[PATCH v2 3/3] ARM: Add barriers to the I/O accessors if ARM_DMA_MEM_BUFFERABLE

[Russell King - ARM Linux]

On Fri, Jul 09, 2010 at 12:41:36PM +0100, Catalin Marinas wrote:
> This part requires a small change. It looks like the e1000e driver
> doesn't like the do {...} while (0) constructs in the write*() accessors
> (the E1000_WRITE_REG_ARRAY macro). Thanks to Giuseppe for finding this.

Hmm.  I think those additional parens in drivers/net/e1000e/hw.h should
be killed off instead.  They aren't serving any useful purpose.

In fact, this is potentially dangerous - it causes write[bwl]() not to
be 'void' - in that if you do use write[bwl]() in an expression that
wants its return value, write[bwl]() will generate a read.  (While you
might not use the value, you're relying on the compiler being able to
spot that you're not using the value.)

Keeping the do { } while() there prevents write[bwl]() being used as
an expression, and therefore avoids nasty surprises when the compiler
decides to read the register after writing.