Re: [PATCH] Document Linux's memory barriers

Andi Kleen
2006-03-07T18:02:07Z

On Tuesday 07 March 2006 18:40, David Howells wrote:

> +Older and less complex CPUs will perform memory accesses in exactly the order
> +specified, so if one is given the following piece of code:
> +
> + a = *A;
> + *B = b;
> + c = *C;
> + d = *D;
> + *E = e;
> +
> +It can be guaranteed that it will complete the memory access for each
> +instruction before moving on to the next line, leading to a definite sequence
> +of operations on the bus:

Actually gcc is free to reorder it
(often it will not when it cannot prove that they don't alias, but sometimes
it can)

> +
> + Consider, for example, an ethernet chipset such as the AMD PCnet32. It
> + presents to the CPU an "address register" and a bunch of "data registers".
> + The way it's accessed is to write the index of the internal register you
> + want to access to the address register, and then read or write the
> + appropriate data register to access the chip's internal register:
> +
> + *ADR = ctl_reg_3;
> + reg = *DATA;

You're not supposed to do it this way anyways. The official way to access
MMIO space is using read/write[bwlq]

Haven't read all of it sorry, but thanks for the work of documenting
it.

-Andi