On Wed, 10 Sep 2003, Timothy Miller wrote:
> >
> >
> >>I just have one quick question about all of this:
> >>
> >>People mention that driver interfaces don't change much in stable
> >>releases, but if memory serves, symbol versioning information changes
> >>with each minor release, requiring a recompile of modules.
> >>
> >>Would it be possible to have a driver module which can be dropped into,
> >>say, 2.6.17 that can also be dropped into 2.6.18 as long as the
> >>interface doesn't change?
> >>
> >
> >
> > Short answer, YES. Anything that can be done is possible. The
> > problem is that different kernel versions end up with different
> > structure members, etc. So, you can't use code for 2.2.xxx in
> > 2.4.xx because, amongst other things, the first element in
> > 'struct file_operations' was added and the others moved up.
>
> That's all fine and dandy. When the kernel changes its interface, then
> you have to recompile (or rewrite) drivers. No problem. I'm just
> trying to avoid having to recompile drivers if the interface DOESN'T change.
>
> >
> > Now, you can make a different module interface that maintains
> > a compatibility level ABI. This has been discussed. Unfortunately,
> > this adds code in the execution path. This extra code gets
> > executed every time the module code is accessed. The result being
> > that the module can't possibly operate as fast as it would if
> > there were no such compatibility layer(s). It might be "good enough",
> > but it is unlikely that the module contributors/maintainers would
> > allow such an interface because the loss of performance is measurable
> > and there has been no requirement to trade-off performance for
> > anything (your and my convenience doesn't count, those are not
> > technical issues).
>
> I am not interested in adding additional layers of abstraction. At
> least not here. I do it plenty of other places, but that's not
> important right now. If someone else wants to make an abstraction layer
> (which seems to have been done here and there), then that's just fine,
> and I may or may not use it.
>
> My point is that I'm not advocating any of the kruft associated with
> backward-compatible interfaces. I'm advocating not having to recompile
> only in the cases where the interface DOES NOT change.
>

In those cases, you simply use "-f" on the `insmod` command line
to force the loading of a module. We have that capability now.
But, the offsets of things your module(s) access can, read will,
be incompatible if the version changed. You "takes you chances and
cross you fingers...." if you use the "-f" option.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (794.73 BogoMips).
Note 96.31% of all statistics are fiction.