Re: IO, ANSI vs GCC structs

On Fri, 25 Jul 2003, Eugene Surovegin wrote:

At 12:22 PM 7/25/2003, Wolfgang Denk wrote:

quoted

quoted

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@qlogic.com      6321 Bury Drive, St 13, Eden Prairie, MN 55346


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