Re: TUNABLE_INT vs TUNABLE_INT_FETCH

On Thu, Aug 23, 2012 at 04:55:05PM +0100, Attilio Rao wrote:
> On Thu, Aug 23, 2012 at 5:05 PM, Luigi Rizzo <rizzo_at_iet.unipi.it> wrote:
> > On Thu, Aug 23, 2012 at 03:52:56PM +0100, Attilio Rao wrote:
> >> On 8/23/12, Luigi Rizzo <rizzo_at_iet.unipi.it> wrote:
> >> > Hi,
> >> > I am a bit unclear on what are the pros and cons of using
> >> > TUNABLE_INT vs TUNABLE_INT_FETCH within a device driver.
> >>
> >> TUNABLE_INT is basically the "statically initializer" version of
> >> TUNABLE_INT_FETCH.
> >> In short terms, you will use TUNABLE_INT_FETCH() in normal functions,
> >> while TUNABLE_INT() in data declaration.
> >
> > The thing is, do we need the data declaration at all ?
> 
> What do you mean with "data declaration"?

i am using your words :)

> We need to mimic a "static initialization" usage, so what we do is to
> use the first SYSINIT() family available (SI_SUB_TUNABLES). You also
> need the env to look for and the static variable to initialize, so for
> SYSINIT's sake you need to pack them up in a single argument.

To explain: as i understand it, kenv variables are created and stored
(presumably as strings) even if not explicitly declared as
TUNABLE_*(). The role of the SYSINIT() block is presumably to
copy the values of interesting entries into C variables
(i suppose at boot time, and perhaps even when kenv runs).
This should be the 'static initialization' you mention.

I think there is only a limited number of cases where this makes sense,
in most circumstances the variables passed through the environment
should be read explictly via TUNABLE_INT_FETCH() to make sure that
they do not change in unexpected moments.

This is why in the documentation I'd probably suggest to use
the TUNABLE_*_FETCH() variant unless you are really really
sure that the variable can change at any time as a result of
a kenv call (or make it clear that it *will not* reflect
the kenv result, i am not sure how it works).

Anyways, thanks for the clarificaiton, now i know what i need to use.

cheers
luigi