Dirk Behme:
Just for the record:
The trick is to ensure that the __arch_putx() containing the volatile is not the last statement in the GCC statement-expression. So, using something like
#define writeb(v,c) ({ __iowmb(); __arch_putb(v,c); v;})
(note the additional 'v;') should result in correct code, too.
Yes, that's good. Also "0" may work, and may be more readable, (or not, according to who reads it).
The patches sent by Wolfgang and Alexander using
#define writeb(v,c) do { __iowmb(); __arch_putb(v,c); } while (0)
do the same with a slightly different syntax, so these patches are fine, too.
It's not just different syntax, it's different semantics.
The ({...}) trick turns statements into expressions, while the "do {...} while(0)" does not. I'd better not forbid statements like
while (reg = readb(addr), reg != VALUE) { .... }
or
if (readb(addr) == VALUE) { ... }
or swtich (readb(addr)) { ... }
While I agree they may in general not be clean, I can forsee meaningful uses. Moreover, I'd better allow expression-looking macros to really behave like expressions -- otherwise error messages are quite hard to understand for the unaquainted.
IMHO, the only reason to use "do {} while(0)" over statemente expressions is being portable but in u-boot we are gcc-specific anyways.
/alessandro