On 30 Apr 2007 17:32:04 +0200, Andi Kleen wrote:
> > From: Juergen Beisert <[EMAIL PROTECTED]>
> >
> > Replace NSC/Cyrix specific chipset access macros by inlined functions.
> > With the macros a line like this fails (and does nothing):
> > setCx86(CX86_CCR2, getCx86(CX86_CCR2) | 0x88);
> > With inlined functions this line will work as expected.
>
> Why do the macros not work?
Delayed evaluation of parameters causing unexpected and broken
interleaving of side-effects. The macros look as follows:
#define getCx86(reg) ({ outb((reg), 0x22); inb(0x23); })
#define setCx86(reg, data) do { \
outb((reg), 0x22); \
outb((data), 0x23); \
} while (0)
So the statement
setCx86(CX86_CCR2, getCx86(CX86_CCR2) | 0x88);
becomes
outb((CX86_CCR2), 0x22);
outb((({ outb((CX86_CCR2), 0x22); inb(0x23); }) | 0x88), 0x23);
which is equivalent to
outb(CX86_CCR2, 0x22); // setCx86
outb(CX86_CCR2, 0x22); // getCx86
tmp = inb(0x23) | 0x88; // getCx86
outb(tmp, 0x23); // setCx86
which the processor doesn't like.
With inline functions the parameters are fully evaluated
before the bodies, so we get
outb(CX86_CCR2, 0x22); // getCx86
tmp = inb(0x23) | 0x88; // getCx86
outb(CX86_CCR2, 0x22); // setCx86
outb(tmp, 0x23); // setCx86
instead.
/Mikael
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