On Friday 12 November 2010, Russell King - ARM Linux wrote:
> On Fri, Nov 12, 2010 at 04:14:51PM +0100, Arnd Bergmann wrote:
> > Some people prefer to express all this in C instead of macros:
> >
> > struct mcde_registers {
> > enum {
> > mcde_cr_dsicmd2_en = 0x00000001,
> > mcde_cr_dsicmd1_en = 0x00000002,
> > ...
> > } cr;
> > enum {
> > mcde_conf0_syncmux0 = 0x00000001,
> > ...
> > } conf0;
> > ...
> > };
> >
> > This gives you better type safety, but which one you choose is your decision.
>
> It is a bad idea to describe device registers using C structures, and
> especially enums.
>
> The only thing C guarantees about structure layout is that the elements
> are arranged in the same order which you specify them in your definition.
> It doesn't make any guarantees about placement of those elements within
> the structure.
Right, I got carried away when seeing the macro overload. My example
would work on a given architecture since the ABI is not changing, but
we should of course not advocate nonportable code.
Normally what I do is to describe the data structure in C and define the
values in a separate enum. The main advantage of using the struct instead
of offset defines is that you have a bit more type safety, i.e. you cannot
accidentally do readw() on a __be32 member.
Using #define for the actual values makes it possible to interleave the
values with the structure definition like
struct mcde_registers {
__le32 cr;
#define MCDE_CR_DSICMD2_EN 0x00000001
#define MCDE_CR_DSICMD1_EN 0x00000002
__le32 conf0;
...
};
whereas the enum has the small advantage of putting the identifiers
into the C language namespace rather than the preprocessor macro
namespace.
Arnd
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