On Fri, 11 Sep 1998, Jason Thorpe wrote:

> Base and offset; you cannot use structs:
> 
> 	(1) different arch's will have different struct padding!
> 
> 	(2) the device may be mapped sparse by the hardware.

You should be able to get away with structures if:

You make sure you use absolute types.  Different ports have different
sizeof(long). 

You place pad fields in every hole so you don't rely on the compiler
padding for you.  Always do:

	struct foo {
		int8_t bar;
		int8_t pad0;
		int16_t pad1;
		int32_t frobitz;
	};

And never:

	struct foo {
	        int8_t bar;
	        int32_t frobitz;
	};

You do not use bitfields (which this particular FreeBSD driver obviously
uses).

You only use the structure to dereference device registers through a
bus_space* accessor, never directly:

	bus_space_read1(tag, &foo_ptr->foo_field, ...)

Never split a register into different fields in a structure.  If it's a
16-bit register always access it through a 16-bit structure field, even if
you only want to deal with only 8-bits of that register.  Otherwise you
may have endianness problems.

=========================================================================
Eduardo Horvath				eeh@one-o.com
	"I need to find a pithy new quote." -- me