Re: [PATCH][2.6] IBM PowerPC Virtual Ethernet Driver

[Andrew Morton]

Keith Owens <kaos@xxxxxxxxxx> wrote:
>
> On Wed, 18 Feb 2004 11:45:20 +1100, 
> Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote:
> >BITFIELDS ARE EVIL !!!
> >The compiler is perfectly free, afaik, to re-order them
> 
> Not it is not.  C standard (ISO/IEC 9899:1999 (E)), section 6.7.2.1.
> 
> 10 ...  If enough space remains, a bit-field that immediately follows
>    another bit-field in a structure shall be packed into adjacent bits
>    of the same unit. ...
> 
> 13 Within a structure object, the non-bit-field members and the units
>    in which bit-fields reside have addresses that increase in the order
>    in which they are declared. ...
> 
> There is no scope for a compiler to reorder the members or the bit
> fields of a structure.

Problem is portability between different architectures, especially little
endian versus big endian.  The 3c59x driver used to use a bitfield to
represent hardware registers and had some lovely ifdeffery to make it work
on ppc hardware.

It's safe enough in arch-specific code but one may as well use the same
idioms as are used in generic code.

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