[PATCH 2/8] pseries: phyp dump: reserve-release proof-of-concept

[Michael Ellerman]

On Wed, 2008-03-12 at 23:29 -0500, Manish Ahuja wrote:
> If Mike and Paul are okay, then I will leave this bit as is and fix all 
> other issues and comments.

Well I still don't like it - it uglifies the code _now_, for a potential
future benefit that may never come. But I don't care that much, if
Paul's happy with it let it go in.

cheers

> Linas Vepstas wrote:
> > On 10/03/2008, Michael Ellerman <michael at ellerman.id.au> wrote:
> >> On Thu, 2008-02-28 at 18:24 -0600, Manish Ahuja wrote:
> > 
> >>  > +
> >>  > +/* Global, used to communicate data between early boot and late boot */
> >>  > +static struct phyp_dump phyp_dump_global;
> >>  > +struct phyp_dump *phyp_dump_info = &phyp_dump_global;
> >>
> >> I don't see the point of this. You have a static (ie. non-global) struct
> >>  called phyp_dump_global, then you create a pointer to it and pass that
> >>  around.
> > 
> > I did this. This is a style used to minimize disruption due to future
> > design changes. Basically, the idea is that, at some later time, for
> > some unknown reason, we decide that this structure shouldn't
> > be global, or maybe shouldn't be statically allocated, or maybe
> > should be per-cpu, or who knows.  By creating a pointer, and
> > just passing that around, you isolate other code from this change.
> > 
> > I learned this trick after spending too many months of my life hunting
> > down globals and replacing them by dynamically allocated structs.
> > Its a long and painful process, on many levels, often requiring major
> > code restructuring.  Code that touches globals directly is often
> > poorly thought out, designed.  But going in the opposite direction
> > is easy: if your code always passes everything it needs as args
> > to subroutines,  then you are free & clear ... if one of those args
> > just happens to be a pointer to a global, there's no loss (not even
> > a performance loss -- the arg passing overhead is about the same
> > as a global TOC lookup!)
> > 
> > So it may look weird if you're not used to seeing it; but the alternative
> > is almost always worse.


-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person
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