IO, ANSI vs GCC structs

[Bret Indrelee]

On Fri, 25 Jul 2003, Eugene Surovegin wrote:
> At 12:22 PM 7/25/2003, Wolfgang Denk wrote:
> > > accessed through a structure?  Is it acceptable to do:
> > >
> > >   #define MYHW  (MY_MBAR+42)
> > >
> > >   ...
> > >
> > >   out_be32(MYHW, 0xco1df00d);
> >
> >...I would not accept such code.
> >
> >There is one fundamental difference: a field in a struct has a  type,
> >i.  e.  the  compiler "knows" how big it is (8, 16, 32, ... bits), if
> >special attributes apply (volatile), etc.
>
> Although functions like out_be32 also *know* the size :).
>
> IMHO, it's not a good idea to access IO remmaped registers simply through C
> struct, it's better and safe to use out_* and in_*.

Wouldn't best practices also include using a base address rather than
a constant, so that when a new board places the hardware in a different
location all you have to do is find it?

out_be32(my_bar + offsetof(myregs_t, sna_reg), 0xdeadbeef);

> There are issues that C struct cannot deal with - load/store reodering (of
> course you can insert wmb() and friends if you know what you are doing :).
>
> I think the best practice is to use C struct (for readability) and
> out_*/in_* on structure members for actual access.

I've always written low level I/O routines for everything in a driver
because it is easier to adapt when someone changes how they want to do
access to the device. The code all calls the low level IO routines.

-Bret

--
Bret Indrelee                 QLogic Corporation
Bret.Indrelee at qlogic.com      6321 Bury Drive, St 13, Eden Prairie, MN 55346


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